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New insight on bird migrations, and how climate change may reshape them

It turns out the longest flights are anything but direct, with long pauses to rest and eat — assuming plentiful food is available.

Transmitters attached to cuckoos weigh less than 5 percent of the birds’ normal body weights, and send data to a satellite every other day in 10-hour transmissions.
Photo by M. Willemoes/Natural History Museum of Denmark

Watching the woods fill up with snow on Wednesday morning, as birds gathered by the feeders, I wanted to congratulate all the goldfinches, crows, woodpeckers and nuthatches on their luck at being stuck here through a bitter Wisconsin winter. Not to mention the eight trumpeter swans I saw conducting an aerial survey of crusted cornfields three weeks ago.

It’s a tough life, but at least they don’t have to migrate.

The extraordinary effort and enduring mystery of avian migration have been on my mind lately thanks to Bernd Heinrich’s new-to-me book, “The Homing Instinct” (2014). To step into the science of this behavior with such a master is, for me at least, to encounter sheer wonder.

This week that wonder is further informed by two brand-new studies, one of them amazingly high-tech, which add considerable complexity both to the story of migrating birds and to how climate change is rewriting it.

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The first bird outfitted for research was an eastern phoebe, banded by John James Audubon himself with a bit of silvered thread in 1804. There are lots of limitations to studying birds this way, not least being that most bands are seen perhaps twice — once when they’re applied, and again when the corpse is recovered.

Of course there have been advances since Audubon’s time, and a while back we had the story of a piping plover who was marked with colored bands of a particular pattern, visible from a distance, which established that it had flown from Sleeping Bear Dunes on Lake Michigan to the Miami area, covering 1,385 miles in less than 44.5 hours.

But what was the route, the pace, the pattern of effort and rest?

It takes electronics — and miniaturization — to actually track a bird, and the early efforts discussed by Heinrich (both in “The Homing Instinct” and “Mind of the Raven,” quoted below) involved fitting clunky gear to uncooperative birds with superglue and such, then trying to extract nuggets of data gold from the dross of imprecision and equipment failure.

Starting June 18 [1992], the next volunteer spent five days doing nothing but tracking. Anticipating that the birds would still not have wandered out of radio contact, I asked him to take a compass reading every three to four hours and to write down the directions of the radio signals from specific predetermined spots. In this way, the movements of the birds could be determined more consistently. …

I eagerly plotted the data. Strangely, one signal, Number 8169, was picked up five times at one station only, but always from the same direction. It was not picked up at all during sixteen other readings at the same station. Number 8510 was picked up every single time the student looked for the birds. He found it from one station only, but from various directions ranging from 70 degrees to 28 degrees from magnetic north. The same applied to Number 9680, but at directions varying from 150 degrees to 350 degrees. In short, the data set said that the birds either had stayed at the nests or had moved only locally for very small distances. But most of the time, they were missing!

Tiny, high-tech trackers

Now comes a fresh example from Danish researchers who fitted out common cuckoos, red-backed shrikes and thrush nightingales with gizmos that enabled detailed tracing of their travels between breeding territories in Denmark and wintering zones in central Africa — trips ranging into the thousands of miles.

Nature being efficient, you might expect the birds to pick direct routes and minimize travel times. You might also expect them to schedule the journeys around seasonal abundance in food supply, which means primarily insects and other invertebrates, with a bit of fruit for variety.

Indeed, there is a term among ornithologists for birds’ propensity to “surf the green wave,” meaning to ride the ripening cycle of vegetation northward from their wintering grounds, which matches their arrivals to big insect blooms.

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It turns out to be much more complicated, but it takes some technology to find that out.

Kasper Thorup of the University of Copenhagen and colleagues fitted their common cuckoos with satellite radio transmitters trailing serious antennae (unlike their curved-clock counterparts, real cuckoos are almost the size of a city pigeon), and the smaller red-backed shrikes and thrush nightingales with tiny “light-level loggers,” which recorded sunshine and moonlight in ways that could be translated into geolocations with time stamps.

From the data, they found that birds make a lot of stops and some of them last for a month or more. And the timing of the trips was more complicated than just following green-up, and may even take into account such local variables as weather patterns and even competition over food with nonmigrating birds and other insectivores. A key passage, slightly compressed:

We propose that, instead of following green vegetation conditions or a constant climatic niche, some migrants might use an alternative strategy of more broadly following pulses of resources, that is, high relative availability of local seasonal resources.

Food availability depends not only on productivity but also on the density of consumers sharing the food resource. Resident populations are thought to be limited by food availability during the season when food availability is at its lowest, whereas clutch size is determined by per-capita food availability during the season when food availability is at its highest. Therefore, migrants may potentially be able to exploit excess or surplus resources in seasonal environments.

In a comment to the Copenhagen Post, Thorup said the birds are overlaying their standard migration behavior, which most ornithologists feel is encoded within their DNA, with adaptive flexibility:

They are bound to their migration programs, so they fly down to the areas that are green, or where they expect it to be green. Just as we have a circadian rhythm, the birds have an internal program — an annual rhythm that they follow.

There have been droughts in some areas of Africa, and we could see there that the birds were still waiting around. They don’t just suddenly come up with a new plan just because it isn’t green.

But short-term drought is not the same as climate change, and the paper raises the question of whether long-term changes could overwhelm the birds’ ability to adapt — while also pointing out that those abilities are little studied as yet.

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Thorup’s call is for more research into this question, perhaps with a view to identifying areas of land that are especially critical to the migration of various species. Of course, that would carry an implication that we might want to put some of them off-limits to development — a force that, for now at least, remains a greater driver of habitat destruction than climate change.

Climate effects already seen

Lest there be any doubt, however, that birds are already feeling the impact of a warming globe’s climate disruption, we have a meta-analysis from researchers at the University of Edinburgh reaching across 413 species, five continents and data sets going back as far as 300 years (including some from the notebooks of Henry David Thoreau).

Their sobering conclusion is that migratory birds are arriving at their summer breeding grounds, on average, about two days earlier with each passing decade. This is a serious problem because, as we’ve just seen, successful breeding depends on arriving at the right site, at the right time, so that food abundance is optimal.

And the longer a species must travel, the more serious the impact can be, simply because the low-food period on arrival may last longer, and whatever food is on hand may already have fallen to earlier-arriving competitors.

Among the cited examples: The British swallow, which winters in South Africa and returns home in April or May after a flight that averages 200 miles per day, at speeds of up to 35 miles per hour.

This paper, too, avoids any firm conclusions about what a shifting climate may mean for long-term survival, other than to observe that the changes observed so far are complex in origins and response. And it’s not that birds haven’t been adapting to all kinds of environmental change over millennia, surviving in forms quite similar to dinosaur times.

But as Thorup has pointed out, none of those climatic changes occurred at the rates we are seeing now, which are only expected to accelerate; the question is whether there will sufficient time for birds to adapt, evolve or both. 

If they can, who knows — they might even outlive humankind. Which maybe would only be just.

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Both papers can be read without charge:

  • “Resource tracking within and across continents and long-distance bird migrants,” as published in the Journal Science Advances, is here [PDF], and
  • “Temporal shifts in temperature sensitivity of avian spring migratory phenology: a phylogenetic meta-analysis,” as published in the Journal of Animal Ecology, is here.