Introduction
Nests are fundamental structures for avian reproduction that determine
egg survival and development (Deeming et al. 1991; Deeming &
Mainwaring 2015). The thermal environment inside these natural
incubators influences an individual’s fitness, and therefore, nest
traits that affect internal conditions are expected to be under strong
selective pressures (Heenan 2013; Mainwaring et al. 2014).
Songbirds (suborder Passeri) and their more than 4500 species represent
the largest extant bird radiation and are known to build a variety of
nest types. The majority of species in this clade build ‘open nests’,
that is, cups or platforms where nest contents are exposed. In contrast,
some species, like Australian lyrebirds, African weavers and penduline
tits, build domed nests, which are structures characterised by an
enclosing roof and a side entrance. Domed nests have often been
suggested to reduce predation and offer thermal insulation, protecting
nest contents better than open nests (Collias 1997; Lamprecht & Schmolz
2004; Martin et al. 2017). Recent evidence highlights their
thermoregulatory advantages in colder environments by enabling parents
to spend more time away from their nests (Lamprecht & Schmolz 2004;
Martin et al. 2017; Matysioková & Remeš 2018), and in extremely
hot environments by shielding nests from solar radiation (Griffithet al. 2016). Given the presumed advantages of domed nests over
open nests, it remains a mystery why most birds in the world
(~70% of all avian families, 72% of songbird species)
build open nests.
For years it was thought that selective pressures, imposed by
temperature or predation rates, would have favoured the transition from
open to domed nests in some clades (Collias 1997; Hansell 2000; Price &
Griffith 2017). The evolutionary history of nests in passerines,
however, suggests the opposite: the common trait of building an open
nest is a derived condition, an innovation. In 2017, Price and Griffith
found evidence that the ancestral type of nest in passerines is domed,
and this result is further supported by other comparative analyses (Fanget al. 2018; Medina 2019). Thus, rather than selection favouring
the occurrence of domed nests throughout the evolution of passerines,
this trait has been repeatedly lost. Furthermore, the transition from
domed to open nests in passerines has been hypothesized to be a key
innovation, given the subsequent diversification, colonization and
expansion of open nest building lineages (Price & Griffith 2017; Fanget al. 2018). We currently ignore why domed nests transitioned
into open structures, but it is possible that costs associated with nest
building could have led to the repeated loss of domed nests. For
instance, while we lack information on whether building domed nests
represents a higher energetic cost over building open nests, nest
building is considered a costly activity and domed nests are heavier
relative to the size of the builder when compared to open nests (Hansell
2000; Mainwaring & Hartley 2013). Yet, we still lack formal analyses at
large taxonomic scales to understand the underlying evolutionary
processes and life-history implications of the transition between domed
and open nests.
The type of nest a species builds could be tightly linked with its
ability to live across different habitats or environments. For instance,
if domed nests offer greater protection across a wider range of
environments, then we would expect species with domed nests to sustain
broader climatic tolerances, thereby enabling them to inhabit a wider
range of habitats. On the other hand, if domed nests are a costly
ancestral adaptation to now scarce specific habitats, then domed-nest
lineages should exhibit narrower and more specialized environmental
tolerances than their open-nest counterparts. An increased availability
of milder habitats (in terms of climate and/or predation) could have
released lineages from the costs of domed nest-building, resulting in
larger populations and larger ranges, and potentially, an increase in
speciation or reduction in extinction rates (Rosenzweig 1995). Indeed,
Australian species with open nests tend to have larger ranges, wider
climatic niches and are less likely to be under an IUCN threat category
compared with species that build domed nests (Medina 2019).
Our main aim is to understand the evolutionary success of open nests by
studying the potential ecological and evolutionary trade-offs associated
with different nest types. First, we compare the building costs between
domed and open nests using published information on time spent
constructing the nest. We then test whether the evolution of domed and
open nests in passerines is associated with differences in range size,
climatic niche breadth, colonisation of novel environments and
differences in speciation or extinction rates. We use a comprehensive
macroevolutionary framework to explore for the first time the joint
evolutionary dynamics of a critical component of the extended phenotype
of birds.