Survival

Schubert (2003) found that adult chickadees at QUBS survived about two winters on average, with 25% of males and 20% of females surviving to a third winter, in general agreement with Smith's (1991) value for average lifespan of 2.5 years. In these analyses, where missing adults were assumed to have died if they were not resighted on the study site, males survived better than females (Cox regression: Wald x2 = 4.17, N = 436 females, 427 males, P = 0.04). This finding is in agreement with other studies of Parid survival (Desrochers et al. 1988; Lens and Dhondt 1993). Interestingly, there was no difference in mean life expectancy of males that attained high rank in their first year compared to those starting at low rank, nor was there a significant difference in survival to second year for males that achieved high versus low rank as yearlings. The latter result contrasts with Koivula et al. (1996) who found social rank predicted survival of juvenile but not adult willow tits.

To examine the effects of age and sex on male and female annual survivorship, Schubert (2003) first used nested, generalized linear models (GLZ) to analyze January recapture data from 1994 to 2002. Analyses were performed in MlWiN using a binomial error structure (Goldstein 1995), again assuming that birds that disappeared had died. Our models revealed no significant effects of age (as measured in years), but significant effects of age class (ASY versus SY), and sex, as well as high annual variability. Models of male survivorship which added variables of rank class (e.g. high, mid, low) or rank score (interactions won divided by all interactions) showed rank to be a marginally better predictor of male survival than age (P = 0.08). To better untangle the effects of age and rank (here categorized as high or low only), which are tightly correlated, a more targeted analysis applied multistrata, capture-mark-recapture models to resighting data from 1997 to 2002 (K. A. Schubert, C. Kraus, D. J. Mennill et al. unpublished data). Population annual survival varied considerably over the 6 years (36-73%), with high-ranked males surviving somewhat better than low-ranked males (annual survival probabilities 0.56 ± 0.09 for high-ranked males vs. 0.50 ± 0.08 for low-ranked males). Moreover, none of the well-supported models contained an age effect ( K. A. Schubert, C. Kraus, D. J. Mennill et al. unpublished data). Taken together, these analyses reveal that rank, independent of age, has an influence on annual survival in male chickadees, even in the face of environmental and other variation captured by modeling year effects.

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