Nest type and other species traits
We extracted species-level nest information from the website Birds of the World (BW) (Billerman et al. 2020) and Beruldsen (1980). We searched for information on all species of songbirds (suborder Passeri), following the taxonomy from Jetz et al. phylogenies (4685 species), with manual resolution of taxonomic names between BW and Jetz et al. For each species we collected information on nest type, which was classified into two broad categories: domed (defined as having a side entrance or a roof, or as being globular) or open (defined as a cup or a platform, or cases where there is only a mat of leaves, or vegetable material). Additionally, we recorded whether the species was a cavity nester or not, whether the species built both domed and open nests, or whether the nest was described as a pouch. This information was collected because in some species (e.g. Icteridae) nests are so elongated that they effectively look more liked domed than open nests. With these variables we generated fine scale classifications (six categories: open, domed, open nest inside cavity, domed nest in cavity, both types and pouch), and this classification was used in initial analyses. Based on results, we further merged data into broader categories, to increase sample size in each category. The main nest categories used were: domed, open or cavity nesters, or just domed vs. open nests (further explained in statistical analyses). Our final dataset included 3175 species with nest information (68% of all Passeri).
Given that response variables like range size can be associated with body size and species latitude, we also collected information on body size for each of the species from Wilman et al. (2014) and information on the mean latitude from their distribution range from Sheard et al. (2020), to control for the effects of these variables. As a measure of colonisation of novel habitats we also collated information of species’ presence in urban environments (data from IUCN, collated in Ducatez et al. (2020). This was recorded as 1 (presence) or 0 (no presence).
To test whether there are differences in the costs of nest building across nest types, we used the time spent building nests as proxy of the energetic cost. We searched for information on the average number of days that takes a species to construct a nest, if a range was given, we used the middle of the range as representative for the species. We also collected information on who builds the nest (females, males or both), given this could affect the time of nest construction.