2.1. Study sites
This study was carried out in 2020 in two different habitats: an urban parkland and a deciduous forest. The study sites are located ~ 10 km apart and are separated by the city of Łódź. The urban parkland study site (51º45’N; 19º24’E) consists of Łódź Botanical Garden, which covers a total area of approximately 67 ha. It is mainly of anthropogenic origin and has fragmented tree and bush cover with very few remnants of natural stands including birches (Betula pendula ), beeches (Fagus sylvatica ) and numerous alien species planted intentionally by garden administrators (Glądalski et al., 2016). The forest study site (51°50’ N;19°29’ E) is an area of approximately 145 ha located in the interior of a rich, mature mixed deciduous forest called the Łagiewniki Forest (1250 ha in total). Oaks (Quercus robur and Q. petrea ) are the dominant tree species in the forest. Both study sites were supplied with standard wooden nest boxes, each made of pinewood (200 in the parkland area and 300 in the forest area). After each breeding season in mid-October, each nest box was cleaned with a wire brush, so there were no visible nest remnants.