eWestern Birds

The Quarterly Journal of Western Field Ornithologists

Vol. 54, No. 2
May 2023
Western Field Ornithologists

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The Potential Identification and Distribution of Ainley’s Storm-Petrel at Sea by Timing of Molt
Desmond E. Sieburth, Ryan Terrill, and Peter Pyle

ABSTRACT: In 2016, the American Ornithologists’ Union split the Leach’s Storm-Petrel into three species: Leach’s Storm-Petrel (Hydrobates leucorhous), Townsend’s Storm-Petrel (H. soccoroensis), and Ainley’s Storm-Petrel (H. cheimomnestes). Leach’s breeds around the Northern Hemisphere during the boreal summer, while both Townsend’s and Ainley’s breed on islets off Guadalupe Island, Mexico, the former in summer and the latter in winter. Although morphological differences between Leach’s and Ainley’s are slight at best, we hypothesized that the difference in breeding schedule may result in a difference between the species in molt schedule, allowing identification of some birds at sea. We examined 528 specimens and hundreds of photographs for molt of primaries, aging each bird by its having juvenile vs. basic flight feathers and the presence of molt clines, and scoring molt by the number of primaries replaced. Using threshold models, we identified ten birds whose timing of molt suggested Ainley’s, most occurring off central-to-southern Mexico. In Leach’s and/or Townsend’s storm-petrels, primaries were being replaced in the second prebasic molt from 21 February (p2) to 15 December (p8) and in the definitive prebasic molt from 24 October (p3) to 22 February (p8). In the smaller sample of Ainley’s, these dates were 23 November (p6) to 22 February (p8) and 11 June (p3) to 7 November (p8), respectively. Thus the timing of molt may help identify Ainley’s at sea, important for the management of this vulnerable species. Genetic analysis of the specimens may confirm our identifications and the applicability of our technique.

Download—The Potential Identification and Distribution of Ainley’s Storm-Petrel at Sea by Timing of Molt