Estimating the stage of incubation for nests of Greater Prairie-chickens using egg flotation: a float curve for grousers

TitleEstimating the stage of incubation for nests of Greater Prairie-chickens using egg flotation: a float curve for grousers
Publication TypeJournal Article
Year of Publication2009
AuthorsMcNew, LB, Gregory, AJ, Wisely, SM, Sandercock, BK
JournalGrouse News
Pagination12 -14
Accession NumberKNZ001288

Introduction Researchers often require accurate estimates of incubation stage for back-calculating the timing of nest initiation or predicting the date of hatching to capture young, to determine the durations of egg-laying, incubation and the construction of breeding phenologies, and to calculate nest productivity. Estimates of nest age are also critical for assessment of the influence of temporal variation in nest survival, and to model daily nest survival as a function of individual-or time-specific covariates (Dinsmore et al. 2002). For example, the timing of nest losses is often related to nest success and the probability of renesting (Schroeder 1997, Pitman et al. 2006, McNew et al. in press). Egg flotation is one of the most common methods employed for estimating stage of embryo development with egg flotation-development relationships documented for many species of birds (Hays & LeCroy 1971, Dunn et al. 1979, Fisher and Sengel 1991, Custer et al. 1992, Brua & Machin 2000, Liebezeit et al. 2007). To our knowledge, there are no published techniques to estimate stage of incubation for nests of grouse. Field biologists estimating stage of incubation for eggs of grouse nests have often used a modified version of an egg flotation technique developed for captive ring-neck pheasants Phasianus colchicus and gray partridges Perdix perdix (Westerskov 1950, Martin & Cooke 1987). It is unknown whether egg flotation can be used to accurately assess age of grouse nests under field conditions. As part of a larger study on the breeding ecology of greater prairie-chickens Tympanuchus cupido; (hereafter "prairie-chickens"), McNew et al. (in press) developed a regression model to accurately predict the stage of incubation for nests from egg flotation angles and egg buoyancy. Methods Prairie-chickens were captured with walk-in traps and drop-nets at leks during March–May of 2006–2008 at three study sites in eastern Kansas, USA (Schroeder & Braun 1991, Silvy et al. 1990). Females were fitted with radio transmitters and located via triangulation >4 times/week during the nesting period (April–July), and daily once it was determined from movement patterns that a female was nesting. Once a female had localized in an area for 3 successive days, we located and flushed the bird so that the eggs could be counted and the nest location recorded with a GPS unit. Females with nests were monitored daily from a distance of >100 m. Nest sites were revisited during incubation to assess clutch size and incubation stage. Figure 1. Estimating the float angle or height of a greater prairie-chicken egg. This egg is floating at ~90° and is not yet buoyant.