Gunnison Sage-grouse Life History
Despite genetic, morphological, and behavioral differences, Gunnison sage-grouse and greater sage-grouse are believed to have similar life histories and habitat requirements. Most research about sage-grouse life history has been performed on greater sage-grouse, and while much of this research is applicable to Gunnison sage-grouse important differences exist between the two species.
Sage-grouse are sagebrush obligate species; they require healthy, functioning sagebrush ecosystems for year round survival. Sage-grouse are also considered indicator species because, if the sagebrush habitat is not healthy, grouse populations also suffer. Due to high levels of natural variation in sagebrush community composition, grouse are adapted to a mosaic of habitats to support their annual cycle (Birds of North America). While most sage-grouse are non-migratory, some birds can move great distances to meet their dietary requirements and find their diverse seasonal habitats (GUSG Range-wide Conservation Plan 2005). Adult Gunnison sage-grouse eat leafy vegetation and will also eat insects in summer. Although sagebrush leaves are their preferred food, grouse will also eat succulent forbs in summer. The winter diet is completely sagebrush based, requiring that some plants in winter habitat reach above the snow. Chicks consume insects and some forbs during brood rearing, and their diet shifts to sagebrush in fall (Birds of North America).
Annual survival rates vary: juvenile survival from hatch to fall is estimated to be 38%. Adult females have higher annual survival rates than males, possibly due to more cryptic plumage and secretive nature, compared to the males' elaborate plumage and display activities (GUSG Range-wide Conservation Plan 2005). As birds age, the male to female sex ratio declines due to lower annual survival rates for males (Birds of North America). Male survival rates are lowest during breeding season and female survival rates are lowest during nesting. Predators include golden eagles, a variety of hawks and accipiters, coyotes, red foxes, and bobcats. Nest predators include corvids, weasels, coyotes and badgers. Nest success is considered the most significant parameter influencing population dynamics (Birds of North America).
The sage-grouse annual cycle begins in mid-March, when the breeding season kicks off. Males congregate on leks, or strutting grounds, which are relatively bare, flat areas in the sagebrush ecosystem with good acoustic qualities and high visibility for females to assess the males' dance and vocalizations. These areas also allow for easy predator detection. Sites are usually located near good nesting habitat and good cover. Males begin to attend leks in early March; however, timing varies depending on snowmelt, weather, and photoperiod. Dominant males establish their position on the lek via wing fights and chases, usually trying to secure positions near the middle of the lek (Birds of North America). Gunnison sage-grouse are highly philopatric, returning to exactly the same spot for generations, even displaying on the ice over the newly created Blue Mesa Reservoir where their lek had previously been (National Park Conservation Association 2011, High Country News 2002). Males congregate to display for females from about an hour before dawn until about an hour after sunrise, with females showing up after males, and leaving while males are still displaying. Males typically do not move more than 1.5 miles from the lek during this time (GUSG Range-wide Conservation Plan 2005).
A lek system is polygamous, meaning females select which male to mate with and the male mates with several females. Approximately 70% of the copulations are by dominant males and only 10-15% of males actually breed (Birds of North America, Gunnison Sage-grouse Local Plan). This has significant conservation concerns because it reduces the effective population size (the number of individuals contributing genes to the next generation) below the absolute population size (the estimated total population) for males. It also limits genetic variation within the population. The lekking season runs from mid-March to late-May, with the highest lek attendance typically occurring in the second week of April. Females begin nesting as soon as they have bred, and the males provide no parental care.
Nesting typically begins late April to the beginning of May; although, in early snow melt years females have laid their first eggs as early as mid-April (Colorado Parks and Wildlife, unpublished data). Research has shown that 70-80% of nests occur within two miles of leks, however, female greater sage-grouse have been found nesting up to 20 miles from their mating lek (Birds of North America). Quality nesting habitat requires live sagebrush with sufficient canopy cover to conceal the nest and a thick forb and grass understory. Often, high vegetative diversity provides the best horizontal and vertical cover, in turn providing greatest protection from visual predators and helping with thermoregulation (Birds of North America, GUSG Range-wide Conservation Plan 2005).
Nests are shallow bowls lined with feathers and leaves at the base of live sagebrush. Clutch size averages 7 to 9 eggs, with yearling females having lower clutch sizes in some populations. Incubation starts once the last egg is laid and occurs for 27 to 28 days (GUSG Range-wide Conservation Plan 2005).
Sage-grouse have some of the lowest nest success rates of all upland game birds, with success rates ranging from 10 to 63% for greater sage-grouse (Birds of North America). Young (1994) reported a 43.2% nest success rate for Gunnison sage-grouse in the Gunnison Basin. Nest abandonment occurs in up to 22% of nests, and is most common when the female is disturbed by human activity (Birds of North America). Re-nesting is infrequent but does occur, with typically lower clutch sizes. Hatching begins in mid-May and usually ends by July, with peak hatch occurring between 10-20 June (GUSG Range-wide Conservation Plan 2005).
Chicks are precocial and leave the nest with the hen within days of hatching. Hens with chicks move to areas containing succulent forbs and insects, often in wet meadows or riparian habitats where cover is tall enough to conceal the brood and provide shade. High quality brood rearing habitat is often classified as having high forb species richness (GUSG Range-wide Conservation Plan 2005). Chick and juvenile survival depend on the availability of food and cover. Insects (primarily beetles, ants and grasshoppers) are the main food source for chicks during the first three weeks of life. From 4 to 8 weeks old, the diet shifts to include plant material, including a variety of succulent forbs and some sagebrush (GUSG Range-wide Conservation Plan 2005). After three months, sagebrush becomes the main dietary component. Adults will also eat some insects during spring and summer; however, forbs and sagebrush dominate their diet.
Unsuccessful nesting females and occasional males will join females with broods in these wet meadows, but are not as dependent on the habitat type. As autumn approaches, flocks and broods intermix as birds move from riparian areas to sagebrush dominated habitats that still provide some green forbs. Sometimes sage-grouse will move into agricultural fields at this time of year. Fringed sagebrush is a common transitional food as grouse shift from summer to winter diets (Birds of North America).
Flock size in winter varies form 15 to 100 birds with flocks often made up of a single sex (GUSG Range-wide Conservation Plan 2005). Many males overwinter within several miles from their lek. From the end of fall through early spring, sagebrush is almost the only component of sage-grouse diets. Almost any kind of sagebrush is consumed, though some birds select various subspecies for higher protein levels and lower monoterpene (plant defense chemical) levels (GUSG Range-wide Conservation Plan 2005). While birds may have low fat reserves after severe winters, individuals gain weight throughout most winters (Birds of North America, Hupp and Braun 1989, Beck and Braun 1978). During harsh winters with high snow accumulation levels, grouse depend on very tall sagebrush that is exposed above the snow. In extreme winters, sage-grouse burrow into snow roosts for protection against the elements. Grouse may associate with ungulates that uncover sagebrush to browse on during the winter (Birds of North America, Patterson 1