Leg length and temperature determine the use of unipedal roosting in birds

Abstract

The function of standing on one leg in birds has long been attributed to reducing heat loss from the unfeathered legs to the external environment. Whilst a handful of single‐species studies correlate the use of the behaviour with ambient temperature, the degree to which it is used across taxa is unknown. Given that leg‐length varies between species, the length of the leg (relative to body size) may mediate the use of this thermoregulatory behaviour, such that birds with longer legs should roost on one leg more than those with relatively shorter legs at any given ambient temperature. We tested this prediction through field observations and comparative analyses of nine shorebird species, with varying tarsi length relative to body size. Six of the nine species examined used unipedal standing more as temperatures decrease, indicating its role as a heat conservation behaviour. We also found that species with relatively longer legs roosted on one leg more frequently across a wide range of temperatures. Species with shorter leg lengths likely rely less on this posture to insulate the relatively smaller surface area of the legs. Our findings showed that the long accepted notion that birds stand on one leg more at colder temperatures holds, and that species with smaller relative leg length were less reliant on this behaviour to minimise heat loss from these bare appendages.

Shape-shifting: changing animal morphologies as a response to climatic warming

“…In birds, appendages such as bills are highly vascularised surfaces not insulated by feathers and, as such, represent sites of significant heat exchange (Figure 1; [8,[28][29][30]). Similarly, unfeathered or unfurred legs in birds and mammals are linked to thermoregulation [31][32][33][34]. In some animals, such as rabbits and mice, ears and tails, respectively, are linked to heat exchange [34].…”

The importance of wetland margin microhabitat mosaics; the case of shorebirds and thermoregulation

“…At very high temperatures, it is possible that either the insulative nature of the feathers may begin to overheat the legs or that the substrate may either dry out or become hotter than the ambient temperature. As such, standing on both legs would provide for better heat dissipation (Ryeland et al., 2019) or increase evaporative cooling if the legs have been wet from previous sitting bouts. A similar pattern of behaviour was found in a previous study, where there was a polynomial relationship between sitting and ambient temperature, with increased time spent sitting until the ambient temperatures became higher than the body temperature (~35°C), at which point the birds moved into the standing position (Ferns, 1992).…”

“…Whilst species may evolve physiological and morphological adaptations to the climatic range across which they occur (Blackburn et al., 1999; Nudds & Oswald, 2007; Symonds & Tattersall, 2010; Vanderwerf, 2012), within the species range birds must employ mechanisms that help them maintain their thermal balance under constant temperature fluctuations. The most common behavioural mechanism used by birds in daily thermoregulation is the covering of the uninsulated appendages, the legs and bill, to prevent unwanted heat loss (Brodsky & Weatherhead, 1984; Ryeland, Weston, & Symonds, 2017, 2019). This behaviour occurs more frequently as temperatures decrease and is mediated in part by the species morphology, that is, birds with longer bills and legs must spend more time at low ambient temperatures standing on one leg, tucking the other into the insulating plumage and with the bill back and within the plumage (Ryeland et al., 2017, 2019; Yorzinski et al., 2018).…”

“…The most common behavioural mechanism used by birds in daily thermoregulation is the covering of the uninsulated appendages, the legs and bill, to prevent unwanted heat loss (Brodsky & Weatherhead, 1984; Ryeland, Weston, & Symonds, 2017, 2019). This behaviour occurs more frequently as temperatures decrease and is mediated in part by the species morphology, that is, birds with longer bills and legs must spend more time at low ambient temperatures standing on one leg, tucking the other into the insulating plumage and with the bill back and within the plumage (Ryeland et al., 2017, 2019; Yorzinski et al., 2018). These postures can reduce heat loss across the surface of the appendages by as much 50% in avian species (Dawson & Whittow, 1999), though they may impose costs such as reduced predator detection or impaired escape (Carr & Lima, 2011; Javůrková et al., 2011).…”

The thermoregulatory role of relative bill and leg surface areas in a Mediterranean population of Great tit (Parus major)

“…Thus, having relatively large unfeathered appendages could increase birds’ thermal tolerance (Gardner et al, 2016 ), which is becoming critically more necessary due to the increases in the frequency, severity, and duration of extreme heat events (McKechnie & Wolf, 2019 ; Stillman, 2019 ). In this context, interest in the role of bill and leg surface areas—especially bill size—as effective mechanisms of dry heat dissipation has increased in recent years (e.g., Gardner et al, 2016 ; Greenberg et al, 2012 ; Ryeland et al, 2017 , 2019 , 2021 ; Tattersall et al, 2009 , 2017 ).…”