Inadequate thermal refuge constrains landscape habitability for a grassland bird species

Abstract

Ecologists have long recognized the influence that environmental conditions have on abundance and range extent of animal species. We used the northern bobwhite (Colinus virginianus; hereafter bobwhite) as a model species for studying how microclimates serve as refuge against severe weather conditions. This species serves as an indicator or umbrella species for other sensitive ground-nesting, grassland obligate species. We conducted a mensurative field experiment in the rolling plains of Texas, USA, a semi-arid ecosystem on the southwestern periphery of bobwhite range, to determine whether native bunch grasses, apparently suitable for bobwhite nesting, could reduce ambient temperature below levels harmful for eggs. During the nesting season, we compared temperature and relative humidity readings at daily heat maxima (i.e., the 3 h during each day with highest temperatures) during the nesting season over the course of two years at 63 suitable nest sites paired with 63 random locations (n = 126) using two sensors at ∼10 and ∼60 cm above ground level. Mean temperature at nest height was 2.3% cooler at nest sites (35.99 °C ± 0.07 SE) compared to random locations (36.81 °C ± 0.07 SE); at ambient height, nest sites were slightly cooler (32.78 °C ± 0.06 SE) than random location (32.99 °C ± 0.06 SE). Mean relative humidity at nest sites was greater at nest height (34.53% ± 0.112 SE) and ambient height (36.22% ± 0.10 SE) compared to random locations at nest (33.35% ± 0.12 SE) and ambient height (35.75% ± 0.10 SE). Based on these results, cover at sites that appear visually suitable for nesting by bobwhites and other ground nesting birds provided adequate thermal refuge in the rolling plains by maintaining cooler, moister microclimates than surrounding non-nesting locations. Post-hoc analyses of data revealed that habitat conditions surrounding suitable nest sites strongly influenced thermal suitability of the substrate. Given that eggs of bobwhites and probably other species would experience lethal temperatures without these thermal refuges in the context of proper habitat condition, nesting vegetation is a critical component of niche space for bobwhites and other ground nesting birds in semi-arid regions. Many contemporary land uses, however, degrade or destroy bunch grasses and grassland systems, and thus decrease landscape inhabitability. Conservationists working with obligate grassland species that require bunch grasses in semi-arid regions should develop land management strategies that maximize the availability of these thermal refuges across space and time.

Acute exposure to hyperthermic oscillating temperatures during pre-incubation influences northern bobwhite development, hatching, and survival

“…1583 Hova Bator with circulated air; RH = 60%) with a unique experimental thermal regimen based on the timing of exposure to doses of low oscillating temperature (LT), high oscillating temperature (HT), or commercial temperature (CT) during the 12-d pre-incubation period (Table 1). The peak temperatures of the low and high groups were selected based on nesting studies showing temperatures peaked ≥40° C in non-drought years and ≥45° C in drought years [4, 19, 20, 25]. The nature of each treatment was such that the LT treatment exposed eggs to 92.4 heating degree-hours per day, the HT treatment exposed eggs to 212.4 heating degree-hours per day, and the CT treatment received no heating degree-hours during pre-incubation [4].…”

“…The temperatures of the diurnal oscillations during pre-incubation, for species like northern bobwhites ( Colinus virginianus ), are often extreme and even lethal in southern latitudes [5]; regularly exceeding 40° C, peaking at 45° C in drought years, and even reaching 60° C [4, 5, 12, 19, 20]. Simulating these extreme pre-incubation oscillating temperatures in the laboratory has been shown to decrease northern bobwhite egg hatching rates by ½ compared to simulated normal, or non-drought conditions [4].…”

Can prescribed fires restore C4grasslands invaded by a C3woody species and a co‐dominant C3grass species?

“…Early studies noted the value of this species as winter and early spring forage (Whisenant et al 1984). However, it is unpalatable most of the year and does not provide sufficient structure for nesting cover and thermal protection for many wildlife species, including gallinaceous birds, compared with C 4 midgrasses (Tomecek et al 2017). For these reasons, a landscape dominated by the Prosopis/Nassella association is undesirable for ranches interested in livestock production or for conservationists interested in grassland biodiversity.…”

Efficacy of acclimating and releasing captive-reared and wild-translocated Northern bobwhites

“…Northern bobwhite quail populations (Colinus virginianus) have been in decline for over a century, 1 largely due to climate, habitat loss, and population fragmentation. [2][3][4][5][6][7] For remedy, wildlife managers have typically focused their efforts on quail habitat restoration; however, for declining density-dependent species like northern bobwhite, habitat restoration alone has had mixed results. 8 Thus, wildlife managers are increasingly utilizing population restoration techniques (PRT) like releasing captive-reared or translocating wild birds, to re-establish or bolster quail populations in restored habitat.…”