Discussion
Nest construction is suggested to be an important innovation that facilitated the radiation of modern birds, specially passerines (Collias 1997; Collias & Collias 2014). Our results support the idea that nests could be important in the colonisation abilities of birds and further suggest that the transition from domed to open nests in songbirds could have contributed to the expansion and success of this radiation in the world. We found that species with open nests spend less time building their nests when compared to species with domed nests. Species with open nests also have larger ranges, broader thermal niches, are more likely to colonise urban environments and have lower extinction rates than species with domed nests.
Collias (1964 & 1997) suggested several benefits in building open nests. First, they are often made of fewer materials. Second, because they are smaller, they can be hidden from predators and can also allow a quicker escape from predator attack. Third, they should take less time and energy to build, which would make renesting during the breeding season more likely. Our results support the hypothesis that open nests reduce energy and time expenditure; we found that species building open nests take significantly less days to build, especially when the nest is built by females or a couple. This pattern could be explained by the large size of domed nests, since open cups have a lower weight compared to domed nests, relative to the size of the builder (Hansell 2000). Evolutionary transitions from domed to open nests, hence, could be favoured given the higher costs associated with building domed nests.
Our regression results also show that species with open nests have wider ranges and broader climatic niches compared to species with domed nests, a pattern previously reported for Australian passerines (Medina 2019). These findings support the idea that open nests have been important in the radiation of passerines, as explicitly suggested by Price and Griffith (2017); Fang et al. (2018). A transition to less costly open nests could have allowed further expansion of a clade into novel environments, and their evolutionary success. This interpretation should be treated with caution, though, since the link between range size (or climatic niche) and nest type that we report in the tip-level analyses is not evidence of causation. Another explanation for such link is that the change in nest type occurred after a transition in niche width. As species colonised new environments that are less thermally demanding, there could have been a switch to another type of nest (i.e. an open nest). However, our results from the MuSSE and HiSSE models do not support this latter hypothesis, since transition rates from all analyses suggest that changes in niche width were preceded by changes in nest type. The likely route to the most common combination of characters ­— broad niches and open nests — is from ancestors with narrow niches and open nests, indicating first a change in nest type (to open) and a subsequent expansion in niche (and hence range). Thus, our findings suggest that open nests were critical in the geographical expansion and the success of modern Passeri.
How could a transition to open nests affect range and niche expansion? Our study does not offer insights into the precise mechanism, but differences in the time taken to build nests could affect the number of nests a species builds in a breeding season, leading to differences in fecundity and explaining the macroevolutionary patterns reported here. It has been shown that in North American and European birds there are differences in annual fecundity between species with different nest types, and species with open nests have a higher number of broods per year (Böhning-Gaese et al. 2000) ­— although the nest type categories used in that study combined cavity and domed nests. Furthermore, it is known that fecundity can have important broad scale impacts. Laube et al. (2013) found a global link between life history traits associated with fecundity (e.g. clutch size and clutches per year) and the range size of bird species. Higher fecundity can lead to a higher local abundance, and ultimately allow range expansion (Blackburn et al. 2006). A link between fecundity and range size (and niche breadth) has been shown in other clades as well (Jelbertet al. 2015; Jan et al. 2019). If open nests lead to higher fecundity (because of more re-nesting opportunities), then this could result in larger ranges and broader niches in species with open nests. Testing for differences in fecundity between species with open and domed nests is beyond the scope of our study, but it would be interesting to explore how the costs of nest building could affect fecundity, and whether this path could explain the broad scale patterns reported here.
Our analyses also revealed that clades with open nests present lower extinction rates compared to those that build domed nests, offering additional support to the idea of open nests as important in the passerine radiation. We found differences in extinction rates that are consistent across trees and linked specifically to nest type. Although it has been shown that species with broader ranges tend to have both lower extinction risk and also lower speciation rates (Purvis et al. 2000; Greenberg & Mooers 2017), we found that open nesting species (with either small or wide niches/ranges) have lower extinction rates compared to species with domed nests. We, however, note that estimates of extinction rates from molecular phylogenies should be taken with extreme caution (Rabosky 2010). In any case, our results do not support differences in speciation rates linked with nest type, and show that high speciation rates are mostly related to small range sizes, supporting previous findings (Cally et al. 2021). Therefore, the relative success of open nesting species cannot be attributed to higher speciation rates — supporting Price and Griffith (2017) — but rather to a higher risk of extinction of dome nesting clades.
We also found evidence of higher urban presence for species with open nests, which could potentially be explained by links with range size, since species with open nests tend to have larger ranges, and urban species are also more likely to have large ranges (Reynolds et al. 2019). Our analysis showed, however, that even if range size is controlled for in the model, there is still an independent and significant effect of nest type on urban presence. To our knowledge, this is the first time a global link between nest architecture and urban life has been proposed. This link could be indirect and explained by selection on variables related to nest type. For instance, previous studies have shown that species living in cities have high rates of of annual fecundity (Møller 2009). If urban habitats constrain the presence of species with low fecundity, that could restrict dome nesting species. Also, non-urban species tend to nest on the ground (Sol et al.2014), and species with domed nests more commonly nest on the ground (Hall et al. 2015). Building nests on the ground might be challenging in urban environments. On the other hand, the link between urban environments and nest type could be direct: if open/cavity nests require less material or have higher flexibility in materials than domed nests, then there could be selection against dome nesters in cities. Urban environments could also be more thermally beneficial (e.g. more places with shade), in which case the thermal benefits of domed nests would not be required. In less harsh environments, species can adjust nest site selection to compensate for the lack of a nest roof (Slagsvold 1989; Kauffman et al. 2020). Future studies could investigate what is constraining the presence of dome nesting species in urban environments.
To conclude, our results show a macroevolutionary link between the type of nest that a bird builds and several traits associated with evolutionary success, such as range size, colonisation of urban environments and reduced extinction risk. Our analyses also suggest that transitions from domed to open nest type enabled species expansion of their niche, and not the other way around. Combined, these findings support a scenario explicitly proposed by previous studies (e.g. (Price & Griffith 2017; Fang et al. 2018), where the evolution of open nests is a key innovation in passerines, sensu Stroud and Losos (2016); Rabosky (2017). Namely, although there is no evidence suggesting that species with open nests have higher speciation rates, having an open nest is linked with an increase in ecological opportunity (through an increase in range size), allowing the colonisation of novel habitats, including urban habitats, and potentially decreasing the risk of extinction. The precise microevolutionary mechanisms that have led to the associations reported in this study remain a mystery but a fruitful venue for targeted research exploring the costs and benefits of building different nest types. Overall, our results highlight that nests could be much more important in the evolution and success of birds than previously thought.