Movement Ecology of Animals

Homepage of Dr. Emily A. McKinnon


2 Comments

Connecticut Warblers fly over the Atlantic for 2 straight days in the fall

Yep. Move aside, Blackpoll Warblers. There’s a new trans-Atlantic fall migrant in town. Okay, actually Connecticut Warblers have probably been doing this for thousands of years, but it’s pretty surprising because we didn’t know it until now.

In my Scientific Naturalist paper in Ecology (or download directly McKinnon_et_al-2017-Ecology), I show how 4 Connecticut Warblers flew from the coast of the US for two solid days (minimum of 48 hours) over water to land somewhere in the Greater Antillean islands, probably Hispaniola. To make it even more stunning, they rested for about a week, then again flew almost 800 km over open water to the Gulf of Venezuela. And they weren’t done yet! They continued south to winter somewhere deep in the Amazon basin. The data are a little blurry on the exact wintering sites, but somewhere deep in the green that is the Amazon forest.

In contrast, the ocean-crossing is as clear as day because, well, there are no trees over the ocean! So when we are using light-sensors to track migration, it’s really obvious when the birds are not experiencing any shading at all from trees or other vegetation. The light levels just steadily increase and then decrease as dusk falls. In contrast, when the birds are on land, it’s a mess of light-levels going up and down, even as low as zero when birds are in really shady spots (giving us dozens of artificial ‘sunsets’ that we have to filter out). This is partly to blame for our fuzzy delineation of the exact overwintering sites for these individuals. The Amazon forest understorey is a dark place!

IMG_2571.JPG

Connecticut Warbler with a light-sensing geolocator backpack. This was one we put out this past summer (2016) so we hope this bird comes back this year (2017)! The stalks will hopefully give us better resolution for identifying wintering sites. The original tags we used were stalk-less and had a lot of shading from the feathers.

Back to the fall migration of this species – how did we miss this amazing feat of migration?

I think there are a lot of reasons, actually. First of all, Connecticut Warblers, on a global scale, are not very abundant, so they are not a species commonly captured or seen by banders or birders. They nest in the southern-boreal aspen-transition habitats in Manitoba, Ontario, Quebec, and some parts of the northern Great Lakes states like Wisconsin and Michigan, but they are not common even within their breeding range. Not many people venture out into bogs in June in Manitoba either, and I know why!

IMG_1278

Feeding the healthy mosquito population while catching Connecticut Warblers

Okay, so not a lot of overlap between people and Connecticut Warblers (a range map is in my previous post on these guys). I also think that Connecticuts probably get confused with their more southern-breeding cousins, the Mourning Warblers, or their western cousins, MacGillivray’s Warblers. This would be especially the case in the fall, where the young-of-the-year in those species might actually have a complete eye ring like a Connecticut Warbler (normally they only have semi-circles, as in MacGillivray’s, or none, like the Mourning).

So all of this means that coming across one of these guys and positively identifying it in the fall especially might be tricky.

But we had some hints that they might do something drastic over the ocean. Hurricane Emily brought down 75 individuals on BERMUDA one October. What would they be doing way the heck over the ocean?!? There is also a really neat video capture of a flock following cruise ship lights in the Lesser Antilles. Finally, the famous ornithologist Alexander Chapman commented on how darn FAT Connecticut Warblers were in the fall in the northeastern states… hmmm, suspicious, no?

Now that we have some tracking data, I can see additional reasons why we would miss this. First of all, if their major Caribbean stopover is on Hispaniola, it would be easy to miss. Hispaniola’s landscape is rugged, and humanitarian crises have resulted in few bird surveys there in recent decades. Unfortunately, it’s also lost a lot of forest recently- something like 90% of the original forest cover in Haiti is gone! Yikes.

Now that we know Connecticut Warblers travel through this area, hopefully we can get some more details on what type of habitat they might use or need for these Caribbean stopovers. Clearly their strategy relies on extraordinarily long migratory flights, which suggests they also need extraordinary amounts of fuel in advance of these flights as well as for recuperation after.

For Connecticut Warblers, I hope this study gives them some well-deserved time in the spotlight. They’ve been missing in action for too long, and I don’t want them to end up missing in action for good!

Full citation for the paper plus links to download a pdf and supplemental files:

McKinnon, EA, Artuso, C., and OP Love. 2017. The mystery of the missing warbler. Ecology. 10.1002/ecy.1844

McKinnon_et_al-2017-Ecology

ecy1844-sup-0001-AppendixS1 (1)

ecy1844-sup-0002-AppendixS2

CONWheadshots

Connecticut Warblers of Manitoba.

 

 


Leave a comment

Would you like a side of fruit with your beetles and ants?

One of the interesting things about Wood Thrushes and many other migratory species is that they have an amazing plasticity in diet. Baby songbirds grow up getting half-smooshed arthropods shoved down their throats from Mommy and Daddy. The high-protein and high-fat content of insects and spiders make these a good food source for growing nestlings. There are some exceptions to this, but in general, baby-bird food is animal-based. Adult songbirds have a lot of different strategies. The thrushes and many warblers also eat a lot of ‘bugs’ during the breeding season, when summer productivity is high and there are lots of juicy lepidopteran larvae around. In fall though, many switch to gorging on fruits. Many songbirds are key seed-dispersers actually, and the many red-berried fruits decorating fall forests are a testament to the co-evolution of birds and plants (see the Wood Thrush-American Ginseng example here).

