Some storks are feeding heartily at landfills, spending their winters in garbage dumps instead of migrating. The findings, published in Science Advances this week, adds to an already growing list of migratory species that have either shortened their migration distances or turned into year-round residents in response to anthropogenic environmental changes.
White storks (Ciconia ciconia) are soaring migrants that rarely use flapping flight. They gain altitude by utilizing thermal uplift – or columns of rising air created by solar radiation – and then they move forward by gliding. Because soaring flight is energetically cheaper than flapping, these birds can travel vast distances with minimal energy expenditure. Soaring species actively select their migration routes and times to experience stronger updrafts. White storks breed from Europe to Northwest Africa and Western Asia, and different populations of this same species have different migration patterns because of flexible, opportunistic life history strategies. That makes them good study subjects for exploring the travel costs of their decisions.
A team led by Andrea Flack from the Max Planck Institute for Ornithology attached GPS devices with 3D acceleration sensors to a total of 70 juvenile storks (pictured below) in Armenia, Greece, Poland, Russia, Spain, southwest Germany, Tunisia, and Uzbekistan. The researchers continuously monitored the movements of the young birds for the first five months of their migratory journey. Many juveniles don’t survive this trip, so these first few months could be used to represent their entire lifetime movement costs.
The timing, routes, lengths, and destinations varied drastically among the different stork populations. Some flew 4,867 kilometers (3,024 miles) and wintered north of the Sahara, for example, while others flew 16,550 kilometers (10,283 miles) across the desert to winter in areas of low human density and high vegetation cover. The migration paths of 62 storks are depicted to the right.
Storks from Russia, Poland, and Greece displayed the most traditional route – through the Sahel region of eastern Africa all the way down to South Africa – while the Spanish birds migrated across the Sahara desert to the western Sahel. Meanwhile, birds from southwest Germany overwintered at garbage dumps in Northern Morocco instead of migrating to the Sahel.
The storks that fly along those more traditional routes spend more energy than those that stay on the landfill sites, Flack explains to IFLScience. Birds in areas that have higher human populations were able to lower their daily energy expenditure with shorter daily foraging trips and shorten their migration distance, or suppress migration completely. This may result in both higher survival and greater reproduction success. Although, Flack adds, they’d have to follow storks for many more years and examine more birds to say for sure.
Migratory animals play major roles in ecosystems all along their travel paths – controlling pests, pollinating plants, and participating in complex disease dynamics. While human-induced influences might be beneficial to migratory species, the changes that result could negatively impact their various ecosystems.
GPS tags on juvenile storks from Russia. Jury Galchenkov
Image in the text: Flack et al. Sci. Adv. 2016