Publication Date


Date of Final Oral Examination (Defense)


Type of Culminating Activity


Degree Title

Master of Science in Raptor Biology



Major Advisor

James R. Belthoff, Ph.D.


Jesse R. Barber, Ph.D.


Ian C. Robertson, Ph.D.


During 2012-2013, I investigated the behavioral responses of western burrowing owls (Athene cunicularia hypugaea) in southwestern Idaho to experimental conspecific brood parasitism to determine if they were capable of egg discrimination, and if they would accept or reject a parasitic egg introduced into their nest. I experimentally added one egg derived from source nests to host nests either during the egg-laying period or just after adult females initiated incubation and examined acceptance or rejection of the parasitic egg in relation to control nests where eggs were handled but not exchanged. Burrowing owl responses to parasitic eggs varied with nesting stage. Burrowing owls rejected at 3 of 11 (27%) nests in which an experimental conspecific egg was added during the laying period and accepted the parasitic egg at each of the 11 nests experimentally parasitized during incubation. None of the control nests (n = 17) experienced rejection of eggs or abandonment of the nest after handling of eggs. Early deposited parasitic eggs may pose a greater threat to host fitness because of increased chance of hatching, which may explain why more early deposited parasitic eggs were rejected. Burrowing owls rejected the nest at an additional 3 of 12 (25%) nests in which an experimental egg was reciprocally exchanged. This behavior suggests that hosts were able to perceive that their nest had been parasitized based on the presence of the parasitic egg rather than on any sudden increase in clutch size. Size differences between host and parasitic eggs and the degree of size variability among eggs within host clutches had no effect on rejection. Instead of selectively rejecting the individual parasitic eggs, hosts buried them along with their own eggs. This implies that the hosts could not remove the parasitic egg or they were unable to individually recognize it. I also assessed changes in parental care by evaluating images from motion-activated infrared cameras placed at owl nests. There was no significant difference in burrow attendance of both male and female adult burrowing owls between treatment (male: 15.6 ± 2.14 (SE) min/hr; female: 25.7 ± 1.80) and control (male: 12.9 ± 2.88 min/hr; female: 30.5 ± 3.85) nests; nor was there a significant difference in prey delivery rate (treatment: 0.3 ± 0.15 prey/nestling/hr; control: 0.5 ± 0.34 prey/nestling/hr) or prey biomass delivered (treatment: 5.5 ± 1.82 g/nestling/hr; control: 4.2 ± 1.76 g/nestling/hr). Thus, burrowing owls may either be unable to identify parasitic nestlings or unwilling to decrease parental care if the cost is decreasing their own offspring’s survival. Finally, I assessed laying determinacy by comparing completed clutch sizes of nests where I removed (i.e., source nests for parasitic eggs) or added one egg during laying, to completed clutch sizes of un-manipulated control nests. Mean clutch size was significantly larger at nests where I added an egg (10.7 ± 0.67, n = 9) than at control nests (8.8 ± 0.18, n = 17), while clutch sizes for control and in nests where we removed one egg (8.3 ± 0.44, n = 9) did not differ. These findings suggest 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 in my study area may be described as removal indeterminate and addition determinate. The results of my study are consistent with the notion that conspecific brood parasitism is part of the behavioral repertoire of burrowing owls, and they also may provide insight into how laying determinacy can facilitate conspecific brood parasitism or mitigate its costs in this species.