Which chicks in a nest a parent bird decides to feed depends on the quality of the local environment, finds a study in Nature Communications. This helps to resolve a long-standing question in ecology about whether parents respond to signals of need (such as how much a chick begs), or signals of quality (such as the chicks’ size) when deciding which of their offspring to feed.
While caring for their offspring, birds use up large amounts of energy by foraging for enough food to sustain their brood. Whether these parents choose to feed the chicks that beg the most (for example, this is the case for tree swallows in North America) is highly variable among different bird species, with some species instead choosing to ignore begging chicks in favour of larger offspring in the nest (for example, blue-footed boobies in the Galapagos Islands). However, which signals guide this decision has been the subject of much debate.
Stuart West and colleagues compile available literature - including 306 studies - on the parental care preferences during feeding of 143 bird species across the globe, and analyse how this variation in care relates to the condition and behaviour of their offspring, and also the environmental conditions in the area in which each species is found. They find that, in favourable and predictable environments, parents choose to feed begging chicks that are in poorer condition relative to their siblings, whereas in unfavourable environments they preferentially feed chicks that are in the best condition, regardless of how much other siblings in the nest beg.
This indicates that local environmental conditions can influence the evolution of parent-offspring communication systems, and explains why previous studies that do not consider environmental variation have not been able to identify consistent behavioural patterns across bird species.