Dominance relationships structure many animal societies, yet the process of rank attainment is poorly understood. We investigated acquisition of social dominance in winter flocks and its fitness consequences in male black-capped chickadees (Poecile atricapillus) over a 10-year period. Age was the best predictor of rank, and paired comparisons showed high-ranked males to be older than their low-ranked flock-mates. When controlling for age, morphological variables did not predict male social rank, but high-ranked males were heavier, had lower fat scores and were in leaner condition than low-ranked males. Males that survived between years tended to increase in rank over time; however, the rate of rank advancement varied individually. Rank reversals between familiar contestants were rare, and changes in male social rank were associated with changes in flock membership. Average lifetime reproductive success (LRS) of males and females was variable and best predicted by lifespan. Male rank history also influenced realized reproductive success. Birds with higher average rank over their lifespan were more likely to reproduce successfully. However, among successful birds, average rank did not significantly predict LRS. Thus, birds that lived longer and attained high social rank earlier had higher fitness, but this effect was not manifested as fine-scale differences among successful individuals. Taken together, these findings demonstrate the importance of social factors influencing individual fitness.
WINTER FIELD METHODS
From January until March of 1992–2002, birds were attracted to feeding stations baited with sunflower seeds. Using Potter traps, we captured between 86 and 198 individuals per year (mean ± SEM = 146.8 ± 12.5). Each individual was banded with a Canadian Wildlife Service number band and one to three plastic colour bands in unique combinations.
We measured mass, tarsus, wing, tail, and fat score, and drew approximately 10–80 µL of blood for molecular analyses from captured individuals. We calculated a condition index as the ratio of body mass to tarsus length for each individual. We used rectrix shape and wear to assign birds to second-year (SY) or after-second-year (ASY) age classes (Pyle et al., 1987). Birds first banded as adults were conservatively assumed to be in their second year of life. The sexes of birds were determined using a discriminant function including body mass, wing, and tail lengths (Desrochers, 1989). Sexes were later confirmed from breeding behaviour.
A total of 155 flocks (identified as groups of birds consistently observed together and which associated together in the absence of other birds) were monitored from 1994 to 2002 (mean = 17.2 ± 2.1 flocks year−1; flock memberships were not recorded prior to 1994). Dominance interactions were quantified by scoring the outcomes of the following competitive interactions at winter feeding stations: supplants, chases, resistance to supplants, displays of submissive postures, and feeding while an opponent waited to approach (Ficken et al., 1990; Otter et al., 1998).
SPRING FIELD METHODS
Breeding data were collected from 1992 to 2002, with a total of 536 pairings observed (mean = 60 ± 8.2 pairs year−1, including mate-switches within years). Many individuals reproduced in several years and, accordingly, breeding season data include observations for a total of 294 individual males and 282 females. Pairs were monitored beginning at flock break-up (typically mid-March to late April) to determine territory boundaries and nest-sites. Pairing status was assessed based on affiliative behaviours, including cavity excavation, nuptial feeding, copulation, mate guarding, and territorial defense (Otter & Ratcliffe, 1996). Disappearances or changes in pairing status were recorded. Birds remaining unpaired throughout the breeding season were assumed not to have reproduced (see section below: ‘Structure of long-term data’). Nests were identified from excavation behaviour and confirmed by the lining of cavities, egg-laying, and/or incubation. The onset of egg-laying was apparent when females gave a sex-specific vocalization made at the onset of fertility (the ‘broken-dee’ call; Smith, 1991; Mennill et al., 2004). At approximately day 6 posthatch, 10–50 µL of blood were collected from the tarsal vein of nestlings for paternity analysis.