Hatching Spread in a Cooperative Breeding Bird, the Pūkeko (Porphyrio melanotus melanotus)

Abstract

In birds, the onset incubation determines the hatch spread. Many studies have explored the role of hatch spreads in solitary breeding species, but there is little research on cooperative breeding species. Joint-laying is a rare form of cooperative breeding in which more than one female lay eggs in the same communal nest, and the group collectively cares for the clutch. This system addresses new questions about hatch spreads since joint nests are larger, contain eggs from multiple females, and have more caretakers than single female nests. Here, we present our research on hatch spreads in joint-laying pūkeko (Porphyrio melanotus melanotus). In our first study, we used an eleven-year data set to describe patterns and interactions between clutch size, hatch spread, and hatch order on hatching success and survival. Larger clutch sizes and longer hatch spreads increased the predicted number of fledglings. Lay order was strongly correlated with hatching order and incubation typically started after five eggs were laid. Chicks from earlier hatching eggs had greater rates of survival but the effect of hatch order on survival decreased in synchronous hatching nests. We suggest further exploration on roles of hatch spread on lifetime inclusive fitness. For our second study, we transferred eggs between nests to create artificially synchronous and asynchronous hatching treatment nests. Synchronous nests showed a trend of greater hatching success although this was not statistically significant. There was also no significant difference in offspring survival between treatments. While our results were largely inconclusive, we make recommendations for how to improve future experimental studies on hatching spread.