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During spring migration, certain songbirds hit the road with more fat than they need to survive their long and treacherous journeys. It’s a diet plan that has intrigued scientists for decades. Now, a recent study may have found the reason for that extra chub: Birds, it seems, plump up so that when it comes time for breeding, they can afford the energetic cost of making babies.

Mark Deutschlander, a biologist at Hobart and William Smith Colleges and co-author of the paper,��published this week in ,��​says there had been two prevailing theories for the weight gain. “One theory says they gather fat in case they arrive early to their destination and food availability is not yet optimal,” he says. “And then there’s the hypothesis that they store fat in order to be able to reproduce. We found a link for the latter.”

Deutschlander and the team, which was led by graduate student Jennalee Holzschuh,��obtained and analyzed data from 12 different warbler species—including Black-throated Blue, Nashville, and Yellow-rumped Warblers—that were caught between 1999 and 2012 at the Braddock Bay Bird Observatory in New York. The station is located near the birds’ breeding location in the boreal forests of Canada.

When measuring fat deposits and scaled mass indexes (similar to body mass indexes), the scientists found that females consistently have higher fat and energy reserves than males. They detected the biggest difference in Blackpoll Warblers; female Blackpolls averaged 18 percent more weight than males. This makes sense biologically,��given that it takes much more energy to produce eggs than sperm. 

Based on calculations from the mass indexes, scientists also found that both sexes stored more energy in spring than in fall. Like females, males also need to prepare for the breeding season: “They guard territories vigilantly, singing songs at the top of their lungs for hours on end,” Deutschlander says. “That’s why we think they’re in better shape when they arrive in spring than when they leave in the fall.”

In addition, the analysis found that 11 of the 12 studied species were richer in energy after arriving later in the season,��rather than a few weeks early. This contradicts the theory of using fat as backup fuel in case food is not yet available.

So does this finding mean fatter birds have better, more-frequent sex? Not necessarily. But they do know if they want tons of babies, they'd better keep feasting.