Partial Incubation in Birds: Its Occurrence, Function, and Quantification

Abstract

Early studies of incubation behavior acknowledged the existence of a less regular form of incubation that can occur from the beginning of egg laying to shortly after clutch completion. It has been variously termed “partial incubation” (Putnam , Seel , Ashkenazie and Safriel ), “brooding” (Barth ), “intermittent incubation” (Samuel ), “irregular incubation” (Beer ), or “nonrhythmic incubation” (Morton et al. ). This form of incubation has been described from visual observations of adults sitting on the nest less regularly, and from egg temperatures that are lower and more variable than those typically attained after clutch completion (Barth , Brackbill , Kendeigh , Barrett , Morton and Pereyra ). We refer to this little-understood behavior as “partial incubation” because it is characterized by a reduced amount of time during egg laying that parents sit on and warm their eggs, compared with the amount and intensity of incubation they provide later in the nesting cycle after the clutch is complete. Partial incubation is a common way for birds to initiate incubation. The onset of incubation, the moment at which a bird begins to incubate a clutch, is a critical event in the avian nesting cycle because it has important effects on fitness, as noted more than half a century ago by David Lack (). Incubation onset can affect hatching success and hatching patterns (Nilsson and Svensson , Wang and Beissinger ), nestling growth and development (Slagsvold et al. , Bitton et al. ), fledging success (Hebert , Hebert and McNeil ), and postfledging survival and recruitment (Cam et al. ). Not only can the onset of incubation affect adult reproductive rates, but it also influences and is influenced by parental body condition and survival (Hanssen et al. ). Yet the onset of incubation is rarely described and even less commonly quantified, and methods of determining incubation onset have not been standardized among studies or species. In this review, we () clarify terminology for the onset of incubation, refine the concept of partial incubation, and summarize the prevalence of partial incubation. We then () examine the potential functions of partial incubation, () map individual-level patterns of partial incubation from previous studies onto a recently developed typology of patterns for the onset of incubation (Wang and Beissinger ), and examine intraspecific variability and compare interspecific data with respect to taxonomic and ecological diversity. We also () provide a standardized method for determining the individual onset of full incubation, using complete records of nest attendance or incubation from the start of laying to beyond clutch completion. Our goal is to show how data from disparate studies can be used to test hypotheses about the function of partial incubation. Finally, we () offer questions that may prove to be fertile areas of research and suggest ways to standardize future data collection and analysis to benefit common research goals. TERMINOLOGY FOR THE ONSET OF INCUBATION