Nest size parameters of Great Tits and Blue Tits: a long-term study

Abstract Nest construction behaviour is widespread among bird species. Causes for the occurrence and great variation in nest characteristics between and within populations are still poorly understood. In this study, we present results concerning nest parameters of Great Tits Parus major and Blue Tits Cyanistes caeruleus over a period of 11 breeding seasons (2012–2022) in two structurally and floristically contrasting study areas in central Poland. The nest depth differed significantly between young (yearlings) and older females (≥ 2 years after hatching) in Blue Tits (but not in Great Tits): younger females tended to build deeper nests. The size and mass of the nest were very variable in both tit species. Blue Tits tended to build deeper and heavier nests than Great Tits. The depth of the nest was positively correlated with nest mass in Great and Blue Tits. In Great Tits, the nest mass (but not nest depth) differed between study sites, with nests being heavier in the forest and the nest depth differed between years; in Blue Tits, there was an interaction between year and study site that affected nest depth. The nest mass (but not depth) from the same nestbox tended to be consistently similar within species in Great Tits and Blue Tits. We argue that the large amount of variation in nest parameters between and within populations of both tit species may be a multi-factorial result of female age, breeding area, breeding season, individual predispositions and the size of the nestbox/natural hole.