Ecological and demographic drivers of kin-directed cooperation in a social bird: Insights from a long-term study

The evolution of sociality is one of the major evolutionary transitions in
the history of life and a key step in this transition is the occurrence of
kin associations. Yet, the question of what demographic processes and
environmental factors generate kin-structured populations and drive
kin-directed cooperation remains open. In this review, we synthesize 30
years of studies of the long-tailed tit Aegithalos caudatus,
which has a kin-selected cooperative breeding system with redirected help:
failed breeders may help to raise offspring of conspecifics, typically
relatives, breeding nearby. We describe the use of ecological,
demographic, genetic and behavioural approaches to reveal: (a) how
kin-structured populations (here ‘kin neighbourhoods’) arise; (b) why the
prevalence of cooperation varies among populations and individuals; and
(c) how variation in dispersal and opportunities for cooperation influence
individual fitness. The kin neighbourhoods of long-tailed tits arise from
three processes. First, natal dispersal is limited and sex-biased so many
individuals, especially males, recruit as breeders close to their natal
site. Second, neither dispersal nor migration necessarily disrupts kin
associations because long-tailed tits often move with close relatives.
Third, a small effective population size driven by high nest predation
rates enhances within-population relatedness. Together, these processes
set the scene for kin-directed helping behaviour by causing spatial
clustering of relatives. The prevalence of cooperation within kin
neighbourhoods depends on several factors, both at the population-level
(annual nest predation rate and length of the breeding season) and
individual-level (relatedness, familiarity, sex and condition). However,
limited information on prior social association and the reliability of kin
discrimination cues hampers our current understanding of individual
helping decisions.  Finally, variation in dispersal within and
between sexes affects the probability of interacting with kin, the
likelihood of cooperation, and accrual of the direct and indirect
components of inclusive fitness. We use this comprehensive understanding
of the factors driving cooperative behaviour in long-tailed tits to
highlight gaps in knowledge and suggest future avenues for research in
this system, and to make general inferences about the role of dispersal,
demography and kinship in social evolution.