A previous study reported that climate-mediated increases in the length of the breeding season produced increasingly female-biased fledging sex ratios in Red-winged Blackbirds (Agelaius phoeniceus). Using those same data plus one additional year (11 years in total), I found that this phenomenon was not a result of greater production of females early and late in the season, contrary to what had been proposed. Instead, seasonal sex-allocation patterns interacted with season length. Early and midseason sex ratios became more female-biased as breeding seasons became longer, whereas late-season sex ratios tended to vary in the opposite manner, albeit weakly. Thus, the discrepancy between sex ratios late in the season and those earlier (early plus midseason) was a strong function of season length. Because fledging sex ratios did not vary with nestling mortality, these patterns appear to be a consequence of nonrandom sex-allocation rather than sex-biased survival. It is unclear whether these sex-allocation patterns are adaptive. Because the climatic factor (the North Atlantic Oscillation) associated with longer breeding seasons is also associated with higher winter mortality, however, it is possible that female Red-winged Blackbirds change how they allocate sex in response to changes in the breeding sex ratio. If climate change continues to alter these patterns, documenting how individuals and populations respond will be informative, from both basic and applied perspectives.
Data presented here come from a series of studies of blackbirds conducted at the Queen's University Biological Station in eastern Ontario (45°37′N, 76°13′W). For detailed methods used in those studies, see the references provided in Weatherhead (2005a). Fledgling sex-ratio data were from the same years (1985–1993 and 1995) used in the analysis that identified the association between sex ratio and length of the breeding season (Weatherhead 2005a). Note that other years with sex-ratio data (e.g., 1981) were not included in the previous analysis or here because sex ratios were determined at the end of incubation rather than at fledging. In addition, I collected new data in 2005. Data for all years came from the same group of cattail (Typha latifolia) marshes. Although every site was not used every year, all sites were used in multiple years and multiple sites were used each year.
Each year, every nest at a site was found and its fate determined. This included all re-nests and second nests. Because nesting females were not individually marked in all marshes or all years, first nests cannot be distinguished from re-nests and second nests, but most nests early in the season were likely to be first nests and most late nests were likely to be re-nests (nesting following a successful first nest is rare in this population). Nestlings were usually measured daily or on alternate days until day 8, after which nestlings were usually not handled to prevent premature fledging. Previously, I had used established criteria based on sexual size-dimorphism (Holcomb and Twiest 1970, Fiala 1981, Westneat et al. 1995, Weatherhead and Dufour 2000) to determine fledgling sex ratios (Weatherhead 2005a). For data collected in 2005, however, fledglings were also sexed using DNA-based methods (Kahn et al. 1998) using blood samples taken when nestlings were four to six days old (Weatherhead et al. 2007). These results were used to refine the morphological sexing methods, and these refinements were then applied retrospectively to recalculate the sex-ratio estimates for earlier years. For the new criteria, a nestling was considered to have hatched the day it was first measured (day 0) if it weighed ≤4.0 g. Respective weights used to assign females and males were ≤27 g and >30 g on day 6, ≤31 g and >32 g on day 7, and ≤31 g and >33 g on day 8. For last-hatched nestlings, however, respective female and male criteria were ≤26 g and >29 g on day 6, but thereafter were the same as for other nestlings.