Movement Ecology of Animals

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Jungle life is not always easy for Wood Thrushes

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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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4 thoughts on “Jungle life is not always easy for Wood Thrushes

  1. Pingback: Belize Foundation for Research and Environmental Education Oecologia article on winter ecology of Wood Thrushes - BFREE

  2. Thanks Emily for this write up; fantastic information presented in an engaging manner. R.McNab/Flores Peten, Guatemala

  3. Pingback: Does what happens in the Tropics stay in the Tropics? | A Bird Biologist's-Eye View

  4. Pingback: Would you like a side of fruit with your beetles and ants? | Movement Ecology of Animals

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