Evolution of parental roles in raptors: prey type determines role asymmetry in the Eurasian kestrel

• We video-filmed prey deliveries and handling at 25 nests of the Eurasian kestrel.
• The male provided prey, and the female dismembered them and fed the nestlings.
• Nestlings became able to feed without maternal assistance sooner when prey were small.
• The female could start helping to provide at lower nestling age if prey were smaller.
• The data predict a geographical variation in sexual size dimorphism depending on diet.

Raptors deviate from the norm among provisioning birds by having asymmetric parental roles, with the female brooding and feeding offspring, and the male providing food, assisted by the female from the latter part of the rearing period. To investigate the poorly understood evolution of role asymmetry in raptors at an intraspecific level, we videorecorded prey delivery and handling in 25 nests of the Eurasian kestrel, Falco tinnunculus, with prey types ranging in size from insects via lizards and mammals to birds, and simultaneously observed prey transfer from male to female outside the nest. As the nestlings aged, the male was more likely to allocate prey items directly to them for unassisted feeding, rather than to the female for further processing and feeding of the nestlings. This switch occurred earlier for lizards than mammals and birds, and earlier for smaller than larger mammals. The time needed to ingest a prey item decreased from birds via mammals to lizards, and was particularly short for insects. The switch from nestlings being fed dismembered prey to nestlings ingesting prey unassisted occurred earlier for lizards than mammals and birds, and earlier for smaller than larger mammals, while all insects were ingested unassisted. Thus, the female could be relieved from dismembering prey, and start hunting, earlier if all prey were lizards rather than mammals and birds, if all mammalian prey were smaller, and in particular if all prey were insects. Because providing for the family selects for small body size, extended confinement of the female as a sedentary food processor for offspring would leave greater potential for differential selection on male and female body size. This potential would vary geographically with the composition of the kestrel's diet, and be larger where the diet contains fewer insects and lizards and more mammals and birds.