During the overwintering period, many songbirds continue to eat fruit, and some even take on a nectarivorous diet, like the Cape May Warbler Setophaga tigrina (Latta and Faaborg 2002). When birds do eat a mixed diet of animal and plant-based foods, is there an optimal combination for staying healthy? Or are plant foods just a poor-quality place holder until more arthropods can be found? This is a question that has been looked at in a few species of overwintering migratory songbirds, but with mixed results. Hermit Thrushes (Catharus guttatus) seem to fatten up more easily on diet of arthropods (Long and Stouffer 2003). But in Costa Rica, migrant songbirds as a group seem to increase their consumption of fruits later in the nonbreeding period, a time when they should be starting to fatten up and pack on muscle for migration (Blake and Loiselle 1992).  This got me wondering about Wood Thrushes. I found out that the forests at my study site in Belize dry out seasonally, and the abundance of arthropods declines. Maybe Wood Thrushes would switch to a more fruit-filled diet later in the winter, prior to migration. If so, would there be a cost? Could they still get ready for their spring sprint northwards, feeding on tropical figs instead of ants and beetles?

woodthrushwithbeetlecjimenez

Most awesome Wood Thrush photo ever by Chris Jimenez. All his photos are fabulous but this one is particularly impressive because 1) it’s actually in a tropical forest (in Costa Rica), 2) it’s eating a beetle, and 3) Wood Thrushes are hard enough to see in dense tropical understory, let alone photograph!  I really wanted to publish this paper in a journal with a cover photo so I could use this picture, but alas Journal of Field Ornithology doesn’t do cover photos. 

[Check out more of Chris’ amazing photos here: http://www.chrisjimenez.net/%5D

To figure this out I analyzed the diet of Wood Thrushes from my study site in Belize. It can be challenging to figure out the diet of small songbirds, since they are not easy to watch in the wild. I don’t know how many times I saw a Wood Thrush foraging on a trail, then ran over as soon as it flushed to try to see what it had been eating. Usually there was no sign of anything! It was more obvious when they were eating fruit, since they would join up with dozens of other Wood Thrushes and many other species, like the tropical resident Clay-coloured Robins, Black-faced Tanagers, Gray Catbirds, and even toucans like the Collared Araçari, all in the tops of fruiting trees, generally making an obvious ruckus and spitting out seeds all over the ground under the trees. I saw this most frequently with Ramon or breadnut trees – Wood Thrushes and other birds loved these fruits, and I could catch Wood Thrushes all day in a single net at the base of the tree, as birds from all over came in for the bonanza.

wotheatingfruit

This is a Wood Thrush trying to eat a ramon fruit. These fruits are 90% seed, with a thin coating of fruit around the outside. Wood Thrushes spit out the seeds on the spot. Clearly they are not doing a very good seed-dispersal job. Maybe the bigger birds like toucans are what this tree is really hoping for? 

To analyze the diet of Wood Thrushes, I decided to look at the stable isotope ratios of nitrogen and carbon in their blood. Basically, there are multiple isotopes of these common elements, and animals preferentially incorporate the lighter isotope into their tissues. This means that animals eating things higher up on the food chain will be consuming tissues with more of the heavier isotope. For example, a top predator, like a jaguar, would have a higher stable isotope ratio of Nitrogen compared to a herbivore like a tapir. Carbon works basically the same way, but also with some differences in the isotope ratio of different types of plants (e.g. grasses versus herbaceous). These isotopes are useful for distinguishing a diet of plant- vs. animal foods because the animals should always have higher stable nitrogen isotope ratios (and to a lesser extent carbon ratios) than the plants. That means a Wood Thrush eating more bugs will also have a higher stable nitrogen isotope ratio in its tissues. For this project, I was already collecting a tiny blood sample for genetic sexing so I used the remainder of the blood for the stable isotope analysis.

IMG_2389

Taking a tiny blood sample for genetic sexing and stable isotope analysis.

I also collected a bunch of potential food sources for comparison – beetles, ants, fruits, spiders, and grasshoppers. First I just looked at the isotope ratios of carbon and nitrogen, then I tried to reconstruct the actual proportions of different foods in the diet by using a mixing model.

Here’s the basic food web that I reconstructed:

Slide3.jpg

On the vertical axis is the ratio of stable nitrogen isotopes, which we call delta-15-N, measured in units of permil (that weird percent thing in the brackets). As you go up the food chain, delta-15-N gets higher, which is why spiders and Wood Thrushes are in the top part of the graph, and fruits and grasshoppers at the bottom. The horizontal axis shows the stable carbon isotopes, or delta-13-C. This plot is call a C-N plot. The dots are the average values, and the crosses show the variation around the average. You can see that Wood Thrushes are all the same (no visible error bars) but beetles and spiders are really variable, especially in delta 15-N. This is probably because I lumped a lot of different species of these guys together. 

