Abstract
Differing selection pressures on stationary nest contents compared to mobile offspring mean that the nest-site characteristics resulting in the highest nest success may not be the same characteristics that result in the highest survival of juveniles from those nests. In such cases, maternal nest-site choice may optimize productivity overall by selecting nest sites that balance opposing pressures on nest success and juvenile survival, rather than maximizing survival of either the egg or the juvenile stage. Determining which macro- and microhabitat characteristics best predict overall productivity is critical for ensuring that land management activities increase overall recruitment into a population of interest, rather than benefiting one life stage at the inadvertent expense of another. We characterized nest-site choice at the macro- and microhabitat scale, and then quantified nest success and juvenile survival to overwintering in two declining turtle species, eastern box turtles and spotted turtles, that co-occur in oak savanna landscapes of northwestern Ohio and southern Michigan. Nest success in box turtles was higher in nests farther from macrohabitat edges, constructed later in the year, and at greater total depths. In contrast, survival of juvenile box turtles to overwintering was greater from nests under less shade cover and at shallower total depths. Spotted turtle nest success and juvenile survival were so high that we were unable to detect relationships between nest-site characteristics and the small amount of variation in survival. Our results demonstrate, at least for eastern box turtles, a tradeoff in nest depth between favoring nest success vs. juvenile survival to overwintering. We suggest that heterogeneity in microhabitat structure within nesting areas is important for allowing female turtles to both exercise flexibility in nest-site choice to match nest-site characteristics to prevailing weather conditions, and to place nests in close proximity to habitat that will subsequently be used by hatchlings for overwintering.
Highlights
In egg-laying animals, a female’s choice of nest site must balance a variety of sometimes conflicting selection pressures (Refsnider and Janzen, 2010)
We modeled the relationship between nest-site macrohabitat variables and nest-site microhabitat variables on nest success and juvenile survival to overwintering separately
We modeled effects of nest-site characteristics at the macrohabitat and microhabitat scale on both nest success and juvenile survival to overwintering in two declining turtle species occurring in oak savanna landscapes
Summary
In egg-laying animals, a female’s choice of nest site must balance a variety of sometimes conflicting selection pressures (Refsnider and Janzen, 2010). Nest sites that are optimal for one reason, such as minimizing risk to an ovipositing or incubating female, may be sub-optimal for a different reason, such as maximizing likelihood of the eggs hatching (e.g., Madsen and Shine, 1999; Spencer, 2002; Amat and Masero, 2004) In such situations, a female’s choice of nest site may have to take into account conflicting selection pressures, which may result in a maternal nest-site choice that optimizes the overall benefits to a female’s lifetime reproductive success, while individual components of nest-site choice may appear to be maladaptive if examined in isolation (Martin, 1992; Chalfoun and Schmidt, 2012). The opposing selection pressures on nest location for nest success vs. fledgling survival resulted in a population mean nest-site choice of nests located in close proximity to the shrub-forest edge, where neither nest success nor fledgling survival were maximized, but where the number of young raised to independence from adult care was maximized (Streby et al, 2014a)
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