Long-term changes in the breeding biology of a New Zealand bellbird population suggest plasticity in life-history responses to ecological restoration

Ecological restoration projects provide excellent opportunities to study how animals adapt their life-history strategies in response to changeable environments. A fundamental way animals can optimise reproductive success in changing conditions is trading-off aspects of their breeding system. The New Zealand bellbird (Anthornis melanura) has had a long-term presence on the small restoration island, Tiritiri Matangi Island (Tiri), spanning the islands degraded agricultural past to its current extensively restored state. We studied the breeding biology of this bellbird population to assess how their reproductive life-history strategies have responded over time to the restoration on Tiri. We compared the current breeding data (2012-2016) of the bellbirds with data from between 2001-2010 (including Baillie, 2011, Cope, 2007), and from 1977-1978 (Anderson and Craig, 2003), prior to the islands restoration. We also explored associations between abiotic/biotic factors and bellbird reproductive success for the most recent period (2012-2016). Our main finding was that clutch size significantly declined over time from a mean of 3.6 to 2.4 eggs per nest and this decline correlated with increasing population density. This is consistent with a density dependent effect, although further data are required to empirically test this conclusion. Overall, the earliest spring laying dates were in late August and the latest extended to January, with all chicks fledged by the end of February. Nest success was 47% (range 40 - 54%) across 2012-2016, falling within a similar range as previous studies. We found little effect of year, weather, parental age or morphometrics on reproductive success. We observed directional change in patterns of parental investment between 1977-1978 and 2012-2016; in 2012-2016, parents persisted with raising single broods rather than abandoning and re-nesting to raise larger broods. These results suggest that the bellbirds life-history traits are plastic in response to local conditions which provides an advantage when repopulating a regenerating or changing habitat.