Right away, the data seemed to suggest that Wood Thrushes were not eating that much fruit. Their stable nitrogen ratios were just too high! I analyzed the data to see if the highest stable isotope values were associated with the best body condition in Wood Thrushes. Did birds at the ‘top’ end of the food web have a payoff compared with birds slightly lower down? The answer is a resounding no, although, there wasn’t a lot of variability in the diets of the Wood Thrushes I analyzed. Most of them seemed to be eating primarily arthropods, and the slight variability across individuals didn’t predict their body condition, fat or muscle levels. But there were some interesting patterns in the data that were somewhat unexpected.

First, I did find some differences by habitat. Birds in the driest, early successional forest tended to eat more fruit mid-winter but then actually increased the amount of arthropods they consumed as spring approached. The opposite pattern was found in the more mature forest sites, with the end result that diets were actually very similar across all habitats in late winter. This tells me that Wood Thrushes in different habitats might be fine eating whatever is easiest and available all winter, but as spring approaches they seek out an optimal balance of fruit and arthropods. This habitat difference also likely explains a few differences I found between males and females, since females were more abundant in the dry habitat. The diet differences didn’t relate to body condition though, so it wasn’t that females or birds in the dry habitat were suffering because of their different diets. They just ate different stuff.

So seems like bugs with a side of fruit is what Wood Thrushes go for in the winter (and not fruit with a side of bugs). What exact types of bugs and fruits would be interesting to figure out – maybe something like DNA barcoding will give us more specific information on their diet in future.

Check out the full paper here:

McKinnon, EA, TK Kyser and BJM Stutchbury. 2017. Does the proportion of arthropods versus fruit in the diet influence overwintering condition of an omnivorous songbird? Journal of Field Ornithology. Early online. doi:10.1111/jofo.12187

 

 

 


Leave a comment

The most interesting boreal bird you’ve never heard of

Spring is on the horizon and birders everywhere are starting to get antsy for those first early migrants. This year is a very exciting one for me, as I’m waiting for the return of one of my newest study species – one of the most mysterious boreal songbirds out there. Right now, ‘my’ birds are out there with tiny data-logging backpacks on, fattening up deep in the jungles of South America, waiting to head north.

Let’s see if you can guess who they are. These small warblers are long-legged, but not always found on the ground; they are colourful, but camouflage well; they are loud songsters, but largely quiet on migration, and often missed at breeding sites; they breed in open bogs, mature aspen forest, or mature mixed woods, but most people live far away from their breeding range; they look very similar to two other warbler species, but they are genetically distinct so placed in their own genus. Here’s a recording I made of one this past spring if you need a big hint:

The answer is… Connecticut Warbler (Oporornis agilis)!

First let me just explain that this unfortunately-named species has little or nothing to do with Connecticut, at any point in its life cycle! The name came from a single specimen that was ‘collected’ (i.e. shot) over the state of Connecticut in 1812 by Alexander Wilson. A better name for this species might be something more physically descriptive, like blue-headed or eye-ringed warbler, or maybe something related to habitat, like boreal bog warbler.

IMG_0258

Check out that beautiful eye ring! Thanks to my field tech @KelseyDBell for taking the photo

In general Connecticut Warblers have a reputation for being mysterious, shy, retiring, skulky, rare, or even ‘sluggish’! After my first field season with this species this past summer, I think this reputation is because most birders only ever see Connecticuts during migration, and they are fairly rare and quiet during this period. Connecticut Warblers also migrate later in spring than a lot of other species, so they easily miss peak spring birding in May. Fortunately for me, living in Manitoba, at the edge of the boreal forest, Connecticut Warblers can be found breeding within 45 minutes’ drive of my house! Delving into the scant literature on this species last winter, I realized that we might actually be losing the Connecticut Warbler before we even know anything about it.

Long-distance migratory songbirds as a group are showing population declines across North America – my previous study species, the Wood Thrush, shows a fairly typical pattern of about 2% population loss per year over the last 40 years. The data for boreal songbirds are a little less strong because the breeding bird surveys used to generate population trends tend to peter out as you go further away from human population centres. However, many boreal songbirds are very long-distance migrants (i.e. they go all the way from the temperate boreal to South America), which is a big risk factor, and the trends that we do have don’t look very promising for most, including the Connecticut Warbler. Overall, the Connecticut Warbler is declining by about 1% per year. Most of the Connecticut Warbler breeding range is in Canada, which means that Canadians in particular have a responsibility to protect this bird.

CONWtrendmapBBS.gif

This map shows the Breeding Bird Survey trends over the last 40 years, with areas in red declining the most and areas in blue increasing. Overall the Connecticut Warbler gets a ‘medium’ confidence in terms of how real these trends are – this is because there aren’t as many survey routes in their range as we would like, and their density is generally pretty low.

