We are losing our migratory songbirds. It’s a fact, and there are many intertwined possible mechanisms, including habitat loss, climate change, invasive species, chemical and light pollution, etc. But for conservation practitioners trying to save the songbirds, there is a gaping hole in our understanding of their biology. We do not know where most small songbirds go when they leave their breeding sites.
Take the Wood Thrush, for example. It breeds in eastern North America, where it has been studied for decades. We know that forest fragmentation and acid rain definitely have effects on breeding Wood Thrushes: birds produce fewer young in small forest patches, or where acid rain has depleted the calcium from the soil and therefore lowered the amount of insect food. But every fall, Wood Thrushes take off from their breeding sites and head to southern Mexico and Central America. Each Wood Thrush heads for a patch of tropical forest somewhere between Veracruz, MX, and the Panama canal. That’s an area of over 500,000 square kilometres! This is why bird banding doesn’t work for making connections for most small birds (unless they are really rare and range restricted, e.g. Bicknell’s Thrush). Finding a Wood Thrush wearing a leg band (marking its breeding site) is like a finding a needle in a haystack, only the haystack covers thousands of square kilometres and it’s full of other needles with no leg bands!
Why is it so important to know where each breeding population goes? For starters, there are geographic patterns in the population declines. Wood Thrushes in Canada (and in the north-east of the U.S.) are disappearing faster than they are in the central and western parts of their breeding range. Are these different breeding populations experiencing different threats on migration or at their winter sites? Do they all mingle on the winter grounds, i.e. a bird from Ontario hangs out with breeders from Georgia? Or do different populations have distinct wintering ranges, i.e. all the Ontario birds hang out together with other Ontario birds? We call this idea of sharing neighbours ‘migratory connectivity’. If it’s strong, the birds stick with their breeding buddies in the winter. If migratory connectivity is weak, the birds might be found next to any ol’ Wood Thrush regardless of breeding origin.
The most effective way to figure out patterns of migratory connectivity out is to follow these birds on migration.
Wood Thrushes easily fit into the palm of your hand, and they weigh less than a tennis ball. How do you follow one over 4,000 km of migration? Researchers have been trying for years to indirectly track birds using chemical markers in their tissues, or DNA structure, or even just by banding lots of individuals in one location and hoping that a bird is captured somewhere else. For Wood Thrushes, not one of these techniques has worked. Despite thousands of Wood Thrushes banded, only one has ever been recaptured in the opposite season. This bird was banded by my colleagues in Nicaragua and hit a window of someone’s house in Pennsylvania in 2011 (read full story here). Thankfully someone noticed the thump on the window and the leg band that identified this bird. But one band recovery out of thousands is not enough to paint a full picture of migratory connectivity for Wood Thrushes. Chemical markers have been somewhat more successful in making connections for Wood Thrushes at a very broad scale (read about it here). But it really was the miniaturization of tracking devices called ‘geolocators’ that revolutionized our understanding of Wood Thrush migratory connectivity.
Geolocators are tiny devices (<2 g) that can be attached to a bird like a backpack, except they go over the legs, and not the wings. These devices are very simple: battery + clock + light sensor + chip to record the data. Before you put the tag on the bird, you turn it on and program it with the current time. Once on the bird, it records light levels continually relative to the clock. If the bird moves east, sunrise will be slightly earlier. If the bird moves south in the fall, day length will be longer. By inputing the recorded times of sunrises and sunsets into a computer program, you can estimate the latitude and longitude where the bird was each day. Easy, right? Well not quite. The most challenging thing about these tiny geolocators is that THEY DO NOT TRANSMIT DATA. This means that we put the backpack on the bird, it migrates thousands of kilometres, does its thing in Mexico or Central America for the winter, migrates back in spring, THEN we have to catch it again to take the backpack off to get the data. Seems nearly impossible, but it does eventually work.
The culmination of years of this type of tracking, and hours and hours of effort by graduate students, field techs, volunteers, and of course our project leader, Dr. Bridget Stutchbury, is a map.
Not just any map. This map contains detailed migration data from over 100 Wood Thrushes tracked from 7 breeding sites and 4 winter sites. It tells us exactly where each bird goes, and what route it takes to get there. This is the first time a migratory connectivity map has been produced for a songbird using tracking from both breeding and winter sites (although our lab has done some pretty awesome work with Purple Martins too).
What did we discover? First of all, there is a pattern. Birds from Canada don’t usually hang out with birds from Georgia in the winter. They stick with their buddies from the central and north-east, and hang out in Nicaragua and Honduras. In contrast, Wood Thrushes I tracked from Belize all head to the central and south of the breeding range: Kentucky, Virginia, the Carolinas, a few even bred at the very southern limits of their range in Florida. Overall we call this pattern ‘leap-frog’ because the birds breeding the farthest north actually migrate the farthest south, ‘leap-frogging’ over the southern breeding populations. The connections also tended to be predicted by longitude, so that birds breeding further east (and north) spent the winter further east (and south). So I can tell you that if you are Canadian visiting the Mexican riviera on vacation and you see a few Wood Thrushes – odds are these are not fellow Canucks, but probably birds from the southern US. If you want to see your ‘Canadian’ Wood Thrushes, you would have to head further south – the Mosquito Coast of Nicaragua would be a good option (a little more adventuresome for a vacation too!).
We also discovered some amazing patterns in migration. In fall the Wood Thrushes tend to funnel south along three major routes – either through Florida then island-hop over Cuba to Honduras, 2) cross the Gulf to the Yucatan peninsula diagonally through the Florida panhandle, or 3) cross the Gulf to the Yucatan from Louisiana. The choice of route was generally predicted by where the birds were breeding, i.e. eastern breeders took the eastern (Florida) route, while western breeders were more likely to cross the Gulf from Louisiana.
In spring, it’s a much more interesting story. We found that almost 75% of ALL Wood Thrushes cross the Gulf of Mexico from the tip of the Yucatan peninsula to land in a small area of Louisiana on the northern gulf coast. That means almost the entire global population of this bird uses that one tiny piece of land near New Orleans every spring!
This is why our work is so important for conservation. We know now where the ‘hotspots’ are that are used by the most Wood Thrushes at a global scale, and we also know which areas are important for specific breeding populations. For example, if you want to protect habitat for those Canadian Wood Thrushes – invest in shade-coffee and sustainable forest use programs in Nicaragua. Better yet, contact the local ministry of the environment (MARENA) and figure out how you can help conserve forests in Nicaragua. The truth is, they aren’t really Canadian Wood Thrushes after all – they have duel citizenship!
Our full paper is published in Conservation Biology. If you can’t access it, email me: emilymckinnon12 AT gmail.com or contact me on Twitter @BirdBiologist and I’ll send you a pdf.
Stanley, C. Q., E. A. McKinnon, K. C. Fraser, M. P. MacPherson, G. Casbourn, L. Friesen, P. P. Marra, C. E. Studds, T. B. Ryder, N. Diggs, and B. J. Stutchbury. 2014. Connectivity of Wood Thrush breeding, wintering, and migration sites based on range-wide tracking. Conservation Biology Early online.