title: Brood size constrains the development of endothermy in blue tits.
abstract: Altricial birds are unable to maintain body temperature when exposed to low ambient temperatures during the first days after hatching. Thermoregulatory capacity begins to form as postnatal development progresses, and eventually nestlings become homeothermic. Several factors may influence this development at both the level of the individual and the level of the whole brood, but to our knowledge no studies have focused on the effect of brood size per se on the development of endothermy in individual nestlings. We performed cooling experiments on blue tit (Cyanistes caeruleus) nestlings in the field, to study how different experimental brood sizes affected the development of endothermy in individual nestlings and the thermal environment experienced by the whole brood in the nest. Nestlings from all experimental brood sizes showed a decrease in cooling rate as they grew older, but birds from reduced broods showed an earlier onset of endothermy compared with nestlings from enlarged and control broods. This difference manifested during early development and gradually disappeared as nestlings grew older. The thermal environment in the nests differed between treatments during nestling development, such that nest temperature in reduced broods was lower than that in enlarged broods during all days and during nights at the end of the experimental period. We suggest that the development of endothermy in blue tit nestlings is not ontogenetically fixed, but instead may vary according to differences in developmental, nutritional and thermal conditions as determined by brood size.
authors.affiliation: Department of Biology, Section for Evolutionary Ecology, Lund University, Ecology Building, Lund S-223 62, Sweden fredrik.andreasson@biol.lu.se.
authors.forename: Fredrik
authors.identifier: 0000-0001-7631-4258
authors.initials: F
authors.lastname: Andreasson
authors.affiliation: Department of Biology, Section for Evolutionary Ecology, Lund University, Ecology Building, Lund S-223 62, Sweden.
authors.forename: Andreas
authors.initials: A
authors.lastname: Nord
authors.affiliation: Department of Biology, Section for Evolutionary Ecology, Lund University, Ecology Building, Lund S-223 62, Sweden.
authors.forename: Jan-Åke
authors.initials: JÅ
authors.lastname: Nilsson
mesh_terms: D000818:Animals; D001835:Body Weight; D003080:Cold Temperature; D016014:Linear Models; D009425:Nesting Behavior / Q000502:physiology*; D046109:Passeriformes / Q000254:growth & development / Q000502:physiology*
keywords: Cyanistes caeruleus; Homeothermy; Thermal imaging; Thermoregulation; Thermoregulatory capacity