BARN OWL REPRODUCTION - PATTERNS AND VARIATION NEAR THE LIMIT OF THE SPECIES DISTRIBUTION

I studied reproduction of the Barn Owl (Tyto alba) in irrigated farmla nds of northern Utah for 16 years documenting 391 nesting attempts. Mo st Barn Owls began nesting at one year of age and produced one brood p er year. The owls rarely produced second broods or replaced failed fir st clutches. Complete first clutches averaged 7.17 eggs (n = 275). Rep lacement (xBAR = 5.81, n = 16) and second clutches (xBAR = 5.79, n = 1 9) were significantly smaller than first clutches, but first (xBAR = 5 .45) and second broods (xBAR = 5.37) did not differ significantly. Rep lacement broods (xBAR = 3.83) were significantly smaller than first. O f all nesting attempts 88% produced full clutches and 71% yielded at l east one fledgling. Successful nests on average produced 5.09 fledglin gs per first brood, 4.94 per second brood, and 3.60 per replacement br ood. Second attempts were more likely to produce fledglings than eithe r first or replacement attempts. Sixty-three percent of all eggs laid hatched and 55% produced fledglings. Of eggs that hatched, 87% survive d to fledging. March 13 was the mean date for initiation of egg laying and latest second clutches hatched on 4 October. Persistent snow cove r and low winter temperatures significantly delayed onset of egg layin g and reduced the number and success of breeding attempts. Clutch size , however, did not differ significantly among years or among nest site s.