I investigated responses of Burrowing Owls (Athene cunicularia) 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.
Owl responses to parasitic eggs varied with nesting stage. Egg rejection predominated in nests where I introduced a parasitic egg during laying, yet owls accepted parasitic eggs added during incubation. 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.
Responses to early parasitic eggs did not differ when I added an egg (clutch size increased) or when I swapped the parasitic egg for a host egg (clutch size did not change). This suggests that hosts rejected the parasitized nest based on presence of the parasitic egg and not solely based on an increased clutch size.
Size differences between host and parasitic eggs, and degree of variability among eggs within host clutches, had no effect on rejection. Instead of selectively rejecting parasitic eggs, hosts buried them along with their own. This implies that the hosts could not remove the parasitic egg or were unable to recognize it.
I also assessed changes in parental care by evaluating images from infrared cameras placed at owl nests. There were no differences in parental behavior between parasitized and non-parasitized nests. 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 unmanipulated controls. Clutch sizes at removal nests did not differ from controls, while clutch sizes at addition nests were significantly larger. Thus, Burrowing Owls in my study area may be described as removal indeterminate and addition determinate.
The results of this study provide evidence that conspecific brood parasitism may be a 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. What is now needed is a comprehensive evaluation of Burrowing Owl family data to lend molecular support to the behavioral evidence of my research.