Invertebrate food supply and reproductive success of two native forest passerines along an elevational gradient

Predation by mammals has been identified as the primary limiting factor of Aotearoa New Zealand native birds. Consequently, the ranges of many native forest bird species have contracted to cooler and higher elevation tracts of forest that support fewer introduced mammals. However, lower elevation forests are likely to be intrinsically more productive and able to sustain larger bird populations if control of mammalian pests removes predation as a primary limiting factor. We wanted to determine whether higher elevation forests provide less food for rat-sensitive, sedentary native insectivorous bird species, resulting in their reduced reproductive potential at higher elevations. In spring and summer 2020/21, we sampled invertebrate prey while simultaneously monitoring nest survival and number of fledglings produced by tītitipounamu / rifleman (Acanthisitta chloris) and miromiro / tomtit (Petroica macrocephala) across three elevational bands on Mount Pirongia, where mammal predators were suppressed. Tracking and camera indices together indicated that introduced mammals were at low relative abundance at all elevations, allowing us to investigate other habitat effects. Biomass of ground-dwelling invertebrates decreased with increasing elevation during the bird breeding season, but was similar across elevational bands later in the season. In contrast, biomass of flying or folivore invertebrates was independent of elevation and only showed clear seasonal trends. In both tītitipounamu and miromiro, nest survival rates marginally decreased with increasing available invertebrate prey, while the number of fledglings of successful nests marginally increased. There was no strong relationship between elevation and nest survival or number of fledglings. These results indicate no clear trend in the reproductive potential of tītitipounamu and miromiro with elevation but do suggest that food supply needs to be considered as an aspect of habitat quality that is related to reproductive success.