Why do grebes cover their nests? Laboratory and field tests of two alternative hypotheses

Abstract

Egg predation is a common feature influencing the reproductive success of open nesting birds. Evolutionary pressure therefore favours building cryptic, inconspicuous nests. However, these antipredatory pressures may be in conflict with thermoregulatory constraints, which select for dry nest material maintaining optimum temperature inside a nest cup during the absence of incubating parents. Here we examined possible trade-offs between nest crypsis and thermoregulation in Little Grebes (Tachybaptus ruficollis), which lay their eggs in floating nests built from wet plant material. As this species regularly covers its eggs with nest material, we experimentally examined (1) the rates of egg predation on covered and uncovered artificial nests and (2) possible thermoregulatory costs from nest covering by comparing temperature and relative humidity changes inside the nest cup. Results revealed that covering clutches is beneficial in terms of deterring predators, because uncovered eggs were more vulnerable to predation. Moreover, covering clutches also had thermoregulatory benefits because the mean temperature and relative humidity inside nest cups covered by dry or wet materials were significantly higher for covered compared to uncovered treatments. Covering clutches in Little Grebes therefore does not pose thermoregulatory costs.

Effect of intermittent incubation and clutch covering on the probability of bacterial trans‐shell infection

“…). This result is attributed to not only their thermo‐insulation function (Prokop & Trnka ) but also to the fact that nest‐lining feathers can reduce microbial loads on eggshells and therefore reduce the probability of BTSI (e.g. Peralta‐Sanchez et al .…”

“…Although clutch covering is usually considered as providing insulation against unstable ambient temperatures (Pinowski et al . , Prokop & Trnka , Dawson et al . ) or reducing conspicuousness of a clutch to visually orientated predators (Kreisinger & Albrecht ), several recent studies suggest that this behaviour may also have an antimicrobial effect.…”

Dual function of egg-covering in the Kentish plover Charadrius alexandrinus

“…Prokop & Trnka (2011) also found that, in the laboratory, cooling rates were higher in grebe nests with uncovered eggs than in nests with covered eggs. However, few studies examined how the interaction between egg-covering and thermal environment may affect egg temperatures suitable for embryogenesis (Howell, 1979;Grant, 1982).…”

“…During the thermal conditions of mid-morning the eggs may be left unattended for longer periods when uncovered than when covered. However, during these periods in which the eggs may be uncovered to alleviate overheating, unattended nests may be easier to locate by predators (Keller, 1989;Opermanis, 2004;Prokop & Trnka, 2011), because the eggs are less well camouflaged. As shorebirds suffer high nest predation rates (e.g., Fraga & Amat, 1996;Neuman et al, 2004;Saalfeld et al, 2011), better egg camouflage may be adaptive because this may improve nesting success (Solís & de Lope, 1995;Lee et al, 2010;Colwell et al, 2011).…”

“…Most studies on birds have interpreted egg-covering as an anti-predation tactic (Summers & Hockey, 1981;Keller, 1989;Kreisinger & Albrecht, 2008;Prokop & Trnka, 2011), and few studies have addressed the effects of egg-covering on egg temperature, or whether egg-covering may reduce the requirements of contact incubation (Howell, 1979;Grant, 1982).…”