Abstract
Population collapses result from drastic environmental changes, but the sexes may differ in vulnerability. Collapse of the endangered Hawaii creeper (Oreomystis mana) at Hakalau Forest National Wildlife Refuge resulted from food limitation associated with increased numbers of an introduced bird (Japanese white-eye, Zosterops japonicus), which competes with the creeper for food. Both creeper sexes had stunted bill growth and the greatest change in molt of native species in the community. With a surge in numbers of white-eyes, a recent cohort of adult females had very low survival after breeding, while adult males from the same cohort, and older females and males, continued to have high survival. Lower female survival resulted in a significantly more male-biased adult sex ratio. Recent low female survival was based on a great cost of reproduction, indicated by molt-breeding overlap that was previously avoided, and lower fat during the lengthy fledgling period. The difference in female survival between cohorts was associated with stunted bills from being reared in and then breeding in an increasingly poor food environment. Trend analysis of survey data indicate that the bird is declining throughout the refuge, with males being 72–80% of adults left six years after the white-eye increased. Competition over time was consistent with that previously documented over space on the Island of Hawaii. Adaptive management to recover the bird in this protected area needs to focus on improving both adult female survival and the adult sex ratio.
Highlights
A population collapse can be distinguished from a long-term decline by the suddenness and rapidity at which it occurs
Food limitation associated with an environmental change could affect adult females more than adult males because of this underlying bias in survival. We explore this possibility with the endangered Hawaii creeper (Oreomystis mana), a Hawaiian honeycreeper (Drepanidinae)
All 29 birds sexed by chromodomain helicase-DNA-binding (CHD) had appropriate wing lengths, suggesting accuracy in measuring wing length and no error in molecular sexing
Summary
A population collapse can be distinguished from a long-term decline by the suddenness and rapidity at which it occurs. Collapses can be documented without regard to sex [6,7,8,9,10,11], in which case it is implicitly assumed that both sexes are declining at similar rates. In some collapses where sex has been considered, adult male mortality was slightly but significantly higher than that of females [12,13,14]. There was much higher adult female mortality [15,16]. Too few collapses have been documented with sufficient ecological detail to determine the conditions that lead to differential vulnerability of the sexes
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