Breeding time trends of the Crested Tit (Lophophanes cristatus) in southern Finland: comparison of data sources

Abstract

We analyzed temporal and spatial variation in timing of breeding of a highly resident bird species, the Crested Tit (Lophophanes cristatus), over the past 86 years in southern Finland. Besides describing temporal changes in breeding times over this exceptionally long time series, we wanted to compare three different data types (nest card data, ringing data, and intensive population monitoring data) for their consistency in relation to temporal changes in laying dates. In the longest time series, the nest card data, the laying dates of L. cristatus show a quadratic trend over the years, delaying from the 1920s and 1930s towards the 1950s and 1960s, and advancing from the 1960s towards the 2000s. L. cristatus now starts laying c. 9 days earlier than in the 1960s and c. 6 days earlier than in the 1920s and 1930s. This is partly explained by increased March–April temperatures in southern Finland, but advancement of laying dates has been greater than expected on the basis of just temperature change. All data types showed very similar temporal trends for L. cristatus laying dates for the overlapping periods. As long as the yearly number of observations is reasonable and spatial variation is adequately controlled, all data types give reliable information on breeding times and their changes. Our results emphasize that, besides nest card data, the ringing data are a valuable source of information on breeding phenology, and allow the avoidance of the potential problem of generalizing temporal changes based on a handful of model species for which intensive monitoring data are available.