Sometimes bird moms and dads have to pick which of their chicks to feed. According to a new Nature Communications study, that decision depends largely on environmental conditions. Parents in a quarter of the bird species studied ignore their begging chicks, and some even neglect smaller chicks that beg more in favor of bigger chicks that beg less.
Birds spend a lot of energy looking for food to sustain their brood, and researchers have long puzzled over how parents decide which of their offspring to invest in, when, and how much. Previous work revealed that raising a brood successfully is as metabolically demanding for breeding birds as cycling the Tour de France is for humans.
Most chicks have evolved behaviors and signals designed to maximize their chances of being fed: vocal calls, begging postures, and bright mouths with colorful ornaments or ultraviolet reflectance, for example. How parents respond varies a lot across species. Some bird mothers, like tree swallows, feed the chicks that beg the most. Others might ignore persistent begging and feed the biggest (and silent) chicks instead, such as the hoopoe. And in siblicidal species, like blue-footed boobies on the Galapagos, bigger chicks beg and get more food, while smaller chicks are left to starve. Exactly how parents choose signals of quality over signals of need (or vice versa) is a mystery.
University of Oxfords Stuart West and colleagues conducted a literature search using Web of Science and Google Scholar on parental care preferences during feeding. They ended up with 306 studies on 143 bird species from around the world. They then analyzed how variation in parents choices are related to factors ranging from chick condition to ecological conditions.
The team found that when the environment is predictable and good, chicks in poorer health beg more, and their parents feed them more. When food is plentiful, all the offspring might survive. But when the environment is unpredictable and less favorable yet an optimistic number of eggs were laid parents tend to feed chicks with the best health, regardless of how much the other nestlings beg. The parents pay less attention to begging, relying instead on signals of quality, such as size.
The findings suggest that ecological variation can lead to different, yet evolutionarily stable, parent-offspring communication systems in different species. Some birds can even adjust their responses: When theres extra food, hihi parents become less sensitive to mouth color in their offspring, and alpine swifts who breed early in the season when theres more food prefer nestlings with lower ultraviolet reflectance.
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