- University of Groningens
- University of Bielefeld
Summary
In dominance-structured animal societies, variation in individual fitness is often related to social status. Like many passerine birds, Black-capped Chickadees (Poecile atricapillus) have a short average adult life-expectancy (∼2 years); however, the maximum recorded life span is >5× as long. Enhanced annual survival could contribute to greater lifetime reproductive success for male Black-capped Chickadees with high social rank. We used multistate capture–mark–recapture models to estimate annual survival of male Black-capped Chickadees in Ontario using resighting and recapture data collected from 1997 to 2002. Our goal was to evaluate support for an influence of rank on annual survival and estimate its effect size for a food-supplemented study site. We also statistically modeled the probability of between-year rank transitions. Model selection based on Akaike's information criterion provided support for an effect of rank on survival. However, multimodel inference revealed that the size of the effect was rather small. Over the six study years, model-averaged estimates of the survival benefit of high versus low rank ranged from 5.0 to 7.3%. As expected, survival was strongly year-dependent, with model-averaged estimates of annual survival probability varying between 0.36 and 0.73. Age was an important predictor of the probability of rank transitions. Low-ranked second-year birds were less likely than older low-ranked birds to advance to high rank between years; likewise, high-ranked after-second-year birds were less likely to drop in rank. Other studies have found larger effects of rank on survival than we observed here. Future research should consider how interactions between social and environmental factors influence annual survival.
Methodology
Study area and population.—Chickadees at the Queen's University Biological Station near Chaffey's Lock, Ontario (44°34′N, 76°19′W), have been studied since 1988. Our analyses included individual records from 1997–2002, because data from earlier years were collected using a much smaller study area. The 2-km2 field site comprised a forested peninsula bordered by a lake, as well as two small islands <200 m from shore. Habitat was mainly mature second-growth forest (Acer spp., Quercus spp., Pinus spp., Betula papyrifera) interspersed with old fields and small streams. Winter temperatures ranged from –40 to 5°C, averaging around –5°C throughout January and February; the ground was typically snow-covered from December through March.
Field methods.—From January through March each year, birds were attracted to between 11 (in 1997) and 15 (in 2002) feeding stations baited with black oil sunflower seeds. Feeders were dispersed across the entire study site, which gradually increased in area over the six study years (as in Koivula et al. 1996). Feeders were always filled, so birds had constant access to food at these sites. In January and February, we caught all birds at the feeding stations using Potter's traps. Individuals were marked with numbered aluminum Canadian Wildlife Service bands (Environment Canada banding sub-permits 10302 AK/AP/AW/BE) and one to three plastic color bands in unique combination (there is no evidence of color-band loss in chickadees). Between 57 and 95 males were captured each year (mean = 74.3 ± 6.2 [SE]), of which 59% were unbanded at the time. We used rectrix shape and wear to distinguish unbanded birds that had fledged the previous season and were entering their second year of life (SY) from older birds (after-second-year, ASY); ∼20% of unbanded birds caught annually were classified as ASY. Technically, birds in both age classes were mature adults by the time of trapping. Newly banded birds were sexed with a discriminant function analysis of body mass, wing, and tail length (Desrochers 1989), and sexes were later confirmed from breeding behavior, by molecular methods, or both (∼50% of males were sexed by molecular methods) (Ramsay et al. 2003). Analyses presented here deal exclusively with male chickadees, because the nature of social relationships in female chickadees is poorly understood (but see Smith 1991, Ramsay and Ratcliffe 2003).
Rank-determination and resighting data.—Around February of each year, when the catching period was over, feeding stations were replaced with dominance boards (0.5-m-diameter circles with a food hopper on one end). We scored dominance relationships between dyads based on competitive interactions for seeds. Males were identified as dominant in a single interaction when they (1) supplanted or chased an opponent, (2) successfully resisted a supplanting attack, (3) elicited a submissive posture in another individual, or (4) fed while an opponent waited to approach (Otter et al. 1998). Interactions at feeders were representative of relationships between flock-mates at other sites during winter (Smith 1991, D. J. Mennill unpubl. data). The period during which dominance data were collected was later used as the resighting occasion in our models (see below).