Storks are elegant birds, but they've survived for 30 million years because they're scrappy, too. And according to a new study, some resourceful storks from Eurasia have adapted their ancient migration patterns so they can gorge on garbage.
The storks in question are white storks (Ciconia ciconia), a widespread species that mostly migrates between Europe and Africa. They've been doing so for as long as humans have kept records, and likely much longer, but now something is different. Many white storks have begun to modify their migration patterns, the study finds, so they can capitalize on human-related food sources like landfills and fish farms.
The study's authors attached GPS bands to 62 young white storks born in eight countries: Armenia, Germany, Greece, Poland, Russia, Spain, Tunisia and Uzbekistan. They then tracked the birds as they migrated, observing how the routes and timing differed from patterns reported in previous studies.
Migratory behavior "varied drastically" among stork populations, the researchers write. Storks from Greece, Poland and Russia mostly followed traditional routes, but those from Germany, Spain and Tunisia often stopped short of where their ancestors went in winter. Armenian storks also made relatively short trips, and Uzbek storks didn't migrate at all, despite historically wintering in Afghanistan and Pakistan.
Migratory patterns of juvenile white storks. (Image: Flack, et al./Science Advances)
White storks' migration is largely a quest for food, since European winters can limit the availability of prey such as insects, amphibians and fish. The trip across Europe and Africa is also dangerous, though, so these opportunistic birds have an eye out for better options along the way — even if it means venturing into civilization.
An increasing number of white storks spend winter at landfills in the Iberian Peninsula, the researchers note, as previous research has shown. Although all the Spanish juveniles they tracked did migrate across the Sahara desert to the western Sahel zone, others from Germany couldn't resist the lure of easy food.
German storks "were clearly affected by these human-induced changes," they write, adding that four of six birds that survived for at least five months overwintered on garbage dumps in northern Morocco instead of migrating to the Sahel.
As for Uzbekistan, the researchers suspect its storks learned to feed off the country's growing aquaculture industry: "Although previous data are lacking," they write, "we hypothesize that human-induced supplementary feeding (that is, feeding on fish farms) could have driven the suppression of migratory behavior of Uzbek storks."
This might be good for storks, the authors say, at least temporarily: "[F]eeding on anthropogenic food sources such as landfills seems to be beneficial because birds can shorten their migration distance and decrease their daily energy expenditure. These changes may result in higher survival and fitness, potentially leading to rapid microevolutionary changes in migratory patterns."
In general, diverse migration patterns buffer birds against hardships, spreading the species' risk across a mix of ecosystems. Species that cram into smaller areas each winter are often more vulnerable to environmental changes than species with stork-like flexibility. In fact, another new paper finds that "partial migrants" — species in which some members migrate and some don't — are less likely to suffer population declines than birds that either always migrate or never do.
"Many species adopt this mixed migratory strategy, including familiar species like blackbirds and robins," says the University of East Anglia's James Gilroy, lead author of that paper, in a statement. "It looks like it could make them more resilient to human impacts — even in comparison to species that don't migrate at all."
Partially migratory species also show more ability to shift their spring arrival dates forward, Gilroy adds. "This trend toward earlier spring arrival might help species adapt to climate change," he says, "by allowing them to commence breeding earlier in the year as spring temperatures rise."
It is encouraging to see ancient species not only adapt to civilization, but thrive in it. There may be downsides to wintering at landfills and fish farms, however, like birds eating inedible trash or food contaminated by surrounding waste. Plus, as the authors of both new studies point out, behavioral shifts in white storks and other migratory birds could have unforeseen ripple effects in their home ecosystems as well as the southerly habitats where they used to spend winters.
"Migratory animals can have fundamental effects on ecosystems by altering ecological networks, influencing pest control and pollination, or affecting infectious disease dynamics," write the authors of the stork study. "Understanding how human actions alter migratory patterns may be the key not only to protecting migratory species but also to maintaining diverse and stable ecosystems."