Do individual breeding experience and parental effort affect breeding season length in blackbirds?

Abstract

Weather is a crucial factor affecting the onset and end of breeding in birds. However, a few studies have clearly indicated that other factors—breeding parameters and/or individual traits—can also exert an influence. Our study aimed to determine how breeding experience and parental investment are related to the start and the end of the breeding season. We used age and pair-bond duration as predictors of breeding experience. We presumed, firstly, that older birds with a longer pair-bond duration would start breeding earlier, and secondly, that greater parental investment (expressed as breeding success in the previous season) favored later breeding. We tested the above hypotheses on an urban population of blackbirds (Turdus merula) in 2 parks in the city of Szczecin (western Poland) over 19 years. Despite the importance of weather variables, breeding experience expressed as age and pair-bond was shown to be a significant factor, too. Older birds with more bond experience started breeding sooner. Toward the end of the breeding season older, more experienced birds prolonged this period. We also observed senescence, which was manifested as the shortening of the breeding season. Those birds with more success in the previous season prolonged their breeding period in the following season; this contradicted our hypothesis. Temperature and precipitation were negatively related to the initiation date of the first clutch but positively to the initiation date of the last one. Our survey clearly showed the importance of breeding experience with the same partner and underlined the advantages of monogamy.