Data were collected for three field seasons between 2016 and 2018 in a gradient of urbanization in the city of Warsaw, Poland. Five hundred Schwegler woodcrete nestboxes (type 1b with 32 mm entrance hole, suitable for great tits and blue tits) were erected in a 50 meters (m)  grid in eight contrasted study sites representative of the urban mosaic: six sites were located within the city borders while two were exurban sites (Figure 1). The total number of nestboxes within each area varied from 21 to 110. While the monitoring of three sites (B, E and H) had started in 2016, these and all other sites were monitored in 2017 and 2018. The location of all study sites, ordered based on their decreasing distance to the city centre, is presented in Figure 1 and described as follows (see also Corsini, Dubiec, Marrot, and Szulkin (2017), Corsini, Marrot, and Szulkin (2019) and Figure S1 for further details):

Suburban village (47 nestboxes—2017, 2018). Palmiry village (20°46'48.9748" E ‐ 52°22'11.3382" N) extends for c. 95 hectares (ha) and is located c. 21 km northwest of Warsaw city borders, in proximity to Kampinos National Park. The site is characterized by residential homes with large gardens.

Natural forest (110 nestboxes—2016, 2017, 2018). Kampinos National Park (20°47'14.3867" E ‐ 52°21'22.5409" N) is a large mixed‐coniferous forest located c. 20 km northwest from Warsaw city borders. It covers a surface of c. 38.500 ha, 15% of which is now under strict protection. The forest is characterized by a dominance of pine trees (Pinus sp., 1753), followed by oaks (Quercus sp.).

Residential area II (52 nestboxes—2017, 2018). Osiedle Olszyna neighbourhood (20°57'39.37097" E ‐ 52°16'23.71883" N) covers c. 19 ha. It is characterized by an alternation of green spaces, recreational facilities and residential buildings (blocks of flats), and is adjacent to Urban Woodland II (Las Olszyna).

Urban woodland II (21 nestboxes—2017, 2018). Las Olszyna (20°57'33.93652" E ‐ 52°16'10.55093" N) is an urban green space composed of a deciduous, wet alder forest and an adjacent playground. The forest covers 3.4 ha and is mainly composed of common alders (Alnus glutinosa), birches (Betula sp.) and oaks (Quercus sp.).

Urban woodland I (91 nestboxes—2016, 2017, 2018). The Jewish cemetery (20°58'23.44285" E ‐ 52°14'52.45584" N) became a dense forest in the post‐war period. It extends for 33 ha and is mainly characterized by a naturally regenerating habitat. With its particular landscape of moss‐covered tombstones and dense tree cover, this wild urban forest is composed of an alternation of native and exotic tree species, which mainly includes Norway maples (Acer platanoides), oaks (Quercus sp.), birches (Betula pendula), and elms (Ulmus sp.). In contrast to all the other study sites, opening hours regulate visitor access to the site.

Office area (28 nestboxes—2017, 2018). The Warsaw University Science Campus (20°59'8.85224" E ‐ 52°12'43.77676" N) extends for c. 9 ha. It is situated in one of the central districts of Warsaw. The presence of offices, university buildings, canteens and dormitories provide a wide range of facilities for students.

Residential area I (46 nestboxes—2017, 2018). The Muranów neighbourhood (20°59'5.74332" E ‐ 52°14'52.17925" N) covers c. 36 ha: as in residential area II, it is a typical housing estate composed of an alternation of blocks of flats and green spaces.

Urban park (105 nestboxes—2016, 2017, 2018). Pole Mokotowskie (21°0'6.98321" E ‐ 52°12'46.66874" N) is an urban green area located in proximity to Warsaw city centre. Its alternation of flowerbeds, grass and trees covers a surface of 65 ha, offering a centrally‐located recreational site for city dwellers.

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