Nest site selection by the endangered black robin increases vulnerability to predation by an invasive bird

Few studies have investigated the impacts of established invasive birds on the native threatened avifauna. However, measuring the levels of mortality in native birdsfrom exotic birds can help with management decisions on whether or not such introduced species need to be controlled. The black robin Petroica traversi is an endangered bird endemic to the Chatham Islands of New Zealand. Black robins build cup-like nests either within tree cavities ('cavity' nests) or in the sub-canopyvegetation ('open' nests). Nest predation, most likely by the invasive European starling Sturnus vulgaris, was the largest cause of nest failure in the main population of the black robin on Rangatira Island. Nest predation rate over five breeding seasons was 20.6% in comparison with 7.74% of nests failing because of severe weather events (the second largest cause for nest failure). Cavity nests experienced a significantly higher rate of predation (36.33%) than open nests (10.82%). The greater frequency of open nests (63.5% out of 244 nests monitored) may be a result of higher selection pressure by starlings on cavity nests. Nest height also influenced predation, with predation risk increasing from 4.88% for nests below 1 mto 31.89%for nests above 3 m. Overall, predation on black robin nests decreased chick production in the population by 15.6% annually. For a population of only 220'240birds, this loss may be limiting continued population growth and we recommend that efforts be made to reduce predation through either the culling of starlings or the installation of nest boxes at the low heights avoided by starlings.