Paul D. B. Skrade

Egg crypsis in a ground-nesting shorebird influences nest survival

Coloration of exposed eggs of ground-nesting birds is thought to reduce predation, with camouflaged eggs that more closely match the area around the nest having greater survival. We tested this hypothesis using digital photographs of 374 Mountain Plover (Charadrius montanus) nests and the substrate surrounding each nest to produce covariates in nest survival models. Covariates included values representing the difference between the color of the eggs and that of the substrate, the average egg and substrate colors, and variation in both egg and substrate color. Nest survival decreased as the difference between the color of the eggs and substrate increased (accounted for by two different methods of quantifying color: βΔE = −0.021, SE = 0.024, 95% CI = −0.069, 0.026 and βΔRGB = −0.004, SE = 0.005, 95% CI = −0.013, 0.005) and increased with increasing variability in substrate color (βΔSDsub = 2.624, SE = 2.846, 95% CI = −2.954, 8.202), although after model-averaging these effects were not well-supported. Model...

Sex-Related Dispersal in the Mountain Plover (Charadrius montanus)

ABSTRACT. The rapid multiclutch mating system of the Mountain Plover (Charadrius montanus) provides an opportunity to examine sex differences in natal, within-year, and between-year breeding dispersal. We used nest locations over a 14-year period from a population of Mountain Plovers breeding on prairie dog colonies in Montana to examine sex-related patterns in natal and within-year breeding dispersal. Additionally, we modeled between-year dispersal distances in relation to sex, previous nest fate, and the occurrence of sylvatic plague in the colonies. We also modeled successive nest fate using dispersal distance and these same covariates. We found no evidence of sex differences in distances moved during natal dispersal (mean = 13.0 km for 16 males; mean = 10.2 km for 22 females) or within-year breeding dispersal (mean = 2.8 km for 22 males; mean = 3.0 km for 26 females). The mean (± SE) dispersal distance was 2.7 ± 0.60 km (n = 115) for males nesting in consecutive years and 4.3 ± 0.87 km (n = 87) for females. Previous nest fate was the only factor that had a strong effect on between-year breeding dispersal. On average, birds that were previously successful moved 3.0 ± 0.55 km (n = 149), whereas unsuccessful birds moved 4.6 ± 1.18 km (n = 53). None of the effects tested was suitable for predicting subsequent nest fate. Our work provides a better understanding of dispersal in an uncommon shorebird, additional insight into a novel mating system, and a basis for testing theories of avian dispersal.

Egg-Size Investment in a Bird with Uniparental Incubation by Both Sexes

Abstract. The Mountain Plover (Charadrius montanus) has an uncommon parental care system in which males and females tend separate nests. To improve their fitness in this system, females have the opportunity to allocate their reproductive investment to male-tended nests and their own nests differently. To examine investment we measured dimensions of plover eggs in Phillips County, Montana, and calculated their volumes. We modeled possible differences in egg volume in male- and female-incubated nests in relation to the effects of sex of the incubating adult, Julian day of nest initiation, and drought conditions. We measured >1000 eggs from 194 nests tended by 131 females and from 213 nests of 148 males. Male- and female-incubated eggs had similar mean volumes (13.20 versus 13.17 cm3, respectively) but differed significantly across the breeding season. The eggs in female-incubated nests tended to be larger than those in male-incubated nests early in the breeding season but were smaller as the season progressed, while the volume of male-incubated eggs peaked in the middle of the season. Egg volumes were affected by drought conditions, being larger during the driest periods of this study. Volumes within a nest were similar and were not influenced by the age of the incubating adult. Larger eggs tended to produce larger chicks. The similarity in the size of Mountain Plover eggs, even between male- and female-incubated nests and under different environmental conditions, provides evidence for stability of this uncommon system of parental care.

Testosterone and prolactin levels in incubating Mountain Plovers (Charadrius montanus)

ABSTRACT The hormones prolactin (PRL) and testosterone (T) are linked to breeding behaviors such as territory defense, incubation, and brood-rearing in birds. The Mountain Plover (Charadrius montanus) is a shorebird with an uncommon parental care system in which males and females tend separate nests. We collected blood from incubating male (n = 38) and female (n = 33) Mountain Plovers in Montana to determine how circulating plasma PRL and T varied by sex, day of incubation, and across the nesting season. PRL levels were similar in males and females. There was no relationship between day of incubation or Julian day and circulating PRL for either sex. T concentrations were higher in males than in females and tended to decrease across the incubation season for both sexes.