Lifetime reproductive success in barn owls near the limit of the species' range

-I studied 357 nesting attempts by a minimum of 473 Barn Owls (Tyto alba) in northern Utah from 1977 to 1995, and documented lifetime reproductive success for 262 owls. Mean age of first breeding was 1.06 years (range <1 to 3), mean number of years breeding was 1.30 (range 1 to 7), and mean number of years breeding successfully was 1.03 (range 0 to 6). Eleven percent of the pairs produced two broods in one year. Mean number of eggs produced in a lifetime was 9.76 (range 1 to 66), and mean number of young fledged was 5.58 (range 0 to 50). Eight percent of the females laid 25% of the population's eggs, and 55% laid 75%. Of the females that laid eggs, 22% produced no fledglings. Twelve percent of the females left breeding descendants in the population with up to four generations traced; the number of direct descendants from these females ranged from 3 to 69. Longer-lived owls produced more eggs and fledglings in their lifetimes, but age that breeding began did not strongly affect lifetime reproductive success. Breeding age had a weak but nonsignificant effect on clutch size and the number of fledglings produced in a breeding season. Habitat variability did not affect LRS, but sites with higher usage were correlated with higher nesting success. Variability in the severity of winter weather had a strong influence on LRS through mortality of adults, reduction in clutch size, and in the likelihood of producing two broods in one season. Severe winters, though, had little effect on the number of fledglings in a brood in the following breeding season. Age and sex of Barn Owls had very little influence on individual LRS. Received 5 July 1996, accepted 31 March 1997. VARIATION IN REPRODUCTIVE SUCCESS among individuals has attracted considerable attention, and the evolutionary significance of lifetime reproductive success (LRS; the total production of offspring during an individual's life) is still debated (Newton 1989b, Murray 1992, Barrowclough and Rockwell 1993). Newton (1985) made a strong case that measurements of LRS offer several advantages over cross-sectional studies in understanding a species' reproduction. First, better than other measures, LRS reveals the extent that reproduction varies among individuals because a few individuals produce a disproportionate percentage of the next generation, and small differences in annual success among individuals may become large differences over entire life spans (Newton 1985, 1988, 1989a; Clutton-Brock 1988). Second, lifetime measures are not affected as much by short-term factors, e.g. poor breeding years brought on by low prey numbers or bad weather conditions (Newton 1989b). Information on LRS is valuable for understanding reproductive strategies, but determining LRS is difficult because many marked 1 E-mail: cmarti@weber.edu individuals must be followed throughout their reproductive lives. Largely for this reason, LRS has been reported in only six raptor species: Eurasian Sparrowhawk (Accipiter nisus; Newton 1985, 1988, 1989a), Osprey (Pandion haliaetus; Postupalsky 1989), Tawny Owl (Strix aluco; Wallin 1988), Ural Owl (Strix uralensis; Saurola 1989), Boreal Owl (Aegoliusfunereus; Korpimaki 1992), and Eastern Screech-Owl (Otus asio; Gehlbach 1989). Most of these species are longlived, and all but the Osprey are forest inhabitants. In contrast to the above species, the Barn Owl (Tyto alba) is short-lived and inhabits open lands. Thus, its pattern of LRS might differ from other raptors. Barn Owls are amenable to the study of LRS because they: (1) readily accept nest boxes for breeding, permitting easy access for documenting reproductive performance; (2) tolerate human presence; and (3) are very sedentary as breeders (Marti 1994). Barn Owls breed at an early age, lay large clutches in relation to their body mass, can produce two or even more broods per year, and often breed only once in their lifetime (Marti 1992, Taylor 1994). Thus, the LRS of a Barn Owl population makes an interesting contrast to species with