The twice-annual long-distance migration of birds is one of nature’s most incredible phenomena. In addition to making impressive feats of navigation, birds must acquire adequate fat to fuel their flights, typically replenishing their reserves en route.

Arriving on northerly breeding grounds with extra fat — more than necessary to complete the migratory journey — could bring potential benefits. For example, it might provide a buffer in case of poor weather conditions or uncertain food resources, a theory dubbed the “insurance hypothesis.” Or that extra fat could furnish additional energy needed for nesting activities, called the “breeding performance hypothesis.”

Researchers from New York’s Braddock Bay Bird Observatory set out to see if birds did indeed have extra fat at the end of their spring migration, and whether they could find evidence for either theory. Jennalee A. Holzschuh and Mark E. Deutschlander analyzed the condition of hundreds of warblers of 12 species banded on the south shore of Lake Ontario, presumed to be one of the last stops before the birds arrived on their breeding territories.

The researchers report in The Auk: Ornithological Advances that 11 of the 12 warbler species were in better condition when they arrived at the site in spring than when they departed there in the fall. Energetic condition of most of the warbler species increased as spring migration advanced, which is contrary to the insurance hypothesis. For all 12 species, females were in better condition than males in both spring and fall.

The results lend support to the breeding-performance hypothesis, in particular since females have higher energetic demands during egg-laying and may benefit more than males from extra reserves. However, the fact that females were heavier in both seasons indicates that other factors are at play. Further studies at different sites, especially on the breeding grounds, could help clarify the role of energy reserves in springtime. – Julie Craves

A version of this article appeared in the October 2016 issue of BirdWatching. Subscribe.

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Originally Published September 21, 2016