Movement Ecology of Animals

Homepage of Dr. Emily A. McKinnon

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Year of the (female) Bird Biologists

I am so excited that Audubon, BirdLife International, Cornell Lab of O, and National Geographic have banded together and decided that 2018 is the ‘Year of the Bird’. This means extra special birdy events going on all year, and extra special attention to the organisms that have fascinated me for the last 10+ years (ahem, every year is year of the bird for this family of ornithologists).

This kind of publicity is what birds need right now; they are the proverbial canaries warning of problems with our planet. Altering their timing of migration in response to climate change, singing a different song to be heard over city noise, accumulating toxic chemicals while breeding in the seemingly pristine Arctic

Birds are telling us that we are impacting our planet with our fossil fuel addiction, rapidly increasing urbanization, and rampant use of chemicals. But birds also tell us about resilience, about facing an entire ocean with no land in sight and launching into the air, landing days later at a tropical destination.

We all need to pay more attention to birds.

As a subscriber of National Geographic I was excited for the special Year of the Bird feature articles. And for March, a whole article on bird migration! My own personal research subject and, in my mind, one of the most mind-blowing behaviours that birds pull off, season after season. The issue even came with a migration poster! Score!

The article describes the epic migrations of godwits, amazing work on migration timing by postdoc Dr. Jesse Conklin and colleagues. The author speaks with Dr. Ben Winger, expert in biogeography and evolution of migration, about the origins of migratory behaviour. Peter Berthold, who has literally written the book on controls of migration is also interviewed to describe how migration might have evolved as climate in sub-saharan Africa changed over millennia. Henrik Mouritsen, Martin Wikelski, Pete Marra– all ornithologists whose work I have studied over the years.

However, the more I read this article, the more I had the nagging feeling that something was missing.

Where are all the female bird biologists?

I know there is a bias in Science where females are still underrepresented. But I always felt that in my field, behavioural ecology of birds, the problem wasn’t as bad, as say, in Math and Engineering (in Canada, anyway, there were actually more women university graduates in general sciences than men). So my impression was (and still is) that there are plenty of amazing female scientists studying birds and migration behaviour.

Therefore I was surprised that out of all the researchers (both early career postdocs and senior scientists) mentioned in this Nat Geo article (n = 18, not including Andrew Farnsworth who consulted on the infographic figure), only 2 are women, and both were mentioned in the context of a team (Bob Gill & Lee Tibbetts of USGS, and Wolfgang & Roswitha Wiltschko of Goethe University in Frankfurt).

Anyway, I thought I would flesh out the article with some of the amazing work by female ornithologists that I know of who have taken the field of migration ecology forward.

Two caveats: 1) This is not an exhaustive list – just a few names that would have fit nicely into the theme of Nat Geo’s original article, and 2) This is not to detract at all from the work of the amazing ornithologists interviewed/featured in the article already. Male ornithologists are awesome too. I should know, I’m married to one (Kevin Fraser).

Click the names to see webpages and links to papers by these folks:

Susanne Åkesson

It’s pretty nutty that godwits can fly from Alaska all the way to New Zealand – 8 days of non-stop flapping! Makes me tired just thinking about it. But Dr. Susanne Åkesson’s Common Swifts are possibly even more amazing – her team has shown, by using tracking devices equipped with accelerometers – that the swifts don’t land for the entire winter while they are in Africa. If that doesn’t blow your socks off, I don’t know what will. She has done an incredible amount of work on migration for the last 20+ years, looking at proximate drivers of navigation and orientation in songbirds and shorebirds.

Debra Arlt

Wheatears have some of the most amazing migrations of all songbirds. From Canadian Arctic eastward to Africa? No problem. From Alaska to Africa (the long way – westward) – also no problem. Debra Arlt has been using the differential migrations of populations of wheatears to explore stopover biology as well as the effects of tags on fitness of the birds.

