SOCIAL CONSTRAINTS ON THE ONSET OF INCUBATION IN A NEOTROPICAL PARROT- A NESTBOX ADDITION EXPERIMENT

We examined whether the early onset of incubation serves to protect eg gs from the dangers imposed by conspecifics in the green-rumped parrot let, Forpus passerinus, a small neotropical parrot that lays a large c lutch and begins incubation on the first egg. Nestboxes with eggs were installed and their fate was followed for 72 h to determine whether e gg destruction and nest site take-overs occurred as predicted by the E gg Protection and Limited Breeding Opportunities Hypotheses, or whethe r additional eggs appeared in the boxes as predicted by the Brood Para sitism Hypothesis. Eggs were destroyed at 40.6% of 69 experimental box es but at only 4.5% of control nests occupied by laying pairs. No eggs were laid in the experimental boxes. Egg destruction at experimental nests occurred during daylight hours and all mortality was caused by g reen-rumped parrotlets. Over 75% of the nests were destroyed by male-f emale pairs prospecting for nest sites, and the remainder were destroy ed by male-male pairs. Lone males never destroyed eggs, although they frequently visited experimental boxes. Two of three failures at contro l nests were the result of nocturnal predators, and the other nest was apparently destroyed by parrotlets. There was no significant differen ce between experimental and control boxes in the frequency of visitati ons by lone males, male-female pairs and male-male pairs. Although exp erimental boxes that parrotlets visited were discovered quickly after placement, parrotlets were usually slow to enter them ((X) over bar = 5.8 h after discovery, range 0.3-23.5 h). Control nests were rarely le ft unattended: females spent nearly 75% of their time in the box, and pairs were typically absent for short intervals (median=7.5 min). Cont rol females responded to intruding parrotlets by remaining in the box 94% of the time when alone, whereas males actively displaced and chase d intruding parrotlets 66% of the time. Parrotlets that visited contro l nests approached the box significantly less often than those visitin g experimental boxes. To ensure the survival of eggs, parrotlet parent s must begin incubating eggs or guarding nests soon after laying to mi nimize destruction of clutches, loss of nest sites, a decline in the v iability of their eggs and the time that all nestlings are exposed to predators. (C) 1998 The Association for the Study of Animal Behaviour.