Temporal and scalar variations affect resource use of northern bobwhite broods
Kubecka, Bradley; Martin, James; Terhune, Theron (2022), Temporal and scalar variations affect resource use of northern bobwhite broods , Dryad, Dataset, https://doi.org/10.5061/dryad.vt4b8gtrn
Disparate resource use originating from phenology of biotic resources, abiotic conditions, and life cycles of exploiting organisms underscores the importance of research across time and space to guide locally relevant management practices. Our goal was to evaluate resource use of northern bobwhite (Colinus virginianus; bobwhite) at two spatial scales and across three age classes, from hatching through a period of the post-juvenile molt. Our study was conducted at Tall Timbers Research Station, Tallahassee, FL, USA– situated in a pyric landscape subjected to biennial prescribed fire. We predicted prescribed fire, disking, and supplemental feeding would dictate resource use, but effects would depend on time since fire, brood age, and time of day. We predicted vegetation and temperature would govern roost use by broods, but these effects would also depend on age. We radio-tracked 62 broods 21–35 times / week during May – October 2018 and 2019. Broods were less likely to use areas with large proportions of hardwood drains but favored sites with greater proportions of burned uplands, regardless of the time of day. Broods were less likely to use areas at greater distances from supplemental feed; this relationship had no interaction with age. The effect of supplemental feed was stronger later in the nesting season (> July 15). Broods were more likely to use areas with greater proportions of fallow fields during the day than for roosting. Broods used roosts with more woody cover and visual obscurity than at available sites. Roosts consisted of less grass and bare ground. However, these effects interacted with age; broods used sparser cover at older ages. Neonate broods were more likely to use cooler roosts with greater thermal stability, but this effect was reversed for juveniles. Broods may alter resource use with changes in vulnerabilities to threats such as thermal risks and predation.
Adult bobwhites were trapped in walk-in funnel traps during January, March, and November as part of a long-term population monitoring study at Tall Timbers (Palmer and Sisson 2017, Palmer et al. 2019, Terhune et al. 2019). A subsample of trapped individuals was marked with 6-g necklace-style VHF transmitters (American Wildlife Enterprises, Tallahassee, FL, USA) and monitored via radio telemetry at a rate of 3 times / week during October – April and daily during the nesting season (May – September). Locations were obtained via homing (White and Garrott 1990) and recorded on a portable-document format (PDF) map (Avenza Systems, Inc., Toronto, ON, Canada) using a GPS-enabled mobile device (e.g., iPad, iPhone, or Android device). Maps were georeferenced with respective landcover types (e.g., burned upland, drain, disked field, etc.). When individuals were documented in the same location after 2 consecutive visits, we assumed they were incubating. Incubating bobwhite were monitored daily until a nest fate was recorded (e.g., hatch, depredation). Upon a hatched nest, the incubating adult was tracked at a rate of 3 times per day. One location was obtained during the morning (0800 hrs –1130 hrs), one in the afternoon (1300–1700 hrs), and one at night during roosting hours (2000 hrs – 0700 hrs).
At 10–12 days of age, broods were captured using a corral method (Smith et al. 2003). All chicks that were captured were marked using patagial wing tags (National Band and Tag Co., Newport, Kentucky, USA) (Carver et al. 1999). If adults were captured without a brood, tracking ceased for that brood and tracking reverted to adult monitoring protocol. For broods captured with greater than 5 chicks, a subsample was marked with 0.75-g VHF radio tags (American Wildlife Enterprises, Monticello, FL, USA), using a modified suture technique (Terhune et al. 2017, Lunsford et al. 2019, Terhune et al. 2020). Chicks were tagged individually to ensure tracking persisted despite brood amalgamations, which are common among bobwhite (Faircloth et al. 2005, Brooks and Rollins 2007, Faircloth 2008). Thus, if chicks were adopted by another brood or orphaned by the brooding adult, we retained the ability to track chicks without the presence of a marked adult. After chicks were individually tagged, tracking intensity increased to a rate of 5 locations per day during the week and 3 times per day on the weekends. The tracking schedule during the week consisted of 5 time slots: early morning (0700 – 1000 hrs), mid-morning (1000–1200 hrs), early afternoon (1200–1500 hrs), late afternoon (1500–2000 hrs), and roost (2000 – 0700 hrs). During weekend checks, the times slots included a morning (0700–1200 hrs), afternoon (1200–2000 hrs), and roost location (2000–0700 hrs). Daily tracking of broods continued until all chicks within a brood died or until 42 days of age. If brood size was less than 5 during capture and individual chicks were not tagged, we continued to monitor broods until 42 days of age by tracking the adult and ensured brooding status by the occurrence or absence of brood feces at the roost.
We sampled ground surface temperature at roost sites at predefined age intervals. At 3, 7, 14, 21, 28, 35, and 42 days of age, we marked roosts with flagging tape in each cardinal direction so they could be found the following morning. The morning (0730 hrs) following locating roosts, observers returned to the marked location and located the roost disk (identified by fecal dropping) within the flagged area. To evaluate the relative differences in temperature at roost sites to paired available locations, we deployed a DS1921G-F50 Thermochron® iButton® (Maxim Integrated Products, Inc., Sunnyvale, CA) temperature sensor within the center of the roost disk and at a site 15 m in a random azimuth (determined by spinning a pencil). At 15 m from the roost site, the observer tossed the pencil in the air above and the point of its landing served as the precise random location. Sensors were glued to a 5-cm, 11-gauge roofing nail which secured its position flush with the ground surrounding it. Sensors were calibrated to collect a temperature reading every 20 minutes from 2100 hrs to 0600 hours (28 readings). The sensor operated under conditions ranging from -40° C to 85° C at 0.5°C increments. Accuracy of temperature readings within this range is 1 °C (Maxim Integrated Products, Inc., Sunnyvale, CA).
Authors characterizing temperatures by bobwhite in the literature commonly report estimates of operative temperature by using temperature loggers fixed inside black, steel spheres (Carroll et al. 2015, Carroll et al. 2016, Carroll et al. 2018, Olsen et al. 2018, Kline et al. 2019). Operative temperatures represent the temperature experienced by an animal by accounting for solar radiation and convection. However, ambient temperature and operative temperature should be similar at night (i.e., given lack of solar radiation) assuming a constant wind speed (Guthery 2002). Thus, we did not adjust temperatures recorded by the iButtons and assumed constant convection.
We collected measurements of vegetation during the same age intervals and mornings temperature sensors were deployed. At the roost site, we estimated visual obscurity (%) using a 1-m2 cover board and vegetation composition (%) for 5 functional groups within a 1-m2 quadrat frame. Functional groups which comprised vegetation composition included grasses, forbs, bare ground, litter, and woody/shrub cover. Species such as greenbriar and sand blackberry were categorized as woody/shrub cover, when present, due to their suffruticose or frutescent growth habits in the region. We pooled the estimates of bare ground (i.e., exposed soil) and litter (e.g., pine needles) to an index called bareness. Pooled, they effectively represent traversability and lack of vertical vegetation structure.
Visual obscurity and vegetation composition were estimated using an image classification system. Images (3,024 x 4,032 pixel resolution) were classified using the point sampling software SamplePoint to standardize estimation among observers (Booth et al. 2006). Photos were taken nadir (facing the quadrat, 1 m above) for estimates of vegetation composition. Photos were taken 4 m south of the roost at a height of 1 m to estimate visual obscurity. The images were then cropped to size of the cover board and quadrat and imported into SamplePoint. A systematic grid of 225 points was generated across the image and observers assigned a functional group to each point.
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