One of the potential threats listed for this species is habitat loss in the winter range. But critically, we have nearly zero information on migration, winter range, winter habitat, or winter ecology. There are only 5 ebird records for this species in the winter range. One in Bolivia, the rest in Colombia. Records in the literature are equally sparse. It’s hard to believe that a North American breeding bird could be so under-studied. Dr. Jay Pitocchelli and colleagues recently (2012) updated the Cornell Lab of Ornithology’s Birds of North America Species Account for Connecticut Warbler, and it is clear there are still huge gaps in our knowledge of this species. For one, it took 70 years after the original specimen was collected for a nest to be described. What we know about breeding biology is largely from a very detailed study of ONE nest in Michigan in the 1960s. Reading through the BNA Account had me more and more intrigued. Then I read this: “Some evidence for nonstop trans-oceanic migration (Monroe 1968, Wetmore et al. 1984) […] Records for West Indies are scarce, but large numbers were reported on Bermuda during Hurricane Emily on 26 Sep 1987 when 75 were grounded; previous daily high had been 3 (Amos 1991).”

Could Connecticut Warblers be trans-oceanic fall migrants, like Blackpoll Warblers? 

Examining the ebird records for the fall migration period were also suggestive of a trans-oceanic flight. Connecticut Warblers are rarely detected along the gulf coast or in Florida, and there is only one ebird record for all of Central America (Panama). In contrast, they have been spotted throughout the Lesser Antilles and into northern South America. Could they be flying over the ocean from the east coast of the US directly to the Lesser Antilles or even direct to South America?

ConnecticutMigrationEbird.jpg

Examining ebird.org records for Connecticut Warblers shows the potential for a trans-Atlantic flight in fall.

Given the total lack of information on fall migration routes and winter sites, and the potentially mind-blowing fall migration this species might undertake, and the fact that they are a designated stewardship species of the northern forest, they are a prime candidate for migration tracking!

After convincing my supervisor Dr. Oliver Love and mentor Dr. Christian Artuso of Bird Studies Canada that this was worth investigating, last summer I captured over 30 males, and put out tiny 0.5 g data-logging backpacks on 29 of them. These tags are passively recording light levels and hopefully the birds return (with their backpacks intact!) this spring to their breeding sites near Winnipeg, Manitoba, where I will be waiting for them! I suspect they have important stories to tell.

Stay tuned for Part 2: Connecticut Warbler Fieldwork (or, how everything I thought I knew about them was wrong)

 

 


Leave a comment

Does what happens in the Tropics stay in the Tropics?

Does what happens in the Tropics stay in the Tropics? Temperate-dwelling people who ‘migrate’ to the Tropics often show ‘carry-over effects’ of their overwintering period upon their return. Those who spent a week or two at an all-inclusive resort might come home relaxed, tanned, perhaps fatter or fitter, depending on the individual preference. I often returned from my tropical sojourns bug-bitten, tired, but elated at the experience. But what about the birds? How does their Tropical stay affect them, once they leave?

We migrate home from Belize on a plane (some times we even get to co-pilot the puddle-jumpers to Belize City!)... Wood Thrush have to wing it there on their own power.

We migrate home from Belize on a plane (sometimes I even get to co-pilot the puddle-jumpers to Belize City!)… Wood Thrush have to wing it there on their own power.

I’ve already blogged about how Wood Thrushes show a decline in body condition over the winter period, ending up in rough shape (at least in Belize) right before they have to leave on spring migration. So what happens on spring migration? Does it matter if they are in poor condition, or in poor-quality habitat in winter? Or do their migration genes just get them on their way, regardless of what condition they are in?

Come back next year, Wood Thrush (with your backpack still on, please)!

Come back next year, Wood Thrush (with your backpack still on, please)!

I tried to answer these questions by combining our geolocator-tracking data (from Belize and also from across the breeding and wintering range) with body condition data and remote sensing of habitat dryness. For the latter, there is a nifty satellite-derived index of habitat quality called NDVI (Normalized Difference Vegetation Index – see figure below). It basically is a picture of the world from space, and the greenness of each pixel in the picture has a value. The more green, the higher the NDVI value, the more productive (as in plant productivitiy), wet, and full of bugs a particular forest is; conversely, less green = less productive, drier, and correspondingly fewer bugs. We know that Wood Thrush body condition is correlated with habitat dryness and bugginess, so this makes the NDVI a good remote indicator of habitat quality.

NDVI

This is what the Normalized Vegetation Difference Index looks like for the world in March – lots of snow in the North, but green in the Tropics and in the temperate south.

We also have our detailed migration data from geolocators. Just a reminder: these are archival tags (they don’t transmit data) that give us estimates of latitudes and longitudes for each day based on light levels (sunrise/set times and day length). From these data we can determine latitude and longitude, and thus migration timing, i.e. departure date, date crossing the Gulf of Mexico (about the halfway point for Wood Thrushes) and date of arrival at breeding sites. We can also figure out migration distance, speed (overall distance/duration in days), and the number of days spent at stopover sites.

