Songbird migration is amazing.
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.
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.
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!
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):