Current male-biased sex ratios in tara iti (Sternula nereis davisae): compounding survival differences and stochastic events

Sex ratio bias can strongly influence the viability of a small population. In tara iti (New Zealand fairy tern, Sternula nereis davisae), long-term monitoring over 25 years revealed a shift from roughly balanced sex ratios to a pronounced male bias among adults. This skew does not originate at hatching, as chick sex ratios fluctuate around 1:1. Skewed sex ratios develop after fledging through sex-specific survival differences and stochastic events. Random losses, caused by storms or predation, disproportionately affected females. This severely impacted the effective population size of tara iti, where each individual represented a large proportion of the potential breeding pool. These findings highlight demographic risks that should inform conservation planning and recovery strategies.