Does initial suppression of allofeeding in small jays help to conceal their nests?

Abstract

A neglected question in the study of communal breeding in birds concerns why alloparenting begins at variously late stages in the nesting cycle. We studied this phenomenon in the Gray Jay (Perisoreus canadensis), a species in which nonbreeders are excluded from the nest area by parental hostility and begin to feed young only during the fledgling period. We hypothesize that this pattern is favoured because of the risk of nest predation. By initially suppressing allofeeding, Gray Jay parents may reduce the frequency of predator-attracting visits to the nest when the young are most vulnerable. We evaluated this predator-avoidance hypothesis in a six-part meta-analysis using observations of 111 philopatric and immigrant nonbreeders associated with 647 pairs over 33 years and nest-visitation data for 307 other passerines. First, we found indirect evidence that Gray Jays and other corvids with non-allofeeding nonbreeders do reduce nest visitation by excluding nonbreeders. Second, we found that the hostility of adult Gray Jays towards nonbreeders peaks during the incubation and nestling periods, is stronger closer to the nest, and is directed towards both related and unrelated nonbreeders. We then found support for four predictions stemming from the predator-avoidance hypothesis. Compared with species having allofeeding nonbreeders, the Gray Jay and other corvids with non-allofeeding nonbreeders are characterized by (i) smaller clutches; (ii) lower parental nest-visitation rates (a consequence of larger food loads); (iii) a greater increase in parental feeding visits after fledging, paralleling the simultaneous relaxation of parental hostility towards potential allofeeders; and (iv) a smaller body size and smaller social groups, rendering them less able to deter nest predators. We discuss the potential value of the predator-avoidance perspective for understanding the occurrence or absence of allofeeding in other birds.