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

Abstract

Reproductive skew occurs when a few individuals monopolize breeding output,which can act as a mechanism of natural selection. However, when populationsizes become small, reproductive skew can depress effective population size andworsen inbreeding. Identifying the cause of reproductive skew is important for mit-igating its effect on conservation of small populations. We hypothesized that superbparrots Polytelis swainsonii, which strongly select for the morphology of tree cav-ity nests, may be reproductively skewed toward pairs that monopolize access tonests. We use SNP genotyping to reconstruct a pedigree, estimate molecular relat-edness and genetic diversity of wild superb parrot in the Australian Capital Terri-tory. We successfully genotyped 181 nestlings (a census between 2015–2019) andshowed they were the progeny of 34 monogamous breeding pairs. There was astrong reproductive skew – 21 pairs bred only once producing 40% of the nest-lings, whereas 13 pairs bred two to four times, producing 60% of the total nest-lings. Five of these repeat-breeders produced 28% of all nestlings, which wasnearly triple the productivity of one-time breeders. Repeat breeders usually monop-olized access to their nest cavities, but the few pairs that switched nests did notdiffer in fecundity from those that stayed. The cause of nest switching wasunknown, but uninterrupted access to a suitable nest (not minor variations in mor-phology between nests) better predicted fitness of breeding superb parrots. Pedi-grees offer powerful insights into demographic processes, and identifyingreproductive skew early provides opportunities to proactively avoid irreversible lossof genetic diversity via conservation management. We identify new research ques-tions based on our results to clarify the relationship between access to resourcesand breeding success.