Reproductive skew in a Vulnerable bird favors breeders that monopolize nest cavities

Reproductive skew occurs when a few individuals monopolize breeding output, which can act as a mechanism of natural selection. However, when population sizes become small, reproductive skew can depress effective population size and worsen inbreeding. Identifying the cause of reproductive skew is important for mitigating its effect on conservation of small populations. We hypothesized that superb parrots Polytelis swainsonii, which strongly select for the morphology of tree cavity nests, may be reproductively skewed toward pairs that monopolize access to nests. We use SNP genotyping to reconstruct a pedigree, estimate molecular relatedness and genetic diversity of wild superb parrot in the Australian Capital Territory. We successfully genotyped 181 nestlings (a census between 2015-2019) and showed they were the progeny of 34 monogamous breeding pairs. There was a strong reproductive skew - 21 pairs bred only once producing 40% of the nestlings, whereas 13 pairs bred two to four times, producing 60% of the total nestlings. Five of these repeat-breeders produced 28% of all nestlings, which was nearly triple the productivity of one-time breeders. Repeat breeders usually monopolized access to their nest cavities, but the few pairs that switched nests did not differ in fecundity from those that stayed. The cause of nest switching was unknown, but uninterrupted access to a suitable nest (not minor variations in morphology between nests) better predicted fitness of breeding superb parrots. Pedigrees offer powerful insights into demographic processes, and identifying reproductive skew early provides opportunities to proactively avoid irreversible loss of genetic diversity via conservation management. We identify new research questions based on our results to clarify the relationship between access to resources and breeding success.