Individual variation explains ageing patterns in a cooperatively breeding bird, the long-tailed tit Aegithalos caudatus

Abstract

Alloparental care in cooperatively breeding species may alter breeder age-specific survival and reproduction and subsequently senescence. The helping behaviour itself might also undergo age-related change, and decisions to help in facultative cooperative breeders are likely to be affected by individual condition. Helpers in long-tailed tits Aegithalos caudatus assist relatives after failing to raise their own brood, with offspring from helped nests being more likely to recruit into the breeding population. Using data collected over 25 years, we examined the age trajectories of survival and reproduction in adult long-tailed tits to determine how these were affected by the presence or absence of helpers and how helper behaviour changed with age. There was evidence for increased reproductive performance with breeder age, but no effect of age on the probability of survival. We found no evidence of significant senescent decline in survival or reproductive performance, although individuals accrued less inclusive fitness in their last year of life. Lifetime reproductive success was positively related to both reproductive life span and body mass. Within a season, breeders that were assisted by helpers enjoyed greater reproductive success through enhanced offspring recruitment in the following year. We found no evidence that age affected an individual's propensity to help, or the amount of indirect fitness accrued through helping. We found a positive correlation between life span and multiple components of reproductive success, suggesting that individual variation in quality underpins age-related variation in fitness in this species. Helping decisions are driven by condition, and lifetime inclusive fitness of immigrants was predicted by body mass. These findings further support individual heterogeneity in quality being a major driver for fitness gains across the life course of long-tailed tits.

Keywords: ageing; cooperative breeding; demography; helping; inclusive fitness; life history; senescence.

FIGURE 1

The life cycle of long‐tailed tits Aegithalos caudatus and key demographic events. Numbers indicate demographic events we investigated, the models for which are displayed in Table 1. In year n, adult long‐tailed tits produce clutches of around 8–12 eggs (1) in socially monogamous pairs which develop via a nestling and fledgling (2) stage ultimately to recruit into the population in year n + 1 (3). Adults may also survive (4) to year n + 1, with a mean life expectancy of 1.67 years. At the nestling stage, breeders may receive assistance rearing their brood from individuals in the population who have failed to breed—redirected helpers (5). Helpers tend to be related individuals, whose effect is not to increase the number of fledglings at a nest (6), but instead the probability a given fledgling recruits into the population (7). Helpers have been shown to have limited effects on breeder survival (8), while their effects on breeder fitness in year n + 1, that is production of recruits in year n + 2 (9), is unknown.

FIGURE 2

Age patterns of reproduction. Mean ± SE (a) clutch sizes of females, (b) number of fledglings, (c) direct fitness and (d) indirect fitness in relation to individual age. Results are presented both for when individuals aged 4 and older are grouped as 4+ (blue), and as the raw means for age classes 4 to 7/8 (red). Sample sizes for each age class are displayed above each data point.

FIGURE 3

Inclusive fitness in an individual's last breeding season. Mean ± SE inclusive fitness for individuals in breeding seasons throughout their life span (No) compared to their final breeding season (Yes).

FIGURE 4

Age patterns of survival. (a) Mean ± SE probability of adult survival from one breeding season to the next with respect to age. (b) The predicted probability ±SE of survival to the next breeding season as a function of an individual's direct fitness in the current breeding season.