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Songbird incubation dilemmas in the alpine: managing parent-offspring trade-offs in a variable thermal environment MacDonald, Elizabeth Catherine


Small-bodied bird species exhibiting single-sex incubation must expend energy to create a buffered thermal environment for their eggs, while also meeting their own energetic requirements. The resultant trade-off between incubation and foraging is intensified in cold environments like the alpine, where energetic constraints are high. Ambient temperature influences incubation behaviour, with variable relationships across species, habitats and populations. I examined incubation rhythms of an alpine population of Horned Larks (Eremophila alpestris) in British Columbia across four years with different thermal regimes (2005 = moderate, 2006 = warm, 2010 = cold overnight, 2011 = cold during day) to determine whether incubating larks exhibited variable relationships between attentivity and temperature under different thermal conditions and how this related to management of the parent-offspring trade-off. Early in the morning, females had to leave their nest to forage to reduce their energy deficit following 7 h of night incubation in near freezing conditions. Since temperatures at this time were still < 5˚C, embryos were at a high risk of potentially lethal chilling when females left the nest. From 06:00 to 08:00, incubating larks reduced attentiveness to 75% from 94% overnight. However, during this time period, females in 2010 spent more time off the nest as temperatures warmed than in other years. Throughout the study, adults took occasional “extended recesses” (incubation breaks lasting ≥ 59 min). These recesses were longer and more frequent in cold years, particularly 2011, and appeared to be largely related to harsh weather events. There are potential fitness consequences associated with these extended recesses, as egg hatching rate dropped to 81% in 2011 from 94% in the other years, and the incubation period was lengthened in two nests that engaged in multiple extended recesses. Overall, this work demonstrated that incubating larks favour themselves when coping with difficult thermal conditions. Lark embryos appeared more tolerant to long periods of considerable cooling than previously thought, although, the reduced hatching rate in one cold year suggested there may be a limit to their cold tolerance.

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