Results
From all species considered in this study (n= 3174, Figure 1), 60% (1902 spp.) were classified as having an open nest, 25% (790 spp.) were classified as domed nesters and 15% (482 spp.) were classified as cavity nesters. Only 1.8% of the species (60 spp.) build both types of nests. From the cavity nesters 17% build domed nests inside the cavities and 83% build cups. Most species were classified as continental (2596 spp.) and 578 spp. were classified as insular.
Species that build domed nests spend more time building their nests than species with open nests (β =-0.37 [HPD 95%: -0.40, -0.36], t-value=-3.31, p=0.001, Figure 2). Larger species take longer to build nests (β = 0.14 [0.11, 0.14], t-value=2.61, p=0.009) and couples have a weak tendency to take longer to build nests (β = 0.13 [0.15, 0.19], t-value=1.923, p=0.055).
Species that build domed nests have smaller geographic ranges, and narrower temperature niches (PCTEMP), but not precipitation niches (PCPRE), when compared with species with open nests (Table 1, Figure 3). This pattern was similar when comparing domed and open nests built within cavities (Figure S2). There were no differences between open and cavity nesters in range size or precipitation niche width, but there were differences in PCTEMP (Table 1), with cavity nesters having broader thermal niches. Similar trends were found when using either continental or all species, when using whole range or breeding range information and when using alternative classifications of nest type (Tables S2 and S3, Figure S2). Species that live in urban environments are usually species with open and not domed nests (β= -1.858, p = 0.001, Table S4, Figure S3), but there were no differences between open and cavity nesters (Table S4).
There were no differences in speciation rates between species with different nest types when using tip-rates derived from the ClaDS model. Although when using the MCC tree we found significantly higher rates for open nests (Domed vs. Open, β= 0.051, t-value= 2.36, p= 0.017), when we sampled 20 random trees there was no association (HPD interval β= -0.065 to 0.030, p-value= 0.03 to 0.83). The MuSSE analysis showed differences in both speciation and extinction rates between the four categories (Table S5, Figure 4A), where species with domed nests had higher average extinction rates (in 18 out of 20 trees) and species with narrow niches had higher speciation rates (in 12 out of 20 trees). The HiSSE analysis showed similar patterns (Figure 4A, Table S5).
The most likely ancestral state for the clade Passeri was having a small niche and a domed nest. This was evident from the HiSSE analysis but was more ambiguous in the MuSSE analyses, where probabilities of state 1 (small niche and domed nest) and state 2 (large niche and domed nest) were very similar (Table S5). The transition rate matrix in all cases showed that transitions between niche widths (small/large) are more common than transitions between nest types (Figure 4B, Figure S4 and S5), and that the most likely route to the association between open nests and wide niches (the most common and derived state) would have occurred from domed nest/narrow niche, to open nest/narrow niche, and then open nest/ wide niche (Figure 4B). This path is over 11 times more likely than evolving a wider niche and then transitioning to an open nest, and suggests that a transition in nest type, from domed to open, is the first step to then support a wider niche.