Movement Ecology of Animals

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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

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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


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Bicknell’s Thrush nests in regenerating clearcuts in the highlands of New Brunswick

Bicknell’s Thrush (Catharus bicknelli) is one of those birds that real twitchers lust after. It’s hard to identify in the field, especially outside of the breeding season, and it’s range is patchy and limited to northeastern North America and a couple of islands in the Caribbean. Its secretive nature, dense foggy breeding and wintering habitat, and mournful song only add to the allure. Unfortunately, like too many other songbirds, Bicknell’s Thrushes are steadily disappearing. This is particularly so in the highlands of New Brunswick and Nova Scotia, Canada, where I did my Masters research on this species. A recent report by Bird Studies Canada’s High Elevation Landbird Program (HELP) showed a decline of 11% ANNUALLY over a 10-year time period! Yikes! Bicknell’s Thrush is now federally listed as THREATENED in Canada.

Super dense balsam fir forest preferred by Bicknell's Thrush

Super dense balsam fir forest preferred by Bicknell’s Thrush

One potential problem for Bicknell’s Thrush in New Brunswick is a logging practice called Precommercial Thinning (PCT). Bicknell’s Thrush nest in very dense forests that regenerate on mountain-tops in North and North-central New Brunswick. Where I worked, in the Christmas Mountains, the highest peaks are around 600-700m above sea level. Not very big relative to other mountains, but enough, at this high latitude (~46N) to create fairly harsh conditions. Balsam fir and white birch thrive up there, especially after clearcuts. The balsam fir grows back at such a high density that the trees stunt their own growth through crowding. This is no good for logging companies interested in the best bang for their buck. To help improve the growth of the trees in a given patch, loggers go in and ‘thin out’ the forest by PCT, when the trees are about 10-15 years old (about 2-5 cm in diameter). It’s pre-commercial because the trees they cut down aren’t at a size to be of much commercial value – they are just cut and left lying on the ground. What this practice does overall is to remove about 75% of the trees in the forest, with the idea that the remaining 25% will be better off for it.

This practice affects Bicknell’s Thrush because they too love high elevations, and they really really love balsam fir. I collected data on a dozen or so nests of Bicknell’s Thrush during my Masters, and every single one of them was in a balsam fir. The only one that was not entirely in a balsam fir was squished between a fir and a spruce. So when the forests at high elevation are basically perfect Bicknell’s Thrush nesting habitat, loggers come in and cut down 75% of the trees! You can see why this is concerning to people interested in preventing further population declines of this species!

Rolling hills of north-central New Brunswick - few people, lots of logging roads!

Rolling hills of north-central New Brunswick – few people, lots of logging roads!

I wanted to know more about the habitat Bicknell’s Thrush used for nesting, and if they ever nested in forest that had been thinned. Imagine a female Bicknell’s Thrush, returning to North Pole Mountain (yes, the Christmas Mountains have Christmasy names!) in the spring, after spending the winter in the highlands of the Dominican Republic. She gets back to the forest she nested in last year, only to find that most of the trees are gone! Does she go for it anyway? Or fly onward to another mountain? Or just move to the unthinned forest next door in hopes it will stay dense for another nesting season?

I can tell you exactly what one female did.

Finding Bicknell’s Thrush nests is like finding a needle in a haystack, only imagine that the needle is camouflaged and the haystack is several hectares large. Against all odds, we found several Bicknell’s Thrush nests over the course of our project, and in our first year, they were all in unthinned forests. We carefully monitored each nest with minimal disturbance, sometimes using video cameras. We tried to capture all the adults feeding the chicks and band them with a unique colour-combination of plastic rings on their legs. Bicknell’s Thrush are an unusual songbird in that they have multiple ‘dads’ that father and help feed the chicks, so a single nest could have 3 or even 4 parents attending the chicks! One of the few females we captured was nesting in a forest that was scheduled to be thinned. Bicknell’s Thrush, like many songbirds, are highly site-faithful, which means they return to the exact same forest patches for nesting year after year. What would this female (her ID was light-green-mauve-black-silver) do when she came back the next year to a thinned forest?

Can you spot the blue eggs?

Can you spot the blue eggs?

The following spring, we searched and searched in the thinned forests and found no signs of any Bicknell’s Thrushes, let alone nests. In a very small dense patch in a boggy area, right across the logging road from light-green-mauve-black-silver’s original forest patch, we did hear a Bicknell’s Thrush singing. It was such a tiny and miserable forest patch (standing water, dense tangles of dead balsam fir, logging roads on either side of a wedge about 10m wide) that we pretty much gave up finding any nests there and figured the singing male was overly optimistic about his chances. Finally near the end of the nesting season, I decided it would be worth going back to this patch for one final search.

My crew dropped me off and I steeled myself for the scratchy squeeze through the dense boggy patch one more time. I finally popped into a bit of an open area and froze. I had hear a distinct ‘Peer!’ call from very close by. This is a call Bicknell’s Thrush often use, but I noticed on our nest videos that females often gave it when they jumped off the nest, either to let Dad in to feed the kids, or because of some disturbance nearby. Like me. I looked around very carefully. There were no branches with needles on them below my head height – the needled branches were all above me, creating a dense canopy about 5-m tall. Then I saw it – way up high, pressed against the trunk of a balsam fir, a messy clump of nesting material! Could it be? Thankfully balsam fir is pretty easy to climb, so I scaled the tree as quietly as possible and peered in – 3 thrush babies, and pretty old ones too! This nest was close to fledging! But was it a Bicknell’s Thrush nest? It was too high to video, so we instead set up some nets around the nest and tried to catch the parents to confirm that this was indeed a Bicknell’s Thrush nest.

Our first capture that day was one of my favourite field moments ever. It was a colour-banded Bicknell’s Thrush. And low and behold, it was light-green-mauve-black-silver, the same female who nested across the road, about 280m away in the unthinned forest, the year before. I think we had probably only banded 2 or 3 females (they are even more cryptic than males), so it was amazing to see this bird again! She seemed happy and healthy so we released her and left her and her kids alone.

My favorite Bicknell's Thrush, LightGreenMauveBlackSilver

My favorite Bicknell’s Thrush, LightGreenMauveBlackSilver

Colour bands

This bird’s story, and other evidence we collected suggests that Bicknell’s Thrush might be okay in areas with PCT if patches of dense stuff are left for them to use for nesting. It’s an easy way that forest managers can help keep nesting habitat for this species in New Brunswick.

Read our full scientific paper here:

McKinnon, E.A., Askanas, H., and A.W. Diamond. 2014. Nest-Patch Characteristics of Bicknell’s Thrush in Regenerating Clearcuts, and Implications for Precommercial Thinning. Northeastern Naturalist 21(2):259-270.

If you can’t access the full version but want to, tweet me @BirdBiologist or email me emilymckinnon12@gmail.com, and I’ll send you a pdf!