Movement Ecology of Animals

Homepage of Dr. Emily A. McKinnon


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Why young birds don’t get the worm

Songbird migration is amazing.

Imagine this:

You hatch somewhere in eastern North America, let’s say… a nice sugar maple forest in Vermont. You hang out in your nest for a while, happily eating whatever your parents shove down your throat, jockeying with your siblings for space and food, until one day – you hop out with a klutzy fluttering of wings. You then follow Mum or Dad around for awhile, maybe joined by one or two of your four sibs. Eventually they wander off, and you are on your own, happily gorging on berries, growing in your last feathers. Finally, the days get shorter and something in your brain clicks and you know it’s time to move. Without any guidance, you simply take off one evening and point your beak southwards. Days or weeks later (and thousands of kilometres), you arrive in a steamy tropical jungle that somehow, feels like home. 

As spring arrives (probably detected by subtle changes in day length) you get that familiar urge to fly, only this time, it’s northwards. You have no idea of your exact destination, just a vague sense of where you were hatched (must be a good place to breed around there, you survived, after all, right?). You head northwards only to be stopped in a few days by an immense body of water, with no land in sight on the other side. This is the Gulf of Mexico. From the tip of the Yucatan penninsula of Mexico, it’s nearly 1000 km to the U.S. coast on the north side. Yet instinct tells you to go for it – so one evening, you launch straight out over the open water. Hours of flying later (likely well into the next day), you finally spy a wavering outline of land ahead. Soon you touch down on a windswept barrier island at the mouth of the Mississippi, near New Orleans. And this is just the start of your first spring migration.

Wood Thrush - an amazing migratory songbird!

Wood Thrush – an amazing migratory songbird!

I’ve tried to walk you through the first migrations of a songbird like the Wood Thrush, because I think it’s so hard for us thinking apes to get our heads around such insane-seeming instinctive behaviours. Songbirds are really like little programmed robots – their genes are so finely tuned that they can accomplish these amazing feats of migration without ever thinking about it. For first-time migrants, this is even more amazing, since they have no opportunity to learn their routes from their parents, or other adults. In fact, since most songbirds migrate at night, they probably can’t even see the other birds they might be flying with.

Scientists have studied the development of migratory behaviour in the lab, and found that while a lot of it is pure instinct, there is an important contribution of experience. Nestlings raised in captivity start getting the migration fidgets (the academic term for this increased hopping and fluttering around in their cages is zugunruhe, German for ‘nocturnal restlessness’) at the right time to start their migrations southwards. They also know what general direction they should go, i.e. southwest. Lab studies have shown this by the simple but ingenious ‘Emlen’ funnel – a paper funnel with an ink pad at the bottom. Put a bird in (and some screen over the top) and the bird will hop all night in the direction it wants to fly, each time stamping its feet on the paper funnel, which thus records the direction the bird wants to go. But young songbirds do have trouble if they get blown off course. Adult birds figure it out and re-orient the following night, but juveniles keep doggedly on the same course. This is why fall is a good time to see rare birds – juveniles are moving around and sometimes end up in places they shouldn’t be. Presumably, natural selection takes care of any juvenile that gets too far off track with the result that juveniles on their first migration probably have pretty high mortality rates.

By the time birds undergo spring migration, all the juveniles have by definition survived fall migration and spent the winter in an appropriate place. In spring, juvenile birds can re-orient themselves when they are blown off course, so the experience of migrating southwards in fall must have given them some sort of overall map in their brains. However, the exact route that they need to take is not the same as in fall (many birds do a loop migration, where spring and fall migration occur along different routes, probably because of favourable wind patterns). This means juvenile birds have to figure out a new route to get to their inaugural breeding site. Songbirds tend to be site faithful to the same territories after they have bred there once, but juveniles rarely return to the exact territory where they were hatched (that could lead to inbreeding). Instead, juveniles are thought to aim for the general area where they were hatched (leading to the patterns of migratory connectivity we discovered), so that a young bird from Vermont might return somewhere nearby – a few hundred kilometres away. It probably wouldn’t breed in Indiana, for example, but might end up in New York state. So even in spring, when juveniles have a little experience, it’s still pretty amazing that they can find their way back to a breeding site at all. To make things even more challenging, if you are a Wood Thrush, as in my example above, in spring you most definitely want to take the most efficient route back to the breeding grounds, which means dealing with the 1,000-km open-water crossing of the Gulf of Mexico.

I studied the spring migrations of juvenile Wood Thrushes from my study site in Belize (www.bfreebz.org) and also used some data collected by my colleague Callie Stanley during her Masters work in Costa Rica (at La Selva Biological Station). One of the many advantages of studying birds in Belize is that I could catch juvenile birds before they left on migration, and if they survived to return the following year, I could map their very first journey north. To do this I used little bird backpacks called geolocators to track their migrations. See my previous blog post for an explanation of how they work. Basically, they record where the bird is each day and I have to recapture the same individual one year later to get the data.

