Are breeding activities risky for northern bobwhites? An assessment of survival costs of reproduction
Behney, Adam (2023), Are breeding activities risky for northern bobwhites? An assessment of survival costs of reproduction, Dryad, Dataset, https://doi.org/10.5061/dryad.2547d7ww2
Behaviors associated with breeding can increase mortality risk. This increased risk can be thought of as a cost of reproduction. Increased movements prior to breeding are common as individuals search for food and breeding sites. These increased movements are thought to entail greater predation risks as individuals travel through unfamiliar areas but few studies have looked at how these prebreeding movements affect survival, especially at a fine temporal resolution. Costs of reproduction may also occur during reproduction. For birds, incubation and brood-rearing can increase predation risk because individuals spend most of their time at nest sites or with broods, which may make them more easily detected and captured by predators. Using time- and individual-specific predictors of survival, I examined the relationship between survival, movements, habitat use, and breeding status of northern bobwhites Colinus virginianus in Colorado, USA. I found that prebreeding ranges were larger for breeders (29 ha) than non-breeders (18.7 ha) but daily movement distance was not different (163 m). Range size did not affect survival; however, longer recent daily movement distances (within 10 days) resulted in higher survival. Breeding status also affected survival; laying individuals experienced the highest daily survival rates followed by incubating, non-breeding and brood-rearing individuals. Overall, there appears to be a survival cost of reproduction for individuals during brood-rearing, but I found no evidence that increased movements results in decreased survival.
Field crews captured bobwhites using baited walk-in traps (Smith et al. 1981) from February through May. We distributed traps throughout Tamarack to try to capture a spatially representative sample and checked traps twice daily (mid-morning and sunset). To reduce injury to captured quail, we used cloth mesh material (Hex mesh, Joanne Fabric) for the top of the trap (Stoddard 1931, Snyder 1978, Wiley et al. 2012). To maximize capture success, we scraped away litter underneath traps and covered traps with woody and herbaceous debris (Behney et al. 2020). Observers weighed all captured bobwhites using a Pesola 300 g spring scale, aged them as either juvenile (hatched previous year) or adult (hatched prior to previous year) based on primary coverts (Leopold 1939), and affixed a numbered, aluminum leg band (National Band & Tag Company, Newport, Kentucky, USA). We affixed a ≤ 6.5 g necklace-style VHF radio transmitter (Burger et al. 1995, DeMaso et al. 1997, 3.8% of average female mass (170 g), Nelson and Martin 1953) on each female and some males. Bobwhites weighing less than 130 g did not receive a transmitter because it would have resulted in the transmitter weighing greater than 5% of the bird’s mass. Transmitters contained a mortality switch that doubled the transmitted pulse rate when there was no movement for 20 h.
Observers used a homing technique (White and Garrott 1990) to attempt to locate each radio-tagged bobwhite ≥ 3 times a week, alternating between morning, mid-day, and evening in a random order each week. We circled radio-tagged bobwhites at approximately 30 m to estimate the bird’s location based on the compass bearing and distance to the bird using a rangefinder from the observer’s location. We made every attempt to avoid flushing birds. If a bobwhite was observed in the same location on multiple, subsequent days, it was deemed to be nesting and a small piece (2.5 × 5.1 cm) of flagging tape was, inconspicuously, placed low in the vegetation 10 m north of the nest site to aid in future nest relocation. We continued to locate birds ≥ 3 times per week and if a bird was off its nest, observers went to the nest to check its status, count eggs and record an exact location with a handheld global positioning system unit. We also aged eggs using the flotation method (Westerskov 1950), which, combined with hatch date, allowed us to calculate the beginning and ending dates of laying and incubation. We monitored nests until they succeeded (≥ 1 egg hatch) or failed (depredated or abandoned). To avoid creating a path for predators to follow directly to the nest site, observers approached nests circuitously from different routes each visit. For bobwhites with broods, we used homing to estimate a location ≥ 3 times a week without flushing the birds. At 14 days post-hatch and weekly thereafter (DeMaso et al. 1997) we flushed birds to verify broods were still active (≥ 1 chick). If no brood was observed during a flush, we always proceeded with the next weekly brood flush to confirm brood loss.
This study was conducted simultaneously with other research examining the effects of high-intensity short-duration grazing during spring on bobwhite nest and brood site selection and survival (Behney 2021). Details of the grazing treatments can be found in Behney (2021). Because habitat structure can affect survival (Janke et al. 2015, Peters et al. 2015), I included a variable in the survival model noting whether each bird location was in a grazing treatment, to compare with effects related to the cost of reproduction.
Colorado Parks and Wildlife
Northeast Colorado Pheasants Forever