Research letters are short papers (5 printed pages, about 4000 words), ideally presenting new and exciting results. Letters will be given priority, whenever possible, in the publication queue. Contribu- tions should be as concise as possible. An abstract is required. JOURNAL OF AVIAN BIOLOGY 34: 134–139, 2003 RESEARCH LETTERS Sex allocation in black-capped chickadees Poecile atricapilla Scott M. Ramsay, Daniel J. Mennill, Ken A. Otter, Laurene M. Ratcliffe and Peter T. Boag Ramsay, S. M., Mennill, D. J., Otter, K. A., Ratcliffe, L. M. and Boag, P. T. 2003. Sex allocation in black-capped chickadees Poecile atricapilla. – J. Avian Biol. 34: 134–139. Optimal sex allocation for individuals can be predicted from a number of different hypotheses. Fisherian models of sex allocation predict equal investment in males and females up to the end of parental care and predict brood compositions based on the relative costs of producing males and females. The Trivers-Willard hypothesis predicts that individual females should alter the sex ratio of their broods based on their own condition if it has a differential impact on the lifetime reproductive success of their sons and daughters. The Charnov model of sex allocation predicts that females should alter sex allocation based on paternal attributes that may differen- tially benefit sons versus daughters. Because females are the heterogametic sex in birds, many recent studies have focussed on primary sex ratio biases. In black-capped chickadees Poecile atricapilla, males are larger than females suggesting they may be more costly to raise than females. Female condition affects competitive ability in contests for mates, and thus may be related to variance in fecundity. Females prefer high-ranking males as both social and extrapair partners. These observations suggest that females might vary the sex ratio of their broods based on the predictions of any of the above models. Here, we report on the results of PCR based sex determination of 1093 nestlings in 175 broods sampled from 1992 to 2001. Population-wide, we found a mean brood sex ratio of 0.525 0.016, with no significant deviation from a predicted binomial distribution. We found no effect of clutch size, female condition, hatch date, parental rank or paternity. Our results reject the idea that female black-capped chickadees systematically vary sex allocation in their broods. S. M. Ramsay (correspondence), D. J. Mennill, K. A. Otter, L. M. Ratcliffe, and P. T. Boag, Department of Biology, Queen’s Uniersity, Kingston ON K7L 3N6, Canada. Present addresses: K. A. Otter, Biology Program, Uniersity of Northern BC, Prince George BC V2N 4Z9, Canada, S. M. Ramsay, Department of Biology, Wilfrid Laurier Uniersity, Waterloo ON N2L 3C5, Canada. E-mail: sramsay@wlu.ca Over the last fifty years, there has been continuous interest in sex ratio theory. A number of theoretical perspectives have been taken such as natural selection for equal investment in both sexes (Fisher 1958), popu- lation level variation of sex ratio (Hamilton 1967), and optimal sex allocation for individuals when conditions provide some advantage for offspring of one sex over the other (Trivers and Willard 1973, Charnov 1982). In birds, adaptive manipulation of sex ratio has attracted considerable interest because females are the het- erogametic sex and there is the potential for manipula- tion of the primary sex ratio (reviewed in Ellegren and Sheldon 1997). In addition, molecular techniques now provide the necessary tools to address sex allocation in non-invasive ways (Ellegren and Sheldon 1997, Griffiths et al. 1998, Fridolfsson and Ellegren 1999). Numerous studies have found evidence of optimal sex allocation, with females biasing their broods based on a number of different factors. For example, mater- nal condition and environmental factors that influence © JOURNAL OF AVIAN BIOLOGY JOURNAL OF AVIAN BIOLOGY 34:2 (2003) 134