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We explored two hypotheses that may explain intraspecific variability in nest size. The “thermoregulatory” hypothesis states that species adjust nest size to maintain egg temperature and minimize temperature fluctuations in the nest. Recently, the suggestion has been made that nest size may reflect the health status or phenotypic quality of the builder, potentially making it a sexually selected trait (“sexual selection” hypothesis). For two years, we weighed nests of Blue Tit (Cyanistes caeruleus) at initiation of reproduction in a nest-box breeding population in central Spain. We recorded laying date, clutch size, incubation period, and hatching success. We measured and took blood samples of adult females when nestlings reached three days of age. General regression models controlling for potential variables that could affect nest weight revealed that prevalence of Trypanosoma avium and immunoglobulin levels in females were significantly related to nest weight in only one of the study years. Females not infected with Trypanosoma avium built heavier nests than infected ones, whereas female immunoglobulin levels were negatively associated with nest weight. Hatching success and duration of the incubation period were not related to nest weight in either year. Thus, our results do not support the thermoregulatory hypothesis and show that certain measures of female health are related to nest-building effort in some years, probably depending on environmental conditions.  Peso del Nido y Estado de Salud de la Hembra en el Cyanistes caeruleus

Nest Weight and Female Health in the Blue Tit (Cyanistes Caeruleus)