Functional significance of nest size variation in the Rufous Bush Robin Cercotrichas galactotes

Avian nest building behaviour has generally been viewed as a result of natu ral selection, and several functional hypotheses have been suggested to exp lain variance in nest size. These include responses to predation and parasi tism risk, and aspects of nest stability, clutch size, insulation and sexua l display. We studied variation in nest size in the Rufous Bush Robin Cerco trichas galactotes, a songbird in which both sexes contribute in nest build ing. Nest size was marginally negatively associated with the probability of brood parasitism, but no relationship was found with nest predation. Four additional hypotheses of the function of nest size variance were considered . The nest support hypothesis, the clutch size hypothesis and the thermoreg ulation hypothesis did not explain the nest size variation found, but the s exual display hypothesis was partially supported because clutch size was po sitively correlated with the amount of nest material, and this nest feature also tended to be positively correlated with size of prey provided to chic ks by males. Nest building behaviour in the Rufous Bush Robin can be consid ered a post-mating sexual display that reflects the willingness of males to invest in parental care and allows females to adjusts their reproductive e ffort accordingly.