Nest characteristics and breeding performance in great tits Parus major.

For birds, building a nest entails a number of costs but it confers important benefits, so that nest quality is often positively associated with reproductive performance. Our main objective was to study the relationships between nest characteristics and breeding performance, and the possible between-year variability in these relationships. We used great tits Parus major as the model species and studied a population breeding in nestboxes within orange plantations in eastern Spain. The study only considered first clutches, in two consecutive years. Nests were visited as frequently as necessary to record basic breeding parameters. We measured and weighed all nests during the incubation period, took biometric characteristics of parents and nestlings (tarsus length, weight and condition, and parental wing length) and measured the immune response of chicks (phytohemagglutinin test). We used principal component analysis to reduce the number of nest variables to three relevant components. Overall, 2007 seemed to have been a more favourable year, with larger clutch sizes and heavier fledglings produced than in 2006. Laying date was earlier in nests with larger nest cup diameter, hatching success was higher as relative cup depth increased in 2007, and fledging success increased as overall nest size increased in 2007. The only significant relationship between parental traits and nest characteristics was that first year females built deeper nests than older ones. Females with longer tarsi and in poorer physical condition started laying earlier. Increased fledging success was associated with decreasing male tarsus length and fledgling condition was better in nests attended by older males. In general, nest characteristics were positively related to breeding success, although specific relationships between the different components could vary between years. These relationships have been more evident in a trophically “good” year than in a “bad” one.