Brood parasites may use gape size constraints to exploit provisioning rules of smaller hosts: an experimental test of mechanisms of food allocation

We investigated whether a mechanism of gape size limitation could increase the competitive ability of a large brood parasite in a brood of smaller host nestlings. The gape size of hatchling birds may limit the size of prey they can swallow and hence parents should bring larger more profitable prey as their nestlings grow. The relatively large gape of a brood parasite in a brood of smaller hosts may 1) increase provisioning rate to the brood, 2) allow the parasite to swallow large prey, or 3) cause parents to start bringing larger prey at an earlier nestling stage. We added 1 large same-aged great tit Parus major nestling to broods of smaller blue tits Cyanistes caeruleus to simulate a naive brood parasite system and filmed them when 2–3 days old. The cue of a single large nestling did not cause parents to increase provisioning rates nor to change the species of prey, but prey items were larger than in control broods. Large prey were "tested" more often than small prey, that is offered and then removed from the gapes of small nestlings, and 17% of the prey that the great tit nestling received had been previously offered to a blue tit. Our results revealed that prey size brought by parents could further increase the competitive ability of brood parasites.