Nest-Site Selection and Breeding Behavior of the Migratory Rock Wren (Salpinctes Obsoletus) in Western Kansas

During the summer of 1997 I investigated the breeding biology of a primarily migratory population of Rock Wrens (Salpinctes obsoletus) in the Scott Lake State Park area of Scott County and Logan County, Kansas. I made behavioral observations on nesting adults and fledglings and evaluated nest-site selection for 50 different nests. Five banded birds were used as an aid to examine nest chronology and the contributions of each sex. Females in this population began laying eggs during the last week in April and the latest eggs hatched during the first week in August. Five days prior to egg-laying, males were observed giving call notes at potential nest sites, with females following 10 investigate each cavity. Both active and old nests could be located by identifying rock paths built by the wrens at the entrances of nest cavities. I observed a single female wren carry 91 rocks to a nest site in two hours while the male sang. Females initiated egg-laying in a new nest the day offspring fledged from a previous nest. Once females began incubation of new nests, males continued to feed juvenile wrens from previous nests for up to three weeks. Nine days after one clutch hatched, I observed a female cowbird remove a single Rock Wren chick from the nest cavity and drop it on the ground outside. To assess what factors might protect nests from predation and Brown-headed Cowbird (Molothrus ater) parasitism. I measured the height, orientation, and cavity dimensions of the 50 nests. According to univariate tests, no nest structure variable was significantly related to nest success. Discriminant function analysis, statistically insignificant trends in the data, and my observations indicated nests that were more successful had smaller openings and were located deeper within the cavities. Successful nests also appeared to usually be located lower 10 the ground. Over the entire breeding season Rock Wrens on 40 territories produced 206 offspring. Predation did not have a significant effect on reproductive success; only 16% of nests had their contents preyed upon, and although 46% of nests contained cowbird eggs, parasitism caused reproductive failure only when more than one cowbird chick fledged. My results may reflect natural selection for specific types of nests that limit predator and Brown-headed Cowbird predation. When this baseline data from one year of study is combined with data and observations from future years of study on this primarily migratory Rock Wren population, significant effects of nest-structure variables on productivity and a better understanding of Rock Wren breeding biology may emerge