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

Interpreting past trophic ecology of a threatened alpine parrot, kea Nestor notabilis, from museum specimens

When ecosystems are under severe pressure or environments change, trophic position and intraspecific niche width may decrease or narrow, signalling that conservation action is required. In New Zealand, alpine and subalpine ecosystems have been extensively modified through farming since 19th-century European settlement, with consequences for indigenous species such as the kea Nestor notabilis. We investigated feather stable isotope values in the kea and predicted a lower trophic position in modern kea populations, to reflect reduced lowland habitat and a mixed diet with more plant material. We predicted that size and sex would influence trophic values in this sexually dimorphic species, with larger birds more likely to have a high protein diet. We examined potential dietary changes in 68 museum collected kea from 1880s to 2000s, first recording accession details including provenance and sex and measuring culmen length. We used bulk carbon and nitrogen stable isotope analyses (BSIAs) of feathers and a further feather subset using compound-specific stable isotope analyses of amino acids (CSIA-AA) to obtain isotopic values and estimate trophic position. BSIA showed δ¹⁵ N values in kea feathers declined through time and could indicate that early century kea were highly omnivorous, with δ¹⁵ N values on average higher than in modern kea. Variance in δ¹⁵ N values was greater after 1950, driven by a few individuals. Few differences between males and females were evident, although females in the south region had lower δ¹⁵ N values. There was a tendency for large male birds to have higher trophic values, perhaps reflecting dominant male bird behaviour noted in historical records. Nonetheless, CSIA-AA performed on a subset of the data suggested that variation in BSIA is likely due to baseline changes rather than relative trophic position which may be more homogenous than these data indicate. Although there was more variability in modern kea, we suggest caution in interpretation. Stable isotope data, particularly CSIA-AA, from museum specimens can reveal potential change in ecological networks as well as sexually dimorphic feeding patterns within species. The data can reveal temporal and regional variation in species trophic position and changes in ecosystem integrity to inform conservation decision-making.

Mitogenomes resolve the phylogeography and divergence times within the endemic New Zealand Callaeidae (Aves: Passerida)

The biogeographical origins of the endemic birds of New Zealand (Aotearoa) are of great interest, particularly Palaeogene lineages such as Callaeidae, a passerine family characterized by brightly coloured wattles behind the beak and, in some cases, extreme sexual dimorphism in bill size and shape. Ancestral representatives of Callaeidae are thought to have split from their closest relatives outside New Zealand in the Oligocene, but little is known about the timing of divergences within the family. We present a fully dated molecular phylogeny of Callaeidae mitogenomes and discuss the biogeographical implications. Our results suggest that formation of Pliocene marine seaways, such as the Manawatu Strait, are likely to have played a significant role in the differentiation of North Island and South Island kōkako (Callaeas spp.) and saddlebacks/tīeke (Philesturnus spp.).

Recent evolution of extreme sexual dimorphism in the huia (Heteralocha acutirostris; Callaeidae)

The New Zealand wattlebirds (Callaeidae) are an endemic New Zealand passerine family whose species show extreme variation in bill morphology. In particular, the extinct huia (Heteralocha acutirostris) has attracted considerable attention because it exhibited extreme sexual dimorphism in bill morphology. However, the phylogenetic relationships within the Callaeidae, crucial for understanding bill evolution in the family, have not been resolved to date. Here we present phylogenies based on complete mitochondrial genome sequences and nuclear ultraconserved elements. Kōkako (Callaeas spp.) is strongly supported as sister taxon to saddleback/tīeke (Philesturnus spp.) and huia, diverging around 6.8 Ma. Saddleback and huia are estimated to have split from each other 5 Ma, indicating that the extreme sexual bill dimorphism in huia has evolved within this time frame. Our estimates for the divergences within the Callaetidae are similar to, or younger than, those of most other endemic New Zealand avian families, therefore the observed bill variation is not a consequence of a longer divergence time. Instead, the expansion of the huia into the wood-foraging niche, combined with the sexual dimorphism it evolved in order to optimise feeding on this resource, has been the main contributor to the large variation of bill morphologies within this family.