Effect of Nest Characteristics on Thermal Properties, Clutch Size, and Reproductive Performance for an Open-Cup Nesting Songbird

Abstract

Maintaining avian eggs and young at optimum temperatures for development can increase hatching success and nestling condition, but this maintenance requires parental energetic demands. Bird nests, which often provide a structure to safely hold the eggs and nestlings and protect them from predators, can additionally be designed to help maintain eggs’ optimum temperatures by minimising heat loss, especially in climates where eggs cool rapidly when unattended. We collected and measured Prairie Warbler ( Setophaga discolor) nests in western Massachusetts, U.S. in 2009 and conducted a climate-controlled, nest-cooling experiment to determine how nest characteristics affect thermal properties for small, open-cup nesting birds. We then assessed if nests with better insulation properties resulted in any fitness benefits, and also tested if nest structural characteristics affected birds’ fitness. We found that nest characteristics influenced their thermal properties, with thicker, heavier, and larger nests having slower cooling rates and higher predicted equilibrium egg temperatures. Both nest cup depth and clutch size significantly declined over the breeding season, and we observed a trend, although non-significant, that nests with shallower cups had smaller clutches. Contrary to studies on cavity-nesting birds, we found no significant effects of nest thermal properties or nest structure on hatching and fledging success, nestling condition, brood parasitism, or nest survival. Prairie Warblers in our study site may already be adapted to build nests within a range that maximises their fitness. Furthermore, studies have shown that open-cup nests of other species are relatively thicker and more insulated in colder environments at higher latitudes than our study. Instead of building nests to solely minimise heat loss, open-cup nesting birds in temperate climates may also be driven by opposing selection pressures when building their nests, such as to prevent nestlings from overheating during hot days.

Climate as an Evolutionary Driver of Nest Morphology in Birds: A Review

“…However, supposing that parents instead build thicker walls and base, the resulting reduction in the area of the inner cup could limit clutch or egg size (Suárez et al, 2005), and parents would face a trade-off between predation avoidance and offspring production. Although it is also possible that smaller nests have denser walls or that nest size (cup space) is not linked to clutch size (Antonov, 2004;Biancucci and Martin, 2010;Akresh et al, 2017;Malzer and Hansell, 2017). Collating data on nest dimensions and density for a wide range of species from distinct climatic regions and predator regimes would be key to further our understanding on this system by testing two inter-related questions: could predation and thermal selection pressures on open nests lead to lower offspring production?…”

“…Nest lining can comprise a large part of the total nest mass and is among the most flexible of nest traits (McGowan et al, 2004). In particular, species that face changing weather conditions throughout the months (e.g., those with long breeding seasons in temperate regions), show large variation in the composition and amount of nest lining (Mainwaring and Hartley, 2008;Akresh et al, 2017). For example, in cavity nesters, the amount of animal hair in collared flycatcher nests and the diversity of animal hair in great tit and blue tit nests decreases as the breeding season advances.…”

Material Type and Position Determines the Insulative Properties of Simulated Nest Walls

“…In whole nests, there seems to be variation in the significance of the intraspecific relationships between nest mass and insulation, with some bird species exhibiting no relationship (Deeming & Campion, 2018), whereas other species show significant positive relationships (Dickinson et al, 2019). Measures of insulation also seem to correlate with base thickness but not with wall thickness (Gray & Deeming, 2017;Dickinson et al, 2019) but other studies do report significant relationships with wall thickness (Heenan & Seymour, 2012;Akresh et al, 2017). Mass of Microtis vole nests also did not correlate with insulation but their wall thickness did (Redman et al, 1999).…”

Elevation has contrasting effects on avian and mammalian nest traits in the Andean temperate mountains

“…However, this pattern is not ubiquitous, as cup characteristics and clutch size were not associated with the open‐cup nesting Setophaga discolour (Prairie warbler) in the north‐eastern United States (Akresh et al . ). Hatching success has also been linked to nest traits (Lombardo ; Álvarez & Barba ; Glądalski et al .…”

“…; Álvarez & Barba ; Akresh et al . ). Despite the importance and currently inconclusive role of nest composition and structure, the number of detailed studies on nest traits among different populations in the southern hemisphere is limited (e.g.…”