Extreme bill dimorphism leads to different but overlapping isotopic niches and similar trophic positions in sexes of the charismatic extinct huia

Abstract

The New Zealand huia (Heteralocha acutirostris) had the most extreme bill sexual dimorphism among modern birds. Given the quick extinction of the species, the cause of the dimorphism could only be hypothesised to reflect different trophic niches and reduce male/female competition. We tested that hypothesis by combining museum specimens, geometric morphometrics, and isotopic analyses. We used geometric morphometrics to describe bill shape; measured bulk (δ¹⁵N_bulk) and (δ¹³C_bulk) values from feather as proxies of the birds' foraging habitat and diet; and compared compound-specific stable isotopes analyses (CSIA) of nitrogen in amino acids (δ¹⁵N_AA) in male-female pairs to estimate their trophic position. Sexes had significantly different, but overlapping feather δ¹⁵N_bulk and δ¹³C_bulk values, but δ¹⁵N_AA indicated identical trophic positions and δ¹⁵N_bulk was not related to bill shape. Trophic position was less variable among females, consistent with a specialised foraging behaviour and, thus, supporting a partial male/female foraging segregation.

Keywords: Compound-specific stable isotopes; Feeding ecology; Heteralocha acutirostris; Natural history collections; New Zealand; Passeriformes.