Why can parent songbirds lose weight while feeding their nestlings?

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Two wild horses graze on tall grass in the open prairie of Theodore Roosevelt National Park, with rolling badlands visible in the background. — Feeding behaviour Wild horses graze in the Badlands of North Dakota.

Parent songbirds may lose weight while feeding their nestlings because parental care is energetically expensive and represents a significant cost of reproduction.1

Providing food to offspring is a non-shareable form of parental investment that requires substantial time and energy from the adults. When parents prioritize the survival and condition of their brood, they often divert resources away from their own maintenance. This trade-off can lead to a reduction in the parent's own body condition, which is considered a current cost of reproduction. In some species, the effort required to fledge a large brood is so demanding that it can even result in reduced postbreeding survival for the parents.1