Can island specialists succeed as urban pioneers? Pied imperial-pigeons provide a case study

Context Long-term viability of wildlife populations may be influenced by the adaptive or maladaptive nature of behavioural shifts. Yet, in the short term, implications of novel behaviour are often uncertain, as they were for a newly formed urban nesting colony of pied imperial-pigeons (PIPs) on the mainland coast of north-eastern Australia. It represented unprecedented behaviour, as most of PIPs, also known as Torresian imperial-pigeons, Ducula bicolor/spilorrhoa, breed colonially on remote small islands. Aims The present study would (1) determine whether aggregated mainland nesting continued, (2) evaluate reproductive success, (3) evaluate spatial distribution of nests and (4) explore possible association of reproductive success with predator presence and broad indicators of food availability. Methods With assistance from volunteers, we found mainland PIP nests and revisited them intermittently to monitor progress. We calculated quantitative estimates of nest survival by the Mayfield method and evaluated reproductive output in relation to environmental data from independent sources. Key results During the 2012, 2013 and 2014 breeding seasons, we recorded 436, 387 and 417 PIP nest events at the new mainland colony. Daily nest-survival rates declined progressively and estimated fledgling output decreased over successive seasons from 0.66 to 0.44 per nest event. The highest mainland output was below that estimated for an island PIP colony (0.78) on the basis of sparse prior data. Across potential foraging grounds, there was no negative change in land use and no widespread adverse weather to account for diminishing success. We identified important causes of nest failure among mainland nesting PIPs to be predation, predominantly by birds, and anthropogenic hazards, including tree pruning and collisions with vehicles and windows of buildings. Conclusions With ongoing exposure to these hazards, mainland nesting PIPs cannot be expected to increase productivity; hence, the new colony may be a short-term phenomenon. We infer that the historic success of PIP populations in Queensland stems from their selection of breeding sites on remote islands that are largely free of relevant predators and anthropogenic activity. Implications Future conservation of extant PIP abundance will depend crucially on protection of island breeding sites, because multiple hazards of mainland nesting make it an unfavourable alternative strategy.