We expected that birds in poor body condition (smaller than expected based on their size) would show negative carry-over effects on migration. For example, a skinny bird might have to delay its departure, or travel more slowly, stopping more frequently along the way, resulting in a later arrival at breeding sites. The breeding arrival date is really critical because this predicts reproductive success in many species. Late arrivers either get crappy territories (males) or crappy mates (females) and which results in fewer babies for those birds in the long run. Of course really poor condition probably results in not surviving spring migration at all! But we can’t measure this because we only have information from birds that survived to bring us back their migration-tracking geolocator. Definitely a bias, but an unavoidable one at the moment.

So we compared body condition of my Belize birds to their spring migration performance. Surprisingly, we found that birds in poor condition didn’t do anything significantly different from those in the best condition! Their timing, stopover behaviour, and speeds were not significantly related to their condition. If you squint hard at the data, it’s almost the opposite effect – birds in good condition hung on later than those in the worst condition!

Wood Thrushes in good shape (body condition positive) were not earlier to leave than birds in poor shape (negative condition index). There was also no difference in timing later during migration, in speed, duration, or distance travelled.

Wood Thrushes in good shape (body condition positive) were not earlier to leave than birds in poor shape (negative condition index). There was also no difference in timing later during migration, in speed, duration, or distance travelled.

Given that this was a pretty small sample size (under 30 birds) all from the same location (Belize), I thought I would expand my tests using some of our other lab tracking data. This is where the NDVI comes in. I had migration data from Wood Thrushes tagged at a breeding site in Pennsylvania, USA – we knew where these guys were in the winter, and using NDVI we could remotely measure the quality of the habitat. I compared  winter habitat quality to spring migration performance to see if birds wintering in drier sites had poor performance compared to birds in wetter sites.

Birds breeding in Pennsylvania wintered in the central part of the winter range, and I looked at the NDVI for each specific winter site to see if drier forests affected their migrations in spring.

Birds breeding in Pennsylvania wintered in the central part of the winter range, and I looked at the NDVI for each specific winter site to see if drier forests affected their migrations in spring.

I found some support for the idea that winter habitat carries-over to affect spring migration. Birds in drier sites departed significantly later than birds in wetter sites; however, the dry-site birds caught up! There was no difference in timing by the midpoint of migration (at the Gulf of Mexico) or by the time they arrived at breeding sites. If these dry-winter-site birds are catching up, they must be moving faster, and sure enough, I found that lower NDVI was associated with faster speeds and shorter spring migration duration. Overall this shows that birds in very dry winter sites may take longer to depart, but they seem to be able to compensate for this en route and arrive at their breeding site without delay.

The last part of my study involved comparing migration performance of birds from my Belize study site, and a site in Costa Rica (La Selva, where co-author Calandra Stanley did the ground-work; she’s now doing a PhD at the Smithsonian/University of Maryland). These two sites are interesting because they are really different in terms of habitat moisture – Belize is always drier, and it dries out faster than Costa Rica. How do these broad-scale differences in habitat quality relate to migration?

Callie Stanley with a Wood Thrush. Her part of this project was the La Selva Costa Rica work, although now she works in Belize on Wood Thrushes.

Callie Stanley with a Wood Thrush. Her part of this project was the La Selva Costa Rica work, although now she works in Belize on Wood Thrushes.

For starters, the Costa Rica birds migrate about 1000km farther on average than the Belize birds! They also depart later and have later timing along their entire migration. Interestingly, the Costa Rican birds go further but stop for about the same number of nights as the Belize birds, indicating that they are more efficient in their migration somehow. This might be because they have significantly longer wings. Overall this goes a long way to explain the species-wide leap-frog migration pattern in Wood Thrushes. We showed in our previous work that Wood Thrushes from the southern parts of the wintering range (e.g. Costa Rica), migrate to the farthest northeast of the breeding range (e.g. up-state New York), essentially ‘leap-frogging‘ over the birds in the middle (e.g. the Belize birds tend to breed in the southeast). The relationship I show with migration and habitat quality could be what’s maintaining this overall, species-level pattern. Birds in Costa Rica can afford to stay longer because the forest is still relatively productive, and their farther breeding sites would still be under snow cover anyway. In contrast, the Belize birds face an increasingly hostile (i.e. dry) winter site. It likely pays for them to get outta dodge earlier and head for their breeding sites in the southeast US, where spring has already sprung. They don’t need to worry about fuelling up as much as the Costa Rica birds either, because they are covering a lot less ground.

Wood Thrushes in Costa Rica are in wetter habitat, and they migrate on average 1000km further than Wood Thrush from drier habitat in Belize.

Wood Thrushes in Costa Rica are in wetter habitat, and they migrate on average 1000km further than Wood Thrush from drier habitat in Belize.

So overall, what happens in the Tropics doesn’t seem to have much of an effect on individual Wood Thrushes (and where it does, they seem to be able to compensate by speeding up their migration). But the overall patterns of habitat quality (drier in the north, wetter in the south) are correlated with broad-scale differences in migration behaviour (earlier timing, shorter distances in the north; later timing, longer distances in the south) leading to the leap-frog system that we see at the species-level.