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

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

It’s challenging enough to catch those ‘golden’ backpack-wearing birds, but the odds of getting the juveniles (now returning as adults) is even lower. Most juveniles just don’t make it to breeding sites and back. Where exactly most of them get into trouble, we don’t know. Could be they choose their tail winds poorly and get stuck out over the Gulf of Mexico. Maybe they don’t have a healthy fear of cell towers or glass skyscrapers and meet an untimely end that way. Until we have backpacks that transmit the data remotely, we won’t know what happens to all the birds that don’t come back.

After several years, I ended up with a pretty decent sample of 17 first-time spring migration tracks for Wood Thrushes. It’s not a lot, but this is the first time songbirds of any species have been followed from start-to-finish on spring migration! So what do they do?

First of all, they leave late.

The first-time migrants hung out at their tropical wintering sites for almost a week more than adults! One idea was that maybe they are in rough shape after duking it out for food with adult birds the whole winter. So I looked at the body condition of adult versus juvenile birds at my site in Belize: no difference. In fact, the juveniles were in a bit better condition than adults (not significant though). Scratch that idea! So why are they leaving late?

One clue is that not only did they leave late, they got more and more behind the adults as they headed northwards. By the time they arrived at breeding sites, juvenile Wood Thrushes were almost two weeks behind adults!

This is one bird we tracked twice, once as a juvenile and the next year as an adult. Check out how much earlier he arrives when he's an adult! 26 April versus 11 May as a juvenile.

This is one bird we tracked twice, once as a juvenile and the next year as an adult. Check out how much earlier he arrives when he’s an adult! 26 April versus 11 May as a juvenile.

The juveniles start to get more and more behind because they stop more frequently in the U.S. as they travel northwards. Why would they do this? Maybe they have to, if they are in rough shape (although I suspect not). It could be that they are less efficient at flying (they do have shorter wings) or that they have less experience selecting tail winds, so each flight doesn’t take them as far as adults. However, I also found that juveniles were just as likely as adults to cross the Gulf of Mexico, and they didn’t stop for longer before (to prepare) or after (to recover), which seems to suggest that they can perform as well as adults.

One idea is that juvenile birds might actually be programmed to arrive later. There are big costs to arriving at a breeding site first: it could get cold, food could be limiting, and early birds will likely have to fend off more than one rival for that prime territory. In contrast, birds that wait a bit arrive when all the adults have settled on the best territories, and there may be comparatively little fuss when they arrive and set up shop in a lesser territory nearby. The benefits might be that the weather is better and therefore food is probably more predictable, and a later arriving bird might not face as many risky territorial challenges. Later arriving birds may not get the best territory (or mate), so their reproductive success might be low, but maybe instead they ‘prioritize’ making it through their first breeding season alive. If you were a juvenile bird, I suspect this later-arrival strategy could be your gene-driven game plan.

We don’t really know, of course, why juvenile Wood Thrushes took a more leisurely spring migration. But now we know how – they both leave late, and stop more on their way northwards!

Read our full paper (Open Access!) here (email or tweet me if you can’t get it):

McKinnon, E.A., Fraser, K.C., Stanley, C.Q., and B. J. M. Stutchbury. 2014. Tracking from the Tropics reveals behaviour of juvenile birds on their first spring migration. PLOS ONE.

 


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Connecting breeding and wintering sites for a declining migratory songbird

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.

Wood Thrush breeding trends

Breeding grounds population trends for Wood Thrushes measured by Breeding Bird Surveys from 1966-2012. The major breeding regions we used in our study are shown by the dashed lines – Northeast (NE), central east (CE), Southeast (SE) and Midwest (MW).

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.

Light-level Geolocator

Geolocator harnessed for a Wood Thrush. The stalk is at the back and points to the bird’s tail; the white square at the end is the light sensor. The super-strong but soft teflon straps are adjusted to fit each bird.

Wood Thrush wearing a geolocator. Only the tip of the light stalk pokes through the feathers once the geolocator has settled on the bird.

Wood Thrush wearing a geolocator. Only the tip of the light stalk pokes through the feathers once the geolocator has settled on the bird.

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.

ConnectivityMapALLBIRDS

Breeding-wintering connections for Wood Thrushes. Each star is a site where geolocators were deployed on Wood Thrushes, and the round circles are the birds’ sites in the opposite season. Each deployment location is colour coded. Inset photo shows a Wood Thrush with a geolocator.

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.

 

Fall migration routes for Wood Thrushes colour-coded by breeding region. Width of line shows proportion of the entire species using that route.

Fall migration routes for Wood Thrushes colour-coded by breeding region. Width of line shows proportion of the entire species using that route.

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!

Spring migration routes for Wood Thrushes colour-coded by breeding region. Width of lines shows proportion of the entire species that uses that route.

Spring migration routes for Wood Thrushes colour-coded by breeding region. Width of lines shows proportion of the entire species that uses that route.

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.

 

 

 

 

 

 


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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!