Responses of Female Burrowing Owls to Alterations in Clutch Size: Are Burrowing Owls Determinate or Indeterminate Egg-Layers?

—Bird species, including raptors, can often be categorized into two groups depending upon their response to alterations in clutch size while laying. For some, clutch size is predetermined prior to the start of laying (i.e., determinate species). In contrast, the clutch size of indeterminate layers can be influenced by external factors present at the time of laying. Using field experiments, our objective was to examine the egg-laying responses of female Western Burrowing Owls (Athene cunicularia hypugaea) to manipulations of clutch size. To assess whether Burrowing Owls are determinate or indeterminate egg-layers, we altered female clutch size by removing or adding one egg during the laying period. At the time of egg removal or addition, the resident female had 1–5 of her own eggs present in her clutch. We compared the size of completed clutches of both removal and addition nests to the size of completed clutches at control nests. Mean clutch size at removal nests (x̄ 5 8.3 eggs, SE 5 0.44, n 5 9) did not differ from controls (x̄ 5 8.8 eggs, SE 5 0.18, n 5 17), whereas mean clutch size at addition nests (x̄ 5 10.7 eggs, SE 5 0.66, n 5 9) was significantly larger than that of control nests. These findings demonstrate that female Burrowing Owls responded to the removal of an egg by laying a replacement, yet they did not curtail laying in response to the addition of an egg to their nest. Thus, female Burrowing Owls may be described as removal indeterminate and addition determinate. These results have implications for understanding aspects of Burrowing Owl nesting biology, such as selective advantage following the partial or total loss of a clutch of eggs, and they also may provide insight into how this behavior could facilitate conspecific brood parasitism or mitigate its costs in this species.