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Greater sage-grouse (Centrocercus urophasianus) populations have declined across their geographic range during the last century. They were once widespread throughout the Intermountain West, but lower annual productivity, likely caused by degradation and loss of suitable habitat, has greatly reduced their distribution and population densities. Habitat used for reproduction has been well described, but relationships between habitat characteristics and reproductive output are less understood. Nesting success and chick survival are both important factors influencing annual productivity of sage-grouse. Several studies have investigated the effects of vegetation characteristics on nest success, but due to the variability of vegetation communities across the range, further work is necessary to clarify results from these studies. The relationships between habitat characteristics and chick survival are not as clearly understood. We initiated a study in the Montana Mountains of northwestern Nevada to describe nesting and early brood-rearing habitat and compare hypotheses describing potential relationships between habitat characteristics and reproductive success.
In 2004 and 2005, we monitored 84 sage-grouse hens during the reproductive period and quantified fine-scale habitat characteristics at nest and brood sites. We quantified the vegetation structure at successful and unsuccessful nests and related individual habitat characteristics to the odds of a nest hatching successfully. Individually marked chicks were monitored for 3 weeks after hatching to measure associations of forb, grass and sagebrush cover, and food availability with chick survival.
Grass cover beneath the nest shrub was the best predictor of nest outcome, and increasing amounts of grass cover improved the likelihood of a nest hatching successfully. Conversely, grass cover at early brood sites was negatively associated with chick survival. Early brood sites with greater forb cover were associated with higher sage-grouse chick survival. There was a weak relationship between sagebrush canopy cover at the nest shrub and hatch success, but sagebrush cover did not appear to have an effect on chick survival in the Montana Mountains.
Finally, we examined the relative importance of maternally-influenced variables for chick survival. Total plasma protein levels (TPP) of pre-laying hens have been linked to reproductive success and may be an indication of early spring habitat quality. We evaluated the association of TPP levels with sage-grouse chick survival, and also tested chick weight and chick sex to determine if they influenced chick survival. Total plasma protein levels were a good indicator of chick survival and may indicate a relationship between early spring forb availability and chick survival. Chick survival did not appear to be related to sex or weight at capture.
These results are similar to earlier studies that described the importance of herbaceous understory for both nest success and early brood-rearing. Management activities focusing on the restoration and maintenance of vegetation communities with intact herbaceous understories will likely improve sage-grouse reproductive success and annual production. |
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