If you think this is as interesting as I do (although I admit I’m biased!), you might want to read the whole paper, found here:

McKinnon, EA, Stanley, CQ, and BJM Stutchbury. 2015. Carry-over effects of nonbreeding habitat on start-to-finish spring migration performance of a songbird. PLOS ONE.


1 Comment

Winter sex-segregation of Snow Buntings explained: The boys are just bigger

Today I’m going to write about one of my new study species, the Snow Bunting! For those of you sweltering through some hot buggy fieldwork at the moment, hopefully this reminder of windswept and snowy fields will help temporarily cool your thoughts.

Beautiful Snow Buntings in Quebec by Jean/Ange (https://www.flickr.com/people/anjoudiscus/)

Beautiful Snow Buntings in Quebec by Jean/Ange (https://www.flickr.com/people/anjoudiscus/)

I have the privilege of working with some amazing data collected by Citizen Scientists across Canada (and some in the US too) on the most northern-breeding Passerine bird, the Snow Bunting! Amazingly, there are keen folks who like going outside in frigid temperatures and patiently baiting open snowy fields with cracked corn or millet for Snow Buntings during the winter months when these little songbirds are found in southern Canada and the northern US. These stalwart volunteers form the Canadian Snow Bunting Banding Network (CSBN). Once the buntings are baited, they are trapped with ground walk-in style traps. Basically, the birds walk in, and can’t fly out. There are some great YouTube clips of  Snow Bunting trapping. Once captured, volunteer banders can measure, weigh, and band each bird, making note of its sex and age before releasing it to its flock. Some sites have amazing catching success and trap and band literally thousands of buntings per winter!

David Lamble, member of the Canadian Snow Bunting Banding Network, checking a walk-in trap.

David Lamble, member of the Canadian Snow Bunting Banding Network, checking a walk-in trap.

I think part of the reason why you find people willing to brave the wind and snow is that these are some of the most endearing songbirds around.  Nick-named ‘snowflakes’ for their behaviour of flying in twittering flocks over snow-covered fields, Snow Buntings are a cheerful sight and sound on any dull winter day. Plus it’s pretty darn amazing that a 35-gram bird is even out there at all, seemingly content even with -40C nights a regular occurrence. One particularly talented Snow-Bunting aficionado is Marie-Pier LaPlante (also doing her Masters research on Snow Bunting flocking behaviour), who wrote a song about them! Listen to it here: 

So all of this winter banding has generated some pretty neat data on winter distributions of this species. One obvious pattern is that there seems to be a sex-bias in captures at some sites. For example, in Thunder Bay, Ontario, mostly males are captured. In Essex County (southern Ontario), mostly females are captured. My colleague Christie Macdonald wanted to figure out what was explaining this pattern for part of her Masters thesis at the University of Windsor in Dr. Oliver Love’s lab. She hypothesized three possible explanations for this pattern: 1) Males are more cold-tolerant because they tend to be a bit bigger, 2) Males winter closer to their breeding sites so they can get back earlier to claim nesting sites, and 3) Males winter in the ‘best’ habitats and kick females out through social dominance, resulting in an overall sex-biased distribution. In short, we call these: 1) the body size, 2) arrival time, and 3) social dominance hypotheses.

Each dot shows a banding site and the size of the pie shows the number of birds captured. You can see there is a gradient with more females captured in southern Ontario - but there are some exceptions, i.e. in Newfoundland, there are quite a lot of females captured despite it being very far north.

Each dot shows a banding site and the size of the pie shows the number of birds captured. You can see there is a gradient with more females captured in southern Ontario – but there are some exceptions, e.g. in Newfoundland, there are quite a lot of females captured despite it being very far north.

We set out to test these hypotheses using our ginormous banding dataset of nearly 40,000 winter site captures (whoa) combined with migration information from directly tracking 19 of these little guys with geolocator backpacks.

We looked at the ratio of males to females at each wintering site and compared it to the local weather patterns. Since males are bigger, they should be more cold-tolerant, so we predicted more males and bigger birds of both sexes at colder and snowier winter sites. If males were wintering north of females to get back earlier at breeding sites, we predicted they would have shorter spring migration distance. The trickiest hypothesis to test was the social dominance hypothesis, but we figured out a roundabout way to look at it. Basically we went on the premise that birds with less access to food tend to carry more fat as insurance against starvation. So we predicted that if females are being denied access to food by males, they might carry more fat than males, regardless of size or local weather. We had fat scores (relative measures of subcutaneous fat) from a lot of our captures, so we used these measures to test for patterns in female fat levels that could indicate they were being excluded from food.

The first thing we found was that sex ratio didn’t seem to change much over the course of the winter at any given site. Then we looked at the size of birds relative to weather. We used data on average snow depth, snowfall and temperature, and combined them into one measure of weather harshness using a principle components analysis (PCA). This gave us a measure called PC1weather, where higher values = more ‘wintery’ sites. We found that there was a significant relationship between body size (measured by wing length) and weather – bigger birds of both sexes were found at colder sites! Also we found that the proportion of males was related to the weather in the same way. Proportionally more males were captured at more wintery sites.

