Social constraints on the onset of incubation in a neotropical parrot: a nestbox addition experiment

Abstract

We examined whether the early onset of incubation serves to protect eggs from the dangers imposed by conspecifics in the green-rumped parrotlet,Forpus passerinus, a small neotropical parrot that lays a large clutch and begins incubation on the first egg. Nestboxes with eggs were installed and their fate was followed for 72h to determine whether egg destruction and nest site take-overs occurred as predicted by the Egg Protection and Limited Breeding Opportunities Hypotheses, or whether additional eggs appeared in the boxes as predicted by the Brood Parasitism Hypothesis. Eggs were destroyed at 40.6% of 69 experimental boxes 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 green-rumped parrotlets. Over 75% of the nests were destroyed by male–female pairs prospecting for nest sites, and the remainder were destroyed by male–male pairs. Lone males never destroyed eggs, although they frequently visited experimental boxes. Two of three failures at control nests were the result of nocturnal predators, and the other nest was apparently destroyed by parrotlets. There was no significant difference between experimental and control boxes in the frequency of visitations by lone males, male–female pairs and male–male pairs. Although experimental boxes that parrotlets visited were discovered quickly after placement, parrotlets were usually slow to enter them (X=5.8h after discovery, range 0.3–23.5h). Control nests were rarely left unattended: females spent nearly 75% of their time in the box, and pairs were typically absent for short intervals (median=7.5min). Control females responded to intruding parrotlets by remaining in the box 94% of the time when alone, whereas males actively displaced and chased intruding parrotlets 66% of the time. Parrotlets that visited control nests approached the box significantly less often than those visiting experimental boxes. To ensure the survival of eggs, parrotlet parents must begin incubating eggs or guarding nests soon after laying to minimize destruction of clutches, loss of nest sites, a decline in the viability of their eggs and the time that all nestlings are exposed to predators.