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

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 (δ(15)N(bulk)) and (δ(13)C(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 (δ(15)N(AA)) in male–female pairs to estimate their trophic position. Sexes had significantly different, but overlapping feather δ(15)N(bulk) and δ(13)C(bulk) values, but δ(15)N(AA) indicated identical trophic positions and δ(15)N(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. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00442-021-05082-8.