In species with extrapair mating, females may choose a social mate who will contribute to the successful raising of their brood and a sire who can enhance the genetic quality of offspring. Female choice of social mate and genetic sire may thus be independent events, directed toward different types of reproductive benefits. Furthermore, if reproductive benefits derived through mating preferences differ for sons and daughters, a coupling between sex ratio adjustment and mate choice would be favored by selection. In this paper, we examined whether females adjust the primary sex ratio of offspring to the quality of the social male and/or the extrapair sire in a socially monogamous species with frequent extrapair mating, the tree swallow Tachycineta bicolor. Recent evidence suggests that females obtain compatible genes’ benefits through extrapair mating in this species. If genetic quality is more important for male than female fitness and contributes to higher variance in reproductive success among males than females, sex allocation theory would predict a male-biased sex ratio among extrapair offspring. However, we found no indication of sex ratio bias with paternity in mixed paternity broods. Instead, females skewed the sex ratio toward males in broods without extrapair paternity, which probably reflects a higher phenotypic quality of these males. Furthermore, we confirmed earlier findings that females in good condition produce male-biased broods. Thus, our results indicate that female tree swallows adjust the primary sex ratio to the phenotypic quality of their social mate and themselves and not to the genetic sire of their offspring.
We conducted this study on a population of box-nesting tree swallows at the Queen's University Biological Station, near Chaffey's Lock, Ontario, Canada (44°34′N, 76°19′W), during the breeding season of 2006. This study was part of an ongoing research project on this population, which has been the subject of intensive studies over the past 3 decades (cf., Robertson and Rendell 2001; Sæther et al. 2005; Stapleton et al. 2007). The study area consists of 8 grids, ranging in size from 9 to 35 nest-boxes, and an additional 25 solitary nest-boxes distributed along the main road connecting the separate grids. The grids are located on hayfields bordered by mixed deciduous forests, and the nest-boxes on the grids are arranged with an interbox distance of 40 m along a row and 28 m across the diagonal. Of the 162 boxes in the study area, 96 were occupied by tree swallows.We checked nest-boxes every other day prior to the appearance of the first egg, at which point we visited the boxes daily until clutch completion. Twelve days postclutch completion, toward the end of the 14-day incubation period, we checked the boxes daily until hatching. Three days posthatching, we collected a small droplet (5–25 μl) of blood through brachial venipuncture from all nestlings (which we stored in lysis buffer for later genetic analyses) and measured nestling body mass (to the nearest 0.1 g using an electronic balance). Each nestling was toenail clipped for later individual identification. At this point, we also collected any unhatched eggs still present in the nest from which we sampled brain or muscle tissue. We stored tissues in alcohol for later genetic analysis. Twelve days posthatching, we banded nestlings with a Canadian Wildlife Service aluminum band, measured tarsus length (to the nearest 0.1 mm using a digital caliper), and obtained a second measurement of nestling body mass. We calculated nestling growth increment as the change in body mass from 3 to 12 days posthatching. Tree swallow nestlings reach their asymptotic body mass approximately 12–14 days after hatching (Robertson et al. 1992).
We caught adults using mist nests or nest-box traps (Stutchbury and Robertson 1986) primarily after the nestlings hatched. At capture, we banded adults with a Canadian Wildlife Service aluminum band and a color band (blue for males and red for females). We collected a small amount of blood through brachial venipuncture (which we stored in lysis buffer for later genetic analyses) and obtained measurements of body mass, tarsus length, and feather mite infestation. We estimated feather mite infestation by counting the number of mite holes present on feathers of the tail and both wings (primaries and secondaries) (Dunn et al. 1994). Feather mites are common ectoparasites of birds. We followed accepted practice in assuming that feather mites decrease the fitness of their hosts (e.g., Dunn et al. 1994; Thompson et al. 1997; Whittingham and Dunn 2000) but recognize that this assumption is open to question (e.g., Blanco et al. 2001). We also classified adult females and males as second year or after second year on the basis of plumage coloration and wing length (Rendell WB, Robertson RJ, unpublished data), respectively. We confirmed estimates from plumage coloration and wing length when possible using banding records from previous years. In each case, estimates from plumage coloration and wing length were correct.