Depth of artificial Burrowing Owl burrows affects thermal suitability and occupancy

Abstract

Many organizations have installed artificial burrows to help bolster local Burrowing Owl (Athene cunicularia) populations. However, occupancy probability and reproductive success in artificial burrows varies within and among burrow installations. We evaluated the possibility that depth below ground might explain differences in occupancy probability and reproductive success by affecting the temperature of artificial burrows. We measured burrow temperatures from March to July 2010 in 27 artificial burrows in southern California that were buried 15–76 cm below the surface (measured between the surface and the top of the burrow chamber). Burrow depth was one of several characteristics that affected burrow temperature. Burrow temperature decreased by 0.03°C per cm of soil on top of the burrow. The percentage of time that artificial burrows provided a thermal refuge from above‐ground temperature decreased with burrow depth and ranged between 50% and 58% among burrows. The percentage of time that burrow temperature was optimal for incubating females also decreased with burrow depth and ranged between 27% and 100% among burrows. However, the percentage of time that burrow temperature was optimal for unattended eggs increased with burrow depth and ranged between 11% and 95% among burrows. We found no effect of burrow depth on reproductive success across 21 nesting attempts. However, occupancy probability had a non‐linear relationship with burrow depth. The shallowest burrows (15 cm) had a moderate probability of being occupied (0.46), burrows between 28 and 40 cm had the highest probability of being occupied (>0.80), and burrows >53 cm had the lowest probability of being occupied (<0.43). Burrowing Owls may prefer burrows at moderate depths because these burrows provide a thermal refuge from above‐ground temperatures, and are often cool enough to allow females to leave eggs unattended before the onset of full‐time incubation, but not too cool for incubating females that spend most of their time in the burrow during incubation. Our results suggest that depth is an important consideration when installing artificial burrows for Burrowing Owls. However, additional study is needed to determine the possible effects of burrow depth on reproductive success and on possible tradeoffs between the effects of burrow depth on optimal temperature and other factors, such as minimizing the risk of nest predation.

Intraspecific variation in incubation behaviours along a latitudinal gradient is driven by nest microclimate and selection on neonate quality

“…Adult thermoregulatory demands could also mediate incubation decisions (Bartholomew & Dawson, 1979; Nadeau et al., 2015), especially in cavity‐ and burrow‐nesting species. Nest temperatures, even at our warmest study site, are often below the thermoneutral zone of an adult burrowing owl (Coulombe, 1970; Nadeau et al., 2015).…”

“…Adult thermoregulatory demands could also mediate incubation decisions (Bartholomew & Dawson, 1979; Nadeau et al., 2015), especially in cavity‐ and burrow‐nesting species. Nest temperatures, even at our warmest study site, are often below the thermoneutral zone of an adult burrowing owl (Coulombe, 1970; Nadeau et al., 2015). Therefore, incubating adults might either need to invest extra energy in thermogenesis or behaviourally thermoregulate through reduced nest attendance (Haftorn & Reinertsen, 1985; Yom‐Tov & Wright, 1993).…”

“…Second, nest attendance may increase predation risk to adults while sitting on the nest (Magnhagen, 1991; Stoleson & Beissinger, 2001). Finally, nest attendance by cavity‐ and burrow‐nesting species could increase adult thermoregulatory demand if the nest temperature is lower than the ambient temperature and below the adult's thermoneutral zone (Nadeau et al., 2015). Reduced attentiveness, however, could impose costs on the offspring such as increased risk of time‐dependent mortality (Boersma, 1982; Clark & Wilson, 1981; Ricklefs, 1993), decreased ability of the incubating parent to directly defend the nest from predation (Blaker, 1969; Bollinger et al, 1990) and lower average egg temperature that could compromise embryonic development and reduce the offspring's lifetime fitness (DuRant et al., 2013; Qualls & Andrews, 1999).…”

Nest microclimate and limits to egg viability explain avian life‐history variation across latitudinal gradients

“…Each nest box included a single entrance tunnel, and the top of each nesting chamber varied in depth from 12 to 65 cm below the ground surface. We used variation in burrow design to our advantage, because different burrow designs and depths increased the variation in individual microclimates among burrows even at the same study site (Nadeau et al 2015).…”

“…We temporarily blocked the burrow entrance during equipment installation, allowing the female (if present) to take refuge in the entrance tunnel and preventing her from flushing. We installed the equipment in <5 min and opening the burrows for up to 10 min did not affect their internal temperature (Nadeau et al 2015). We kept each camera running until at least 20 d posthatch or until the nest failed.…”