Age-dependent effects of predation risk on night-time hypothermia in two wintering passerine species

Abstract

Small animals that winter at northern latitudes need to maximize energy intake and minimize energy loss. Many passerine birds use night-time hypothermia to conserve energy. A potential cost of night-time hypothermia with much theoretical (but little empirical) support is increased risk of night-time predation, due to reduced vigilance and lower escape speed in hypothermic birds. This idea has never been tested in the wild. We, therefore, increased perceived predation risk in great tits ( Parus major ) and blue tits ( Cyanistes caeruleus ) roosting in nest boxes during cold winter nights to measure any resultant effect on their use of night-time hypothermia. Roosting birds of both species that experienced their first winter were less prone to use hypothermia as an energy-saving strategy at low ambient temperatures when exposed to increased perceived predation risk either via handling (great tits) or via predator scent manipulation (blue tits). However, we did not record such effects in birds that were in their second winter or beyond. Our results suggest that effects of increased predation risk are age- and temperature specific. This could be caused by age-related differences in experience and subsequent risk assessment, or by dominance-related variation in habitat quality between young and old birds. Predation risk could, through its effect on use and depth of night-time hypothermia, be important for total energy management and winter survival for resident birds at northern latitudes.

Age differences in night-time metabolic rate and body temperature in a small passerine

“…Minimum predicted T b (± SD) was attained when T a was around − 5 °C and averaged 38.3 ± 0.7 °C for young birds and 37.6 ± 1.1 °C for old birds. These measures of predicted T b align well with previous studies of night-time hypothermia in undisturbed birds from the same population (Nord et al 2009(Nord et al , 2011Andreasson et al 2019). A few individuals (n = 3) reduced T s substantially to levels below 35 °C, but most birds exhibited a rather modest predicted T b reduction over a 40 °C gradient from 25 to − 15 °C, with an initial decrease in T s below LCT and a slight increase again at the lowest T a (Fig.…”

“…So why do young birds maintain a higher body temperature in ambient temperatures below LCT compared to old birds? A similar difference between the age groups was found in the same blue tit population in response to increased perceived predation risk (Andreasson et al 2019). As the confinement within a respirometer chamber may be experienced as a comparable stressor, it seems that young birds keep a higher body temperature in stressful environments.…”

“…This pattern is also clear in Svalbard ptarmigan (Lagopus muta hyperborea) where young birds have a higher resting metabolic rate, RMR, and C compared to older conspecifics in winter after post-juvenile moult (where they are time constrained), but not the following summer where onset of moult is the same for both age groups (Nord and Folkow 2018). In addition, young blue tits keep a higher T b in cold ambient temperatures when faced with a higher perceived predation risk compared to older birds (Andreasson et al 2019), which would require either amplified heat production, reduced heat loss or both. Thus, physiological and behavioral age-related differences could potentially shape the relation between metabolic rate and T b differently during a cold winter night.…”

“…Despite the potential energetic savings and benefits to winter survival, birds seem to attenuate or avoid nightly hypothermia in milder ambient temperatures (T a ), when food is plentiful and when body condition is high (Reinertsen and Haftorn 1986;Dolby et al 2004;Nord et al 2009Nord et al , 2011. This indicates that there are costs associated with the use of hypothermia, e.g., impaired quality of sleep (Mueller et al 2012), impaired immune function (Nord et al 2013;Sköld-Chiriac et al 2015), and increased predation risk (Laurila and Hohtola 2005;Carr and Lima 2013;Andreasson et al 2019).…”

“…Metabolic rate and nocturnal hypothermia, on the other hand, are open for dynamic and strategic changes that potentially depend on T a and internal states such as size, age and nutritional state. However, although several studies have investigated nocturnal hypothermia in small, northern passerines (e.g., Palmgren 1944;Udvardy 1955;Steen 1958;Haftorn 1992;Hill et al 1980;Reinertsen and Haftorn 1983;Reinertsen and Haftorn 1986) and the proximate mechanisms that determine the depth and use of hypothermia (i.e., ambient conditions, body condition/reserves, food availability and predation risk; Reinertsen and Haftorn 1986;Dolby et al 2004;Nord et al 2009Nord et al , 2011Andreasson et al 2019), we still know relatively little about how increased heat production below thermoneutrality actually affects at which level T b is maintained. Some studies have measured both metabolic rate and T b continuously (black-capped chickadees (Poecile atricapillus): Chaplin 1976;Grossman and West 1977; willow tits (Poecile montanus): Reinertsen and Haftorn 1983;Reinertsen and Haftorn 1984; greenfinches (Chloris chloris): Saarela et al 1995) in T a below thermoneutrality.…”