Barn Owl Reproduction: Patterns and Variation near the Limit of the Species' Distribution

Abstract

I studied reproduction of the Barn Owl (Tyto alba) in irrigated farmlands of northern Utah for 16 years documenting 391 nesting attempts. Most Barn Owls began nesting at one year of age and produced one brood per year. The owls rarely produced second broods or replaced failed first clutches. Complete first clutches averaged 7.17 eggs (n = 275). Replacement (x = 5.81, n = 16) and second clutches (x = 5.79, n = 19) were significantly smaller than first clutches, but first (X = 5.45) and second broods (x = 5.37) did not differ significantly. Replacement broods (x = 3.83) were significantly smaller than first. Of all nesting attempts 88% produced full clutches and 71% yielded at least one fledgling. Successful nests on average produced 5.09 fledglings per first brood, 4.94 per second brood, and 3.60 per replacement brood. Second attempts were more likely to produce fledglings than either first or replacement attempts. Sixty-three percent of all eggs laid hatched and 55% produced fledglings. Of eggs that hatched, 87% survived to fledging. March 13 was the mean date for initiation of egg laying and latest second clutches hatched on 4 October. Persistent snow cover and low winter temperatures significantly delayed onset of egg laying and reduced the number and success of breeding attempts. Clutch size, however, did not differ significantly among years or among nest sites.