Intraspecific variation in group structure arises due to environmentally-mediated directional dispersal in a cooperative breeder

Many cooperatively breeding species live in groups with complex social
structure—large group sizes, low but mixed kin structure, and multiple
breeding pairs—that form as a result of immigration of unrelated
individuals in addition to offspring delaying or foregoing dispersal.
Differences in patterns of dispersal can therefore generate variation in
social structure, even within the same species or population. Yet, the
mechanisms underlying inter-individual variation in dispersal and
intraspecific variation in social structure remain poorly studied. Here,
we examine how environmentally-mediated dispersal patterns inferred from
genetic data influence variation in social structure in the superb
starling (Lamprotornis superbus), a complex cooperative breeder that
inhabits a spatiotemporally variable savanna environment. Using
genome-wide polymorphic loci and remotely-sensed, fine-scale ecological
data, we find evidence of not only frequent and long-distance dispersal in
both sexes (low isolation-by-distance and weak genetic structure), but
also directional dispersal from small groups occupying low-quality
territories to large groups occupying high-quality territories. Moreover,
we find stronger genetic structure among groups in low-quality
territories, but higher genetic diversity and lower overall relatedness of
groups in high-quality territories. Long-distance, directional dispersal
can thus lead to smaller and simpler kin-based social groups in
low-quality territories, but larger and more complex mixed-kin groups in
high-quality territories. Thus, individual-level dispersal decisions from
lower to higher quality habitat that maximize fitness can generate
intraspecific variation in social structure, even within the same
population. Such within-population variation in social structure could
have profound implications for the mechanisms underlying the formation of
cooperative societies more broadly.