Melissa Bowlin

I love the story of Bill Cochran and Martin Wikelski chasing down their radio-tagged Swainson’s Thrushes (pre-geolocators, pre-Motus) all night, trying to figure out the proximate rules for songbird migration. Dr. Melissa Bowlin has continued this amazing work, and shown how wing shape in thrushes affects aerodynamics of flight, among other things. She continues to use the chase-car strategy to look at flight patterns in migrating thrushes, and has discovered some crazy patterns in flight altitude that are still a mystery.

Kira Delmore

It’s really interesting to think about how long-distance migration might have evolved, and Ben Winger has done some really neat stuff looking at origins of migration (i.e. the southern vs. northern home theories). One researcher I think of when it comes to evolution of migration patterns is Dr. Kira Delmore (postdoc at Max Planck). She did a great study using geolocators on Swainson’s Thrushes in a hybrid zone between western and eastern subspecies, and she found that the hybrid individuals had a sub-optimal migration route compared to both parental types. Migration as a post-zygotic barrier to (sub)species fusion! How awesome is that. She continues to explore genetics (and epigenetics!) and migration behaviour in a search for the genetic basis for migration.


Camila Gomez

While working on her PhD in Colombia at the University of the Andes, Ms. Gomez has produced some rock-solid research on stopover biology of thrushes. Along with colleagues working at a stopover site in northern Colombia, she has shown that thrushes have the fuel to make it all the way to their breeding sites in a single mega-flight. To me, her work is starting to tip the scales in terms of how we think about songbird migration – we thought most used a short-hop strategy, stopping frequently to refuel – however, Ms. Gomez’s work and others is starting to point to a more shorebird-like long-jump strategy. I suspect there will be more amazing discoveries in the future that will add to this picture.

Elizabeth Gow

A Stutchbury alumna (see Bridget below), Dr. Gow has studied the differential migrations of male and female woodpeckers. That is, after she figured out how to get them to stop ripping off the light stalks from the geolocators! #woodpeckerfieldworkproblems She is now working on a huge migration dataset for tree swallows as a postdoc in the Norris lab.

Yolanda Morbey

Dr. Morbey has done some nice work on timing of migration, especially protandry – the idea that males arrive at breeding sites before females- with both theoretical and field-based studies.

Rachel Muheim

Another female ornithologist who has really explored bird navigation systems is Dr. Rachel Muheim. Both in the field and the lab, in well-designed experiments, she has studied the magnetic compass systems that birds use to figure out where to go when they migrate.

Janne Owehand

Dr. Owehand is an up-and-coming bird biologist who I first met in Latvia at the EOU in 2011. Her PhD work resulted in some amazing papers exploring constraints on long-distance migrations of Pied Flycatchers. I will forgive her early papers on earthworms and bats because her PhD work is just so cool. J If you want to know more, check out her TEDx talk:

Bridget Stutchbury

Full disclosure, Bridget was my PhD advisor, but the reason I wanted to work with her was the game-changing Science paper showing the first geolocator-based migration tracks from Wood Thrushes and Purple Martins. Bridget didn’t really start off as a migration ecologist but her behavioural ecology background and strong interest in conservation of songbirds led her directly to the tracking work that caught my eye. She has been exploring migration as it relates to conservation of songbirds for the past 10 years, and is now doing some really interesting work with another rockstar bird biologist, Dr. Christy Morrissey, on how pesticides affect songbird migration.



These are just a few of the amazing researchers that I know of whose work would have fit well into the Nat Geo article. There are lots more awesome bird biologists (who just happen to be female) that are studying aspects of bird migration more generally, e.g. Emily Cohen, Hanna Kokko, Jen Owen, Kristina Paxton, Jill Deppe, Kristen Covino, okay somebody stop me!  In the ornithology textbooks of the future, I’m sure you will see their work highlighted for the important contributions they and many others are making to the field. In fact, I just might have to go write that textbook myself to make sure it’s done right!


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!


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!


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


ecy1844-sup-0001-AppendixS1 (1)



Connecticut Warblers of Manitoba.



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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.


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.


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?


Examining 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)



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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.


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.


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 (

Beautiful Snow Buntings in Quebec by Jean/Ange (

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