Bill size mediates behavioural thermoregulation in birds

Abstract

Summary Bird bills have been the subject of classic evolutionary studies that demonstrate how morphological adaptations are driven by foraging ecology. As significant areas of heat loss, bills are also under selection with respect to thermoregulation, yet the ecological consequences of this function have been largely ignored until recently. Given the role of bills in thermoregulation is important, we predict that birds should behave to minimise heat loss through their bills at lower ambient temperatures by attempting to insulate their bill in their plumage. Our second prediction is that this thermoregulatory behaviour should be utilised more by species with relatively larger bills, because they will be more prone to heat loss. We demonstrate both these predictions to be correct using field observations and a comparative analysis of nine shorebird species. First, the placement of the bill within the back plumage while roosting (‘back rest’ behaviour), which insulates the bill, becomes more frequent at cooler temperatures. Second, species with large bills relative to their body size do indeed use back rest behaviour more frequently across a range of temperatures. Our findings show that behavioural thermoregulation can be mediated by bill size, and that bill morphology is not only relevant to foraging ecology but also influences behavioural adaptations to climate. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12814/suppinfo is available for this article.

Morphology and geography predict the use of heat conservation behaviours across birds

“…These results both fit with our prediction that birds found in colder climates should be more likely to use these behaviours. Many of the species at higher latitudes have smaller bills and legs (Nudds & Oswald, ; Symonds & Tattersall, ), emphasising the role that ambient temperature may play in determining use of back rest, standing on one leg and sitting (Bouchard & Anderson, ; Brodsky & Weatherhead, ; Reebs, ; Ryeland et al, ; Wellmann & Downs, ). We cannot entirely rule out the possibility that our observed pattern is due to species in temperate latitudes being more well‐studied (there are more temperate than tropical species in our analysis).…”

“…Given the thermoregulatory nature of the behaviours, we predicted that birds found in colder climates (e.g., higher latitudes and altitudes) would be more likely to have been observed using these behaviours. This prediction derives from the existing observations within species that the use of these behaviours increases in colder conditions (Anderson & Williams, ; Brodsky & Weatherhead, ; Ryeland et al, ; Wellmann & Downs, ).…”

“…Brodsky and Weatherhead () identified back rest, standing on one leg and the tendency to sit down (thereby covering both legs in plumage) as part of a suite of behaviours employed by Canadian black ducks ( Anas rubripes ) during cold winters. A recent field study of shorebird species in Australia showed clear negative relationships between use of back rest and ambient temperature (Ryeland, Weston, & Symonds, ), and species as diverse as hummingbirds and cranes have been observed to withdraw their legs into the belly feathers in cold temperatures (Alonso & Alonso, ; Udvardy, ). Dawson and Whittow () estimated that as much as 50% of the heat dissipated by unfeathered legs can be saved by sitting.…”

“…Consequently, we predicted that species with relatively longer bills should be more likely to use back rest, and species with relatively longer legs more likely to stand on one leg and/or sit when resting. Existing evidence for such a link comes from shorebirds, where species with larger bills (relative to their body size) use back rest for a greater proportion of time while roosting, across a range of ambient temperatures (Ryeland et al, ).…”

A test of a corollary of Allen's rule suggests a role for population density

“…Among the sexually dimorphic tidal marsh species, this is perhaps easier, as multiple studies across a large diversity of bird taxa have reported positive correlations between minimum winter temperatures and bill size (Snow 1954, Symonds and Tattersall 2010, but see VanderWerf 2012, Friedman et al 2017, Ryeland et al 2017. Among the sexually dimorphic tidal marsh species, this is perhaps easier, as multiple studies across a large diversity of bird taxa have reported positive correlations between minimum winter temperatures and bill size (Snow 1954, Symonds and Tattersall 2010, but see VanderWerf 2012, Friedman et al 2017, Ryeland et al 2017.…”