Males are always bigger than females, and older birds are always bigger than first-winter birds, but the biggest birds in each age-sex group were found at colder sites (higher values of PC1 weather = more snowfall, deeper snow on the ground and colder temps).

Males are always bigger than females, and older birds are always bigger than first-winter birds, but the biggest birds in each age-sex group were found at colder sites (higher values of PC1 weather = more snowfall, deeper snow on the ground and colder temps).

Higher proportions of males were captured at sites with harsher weather (higher values of PC1weather indicate more snow on the ground, greater average snow fall, and colder temps).

Higher proportions of males were captured at sites with harsher weather (higher values of PC1weather indicate more snow on the ground, greater average snow fall, and colder temps).

So it looks like we are getting lots of support for the body size hypothesis – namely, the bigger you are, the more you can tough it out when the weather is harsh. Because males tend to be bigger than females, this explains why there tends to be more males captured at more northern sites – those sites tend to be the coldest and snowiest.

But males also need to get back to their Arctic-breeding sites early, so maybe that is also driving their choice of wintering site. When we looked at our 19 birds tracked on migration from a breeding site at East Bay Island, in the Canadian low Arctic, we found that males didn’t migrate shorter distances in spring than females. There was a slight trend for males to winter further north but this only translated into ~150km difference on spring migration. Hardly enough to allow males to arrive much earlier at the breeding site (we know buntings can easily cover 150km of migration in a day from Christie’s previous geolocator study). Therefore we concluded that the arrival time hypothesis wasn’t the main reason behind the differential distribution of buntings by sex in winter.

Here's one of our backpack-toting Buntings from East Bay Island

Here’s one of our backpack-toting Buntings from East Bay Island.

Males didn't really winter that much close to their breeding sites than females. Overall spring migration differences were negligible.

Males didn’t really winter that much close to their breeding sites than females. Overall spring migration differences were negligible.

Finally, we tested to see if female buntings carried more fat than males, possibly indicating that they had less access to food via social dominance effects. While our fat-score models were generally pretty poor, we found a trend that females did tend to carry more fat than males, independent of weather effects on fat levels. Older birds also tended to carry more fat that younger birds, which seemed weird at first, since older birds are usually the dominant ones in most species I’ve studied. But, delving into some of the Snow Bunting literature I found a study on dominance in flocks of buntings wintering in Scotland that showed younger birds are actually dominant over adults (Smith and Metcalfe 1997)! So the fat scores do make sense with what we know about dominance status.

At the end of the day, the best answer for why do male snow buntings winter further north than females seems to be that the males are simply bigger, and presumably better able to tolerate those 30-cm dumps of snow and -40C nights. One interesting implication of this relates to climate change. Winters in North America are getting warmer, and less snow will become the norm in may places (Krasting et al. 2013 Journal of Climate). Will this reduce constraints on smaller-bodied female buntings in future? Migrating farther distances from breeding sites is presumably costly, so if they don’t need to go far to find tolerable winter sites, perhaps they will end up wintering further north. Time will tell, and maybe our winter distribution map will have a lot more pink on it in decades to come.

For our full paper, please visit the Journal of Avian biology link below or email/tweet me for a copy:

Macdonald, C.A., McKinnon, E.A., Gilchrist, H.G., Love, O.P. 2015. Cold-tolerance, and not earlier arrival on breeding grounds, explains why males winter further north in an Arctic-breeding songbird. Journal of Avian Biology (accepted). DOI: 10.1111/jav.00689


4 Comments

Jungle life is not always easy for Wood Thrushes

Whenever I tell people that I did my PhD research in the tropical forests of Belize, they tend to roll their eyes and comment sarcastically on how hard that must have been to handle…warm tropical breezes, beautiful beaches, fresh papayas, and cold Belikin beers…

Although I’m not complaining, let me just point out that it ain’t always easy living in the tropics! Yes it’s hotter than Canada in January, but I also got debilitating heat rash from the 35C+ and humidity, leaving me with blistered hands and face. Not fun. Yes, there are awesome tropical fruits (not just papayas! molly apples, soursop, custard apple, pineapple, mango, surinam cherries) and in general the food is delicious (I actually love rice and beans). But there’s always a chance of good ol’ Montezuma’s revenge given all the new fauna that get introduced to your digestive system! I won’t even get into the botflies, ticks, botlass flies, deer flies, sandflies, kissing bugs, scorpions, tarantulas… you get the idea. It’s not all beers on the beach. This is why I never got a tan.

Okay, I admit there were some beers on the beach!

Okay, I admit there were some beers on the beach!

And I figured out during my PhD that, likewise, it’s not just easy living for the migratory Wood Thrushes that make the jungle their home.

I went to Belize to figure out what was important to Wood Thrushes during the non-breeding season (and also how that influenced their migration; but that’s another paper and blog post). Wood Thrushes like forests, and they eat both arthropods (insects, spiders) and fruit. Previous work on the fruit-fly of the migratory bird world, the American Redstart, showed pretty clearly that habitat moisture levels (i.e. rainfall) are the major limiting factor for their success in winter. When it gets dry, insect abundance declines and insect-eating birds have to work harder to get through their day. Plus there’s a seasonal change – the tropics, at least in Central America and the Caribbean, tend to dry out from October to April, over the course of the wintering period for migratory birds. So the environment gets increasingly hostile for an insectivorous bird. Couple this with the fact that they have to power a migration of several thousand kilometres and you can see that late winter is really crunch time for migratory songbirds.

Now this has been shown for redstarts, and also Ovenbirds and Northern Waterthrush. The picture for Wood Thrushes was a little less clear. Unlike their smaller warblers cousins, Wood Thrushes are big chunky birds perhaps less prone to suffering effects of habitat dryness. Plus they can also eat fruit, so if arthropod abundance goes down, maybe they can compensate by gorging on some tropical figs. The other factor is that Wood Thrushes are forest-dependent birds. Unlike the redstarts, which can be found in really dry scrub habitats, Wood Thrushes tend to hang out in more shady, moist areas overall. So is rainfall still a major limiting factor for Wood Thrushes?  Part of my PhD was trying to answer this question.

I wanted to capture the seasonal variation in habitat quality for Wood Thrushes, so I spread out my field work over their entire wintering period  – spanning late October to early April. Over this time, I captured Wood Thrushes and measured their weight, fat levels, muscle levels, as well as their hematocrit (packed red blood cells). I also collected arthropods in pit-fall traps and surveyed for fruit along transects to get an idea of how much food was around at different times of the season and in different habitats. Finally I measured soil moisture levels to get a sense of how dry things were getting.

Collecting a small blood sample from a Wood Thrush. We used this to measure hematocrit, diet, and we used  the DNA to determine sex.

Collecting a small blood sample from a Wood Thrush. We used this to measure hematocrit, diet, and we used the DNA to determine sex.

We dug pitfall traps with old machetes and emptied them daily for 3 days in each location.

We dug pitfall traps with old machetes and emptied them daily for 3 days in each location.

So here’s how the story goes, at least in terms of the habitat. The forest starts off really wet (and anecdotally, full of mosquitos!) and steadily dries out from October to April. Parallel to this decline in moisture there is a decline in arthropod abundance, as you might expect. Bugs don’t like hot and dry, so they either leave, die, or hide when the conditions get rough. I also found (kind of surprisingly) that the fruit abundance is also lower as the season progresses. Hmmm, not looking good for the Wood Thrushes. Less food, more hot and dry, plus they have spring migration looming on the horizon…

Here’s how the story goes for the birds: they start off fat, heavy, with big muscles, and they consistently get lighter, carry less fat and tend more towards emaciated muscle levels! Yikes! Packed red blood cells didn’t change, although you might expect higher levels of red bloods cells in preparation of the marathon of spring migration (like blood-doping athletes, birds with more red blood cells can migrate more efficiently). So it really is crunch time for Wood Thrushes at the end of their overwintering period!

Wood thrush body condition (a) was lower in the dry season, (b) hematocrit was the same, and (c) fat and (d) muscle scores declined. The three habitat types showed basically the same pattern, even though the scrub-forest tended to be  bit drier.

Wood thrush body condition (a) was lower in the dry season, (b) hematocrit was the same, and (c) fat and (d) muscle scores declined. The three habitat types showed basically the same pattern, even though the scrub-forest tended to be a bit drier.

So what does this mean? I think it tells us that Wood Thrushes may be more similar to redstarts than we thought, in that big-scale processes like seasonal drying have the capability to affect their body condition in a big way. Eating fruit or living in a forest didn’t seem to provide any buffer from this seasonal change (although I have data that indicates they aren’t actually eating a heck of a lot of fruit – paper coming soon). I suspect that Wood Thrushes are still able to migrate northwards even if they do decline in condition over the winter- but what effects might this have on their survival or reproductive success?

Another factor to consider is that Belize, like most of Central America, is already experiencing the effects of climate change. Just in the last 20 years or so, the dry season mean temperatures have increased by about 0.5C. And climate projections indicate more warming (and increased frequency of droughts) in the future. Maybe Wood Thrushes are already suffering the effects of hotter, drier, wintering sites. Add in high rates of tropical deforestation and it’s no wonder that the Wood Thrush is listed as a Threatened species in Canada.

So what do we do? Unfortunately climate change doesn’t seem to be going away anytime soon, but I think in the short term, protecting remaining tropical habitat for Wood Thrushes would be helpful. The more fragmented a forest is, the more it dries out, exacerbating the overall seasonal drying. As scientists I think it will be important to look at the climate projections and assess which regions are going to getting hotter and drier faster, to assess where Wood Thrushes and other species may be more affected by these changes. And of course, by buying delicious Bird-Friendly Coffee we can support farmers who create cool shady coffee plantations where Wood Thrushes can find lots of arthropods!

Read my full paper on winter ecology of Wood Thrushes here (or email me for a copy if you don’t have access to Oecologia):

McKinnon, E.A., Rotenberg, J. A., and B.J. M. Stutchbury. 2015. Seasonal change in tropical habitat quality and body condition for a declining migratory songbird. Oecologia Early Online. 10.1007/s00442-015-3343-1