Sex Ratios of Barn Owls in South-Central Pennsylvania Shawn E. Fauth and Karl W. Kleiner York College of Pennsylvania, Department of Biological Sciences, York PA Introduction Not all vertebrate species are bound to a 50/50 sex ratio. Sex ratio bias has been shown in many different species of animals across most classes (West and Sheldon 2002). Bias in the sex ratio is well documented in birds (Millon and Bretagnolle 2005, Kalmbach et all 200, Bensch et al 1999), and especially in sexually size dimorphic birds of prey, such as owls (Desfor et al 2007, Brommer et al 2003, Hornfeldt et al. 2000, Sasvari and Nishiumi 2005). This study examined one particular species of owl of which little is known about sex allocation, the Barn Owl Tyto alba pratincola. A female can increase her own fitness by varying the ratio of males to females in her offspring. There are two main approaches to understanding sex ratio variation: sex allocation at 1) the family level and 2) the population level. Sex ratio variation at the family level is the result of specific environmental factors such as prey availability (Brommer et al 2003, Appleby et al 2007), time of season (Daan et al 1996), and hatching order (Sasvari and Nishiumi 2005). This implies that the condition of the individual female will ultimately play a role in sex ratio determination (Trivers and Willard 1973). In the case of the Barn Owl, the females are reversed sexually dimorphic, males being about 17% lighter than females (Marti 1992). Thus, in times of food scarcity or other adverse environmental conditions, females should produce males, the sex which is energetically less costly. The second approach to sex ratio variation is sex allocation at the population level. Natural selection will favor an equal investment of energy in both sexes by the parents. Therefore, the energetically cheaper sex may be overproduced; however, frequency dependent selection will favor a 50:50 ratio overall at the population level (Fisher 1930). This study examined five Barn Owl nest sites in South-central Pennsylvania over a period of 3 consecutive years. Sex ratios were examined in relation to parity, year to year variation, relationship between brood size and sex ratio, and relationship between date of first egg being laid and sex ratio. We predicted that because Barn Owls are reversed sexually dimorphic, there should be a slight bias toward male offspring, and that sex ratios should change over the three years, possibly due to varying environmental conditions.Methods Five Barn Owl nest sites in Adams, Fulton, Dauphin, and Juniata Counties were sampled based on a clutch size of 3 or greater and recurrence over three consecutive years. DNA was extracted from feather samples from hatchlings at all five nest sites. Gender was determined using PCR and gel electrophoresis,. The percentage of males was analyzed using a G-test for goodness of fit to determine: If the percentage of males differed from parity If the percentage of males changed over the three years Linear regression was used to determine: If there was a relationship between clutch size and percentage of males. If there was a relationship between date of first egg laid and percentage of males. Results The percentage of males did not differ from parity in any of the three years (P > 0.05). The percentage of males increased slightly over the 3 years, though not significantly (P > 0.05). The percentage of males increased as clutch size increased (P < 0.05). There was no relationship between percentage of males and first egg laying date (P > 0.05) Discussion Overall, there is evidence in this study for both sex ratio adjustment at the population and the family level. The results support Fisher’s (1930) theory that the ratio of males to females at the population level will be 50:50. The increased proportion of males in relation to increased clutch size would be expected, as males would be less energetically expensive to produce. This would seem to support sex ratio adjustment at the family level as Trivers and Willard (1973) predicted. The lack of a relationship between date of the first egg being laid and sex ratio supports Daan et al.’s (1996) theory that sex ratio variation should be biased towards the gender which would have a greater benefit from hatching first in relation to age of sexual maturity. Because both males and females mature around 12 months of age, we would expect to see the sex ratio to be about even throughout the breeding season. Acknowledgements Acknowledgements Jamie Flickinger and Dan Mummert, Pennsylvania Game Commission, for access to nest sites and data. Susan Klugman for proofreading. Works Cited Appleby, Bridget M., Petty, Steve J., Blakey, Jeremy K., Rainey, Paul, and MacDonald, David W Does variation in sex ratio enhance reproductive success of offspring in tawny owls (Strix aluco)? Proc. R. Soc. Lond. B 264: Bensch, S., Westerdahl, H., Hansson, B., and Hasselquist, D Do females adjust the sex of their offspring in relation to the breeding sex ratio? Journal of Evolutionary Biology. 12: Brommer, Jon E., Karell, Patrik, Pihlaja, Tuomo, Painter, Jodie N. Primmer, Craig R., Pietiainen, Hannu Ural owl sex allocation and parental investment under poor food conditions. Behavioural Ecology. 137: Daan, Serge, Dijkstra, Cor, and Weissing, Franz J An evolutionary explaination for the seasonal trends in avian sex ratios. Behavioral Ecology. 7(4): Desfor, Kasi B., Boomsma, Jacobus J., and Sunde, Peter Tawny owls Strix aluco with reliable food supply produce male biased broods. Ibis. 149: Fisher, R.A. The Genetical Theory of Natural Selection. Oxford at Clarend Press Hornfeldt, B., Hipkiss, T., Fridolfsson, K., Eklund, U., and Ellegren, H Sex ratio and fledgling success of supplementary fed Tengmalm’s owl broods. Molecular Ecology. 9: Kalmbach, Ellen, Nager, Ruedi G., Griffiths, Richards, and Furness, Robert W Increased reproductive effort results in male-biased offspring sex ratio: an experimental study in a species with reversed sexual size dimorphism. Proc. R. Soc. Lond. B 268: Marti, C.D Barn Owl. In The Birds of North America, No. 1 (A. Poole, P. Stettenheim, and F. Gill, Eds.). Phildadelphia: The Academy of natural Sciences; Washington, DC: The American Ornithologists’ Union. Millon, Alexandre, and Bretagnolle, Vincent Nonlinear and population-specific offspring sex ratios in relation to high variation in prey abundance. Oikos. 108: Sasvari, Lajos, and Nishiumi, Isao Environmental Conditions affect offspring sex-ratio variation and adult survival in tawny owls. The Condor. 107: Trivers, Robert L., and Willard, Dan E Natural selection of parental ability to vary the sex ratio of offspring. Science. 179: West, Stuart A., and Sheldon, Ben C Constraints in the evolution of sex ratio adjustment. Science. 5560(295): Results Figure 1. Mean (± 95% C.I.) percentage of males by year. Figure 2. Mean (± 95% C.I.) number of males in each sample site by year Figure 3. Mean (± 95% C.I.) clutch size by year. Figure 4. Relationship between brood size and the percentage of males in the clutch. Figure 5. Relationship between the estimated Ordinal date of the first egg being laid and the percentage of males in the clutch. Agar gel displaying PCR products. A single band indicates a male,(ZZ) and a double band indicates a female (WZ).