Long-Term Winter Population Trends of Tits (Paridae) in Relation to Urbanization

Tit species (Paridae) are an important part of urban settlements during winter. We counted wintering tit species from 31 urban settlements along a 920 km latitudinal gradient in Finland during four winters between 1991 and 2020. We observed a total of five tit species, the Great Tit (Parus major), Eurasian Blue Tit (Cyanistes caeruleus), Coal Tit (Periparus ater), Willow Tit (Poecile montanus), and Crested Tit (Lophophanes cristatus) during the surveys. The most common and abundant species were the deciduous forest preferring Great Tit and Eurasian Blue Tit, whereas the coniferous forest preferring species exhibiting a hoarding behavior, the Coal Tit, Willow Tit, and the Crested Tit, were seldom observed, and no Siberian Tits were detected. These results indicated that food-hording coniferous preferring tit species avoided urban areas. The numbers of Great Tit and Eurasian Blue Tit were greater at the end of the study period than in the first two winters studied. The average growth rate (λ) of the Great Tit and Eurasian Blue Tit increased during the winters studied. Our data indicated a greater increase rate of the Great Tit and Eurasian Blue Tit than the Finnish winter bird monitoring work, probably because we only surveyed tits within human settlements. There was a positive correlation between the average growth rate of the Great Tit and the latitude. There was a negative correlation between the changes in average growth rate (λ) of the Eurasian Blue Tit and the changes in built-up area cover within the study areas between winters 1991/1992 and 2019/2020, and vice versa, indicating that the Eurasian Blue Tit population suffered from the increase in built-up area cover. Despite the fact that the total number of winter-feeding sites decreased during the study period, changes in their numbers were not associated with the growth rates of any tit species. The abundance of the Great Tit was negatively associated with building cover and positively associated with winter temperature. The abundance of the Eurasian Blue Tit was negatively associated with building cover and negatively associated with latitude. When controlling for the latitude, the growth rate of the Great Tit increased with the temperature in winter months, indicating that the Great Tit populations have increased in colder study sites. Our results indicated that population trends of tit species may differ regionally, and that changes in urban settlements may modify the abundance of tit species during winter. We did not detect any correlation in population growth rates between species. We recommend conducting more long-term tit research both during the winter and breeding seasons to understand the population dynamics and population trends of tit species across diverse types of habitats in more detail.