The rate at which parent birds deliver food to their young may depend on several factors, both intrinsic (e.g., brood demand) and extrinsic (e.g., time of day). Using PIT tags, we determined patterns of nestling provisioning in the Blue Tit (Cyanistes caeruleus) and the proximate factors affecting them. Over 3 years, we monitored the rates of provisioning by Blue Tits in two types of forest (evergreen and deciduous) in Cabañeros National Park (Spain). Pairs breeding in the deciduous forest visited the nest at rates higher than did those in the evergreen forest. Contrary to expectation, males fed their offspring more than females did, and the rates of feeding were positively related to those of the social partner. Sex roles in provisioning were consistent through the day, suggesting that parents did not work in concert. Provisioning rates also tended to decline through the day and were negatively correlated with ambient temperature. In females, but not in males, the adult's body condition was negatively related to provisioning effort. There was no relationship between nestlings' condition and parental provisioning rates. We found a strong correlation between hourly feeding rates and the total number of feedings registered over a 24-hr period, which indicates that short-term samples are representative of what occurs over a longer time. Overall, our study represents a significant contribution to the understanding of proximate factors influencing parental provisioning strategies in altricial birds with biparental care.
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Vol. 114 • No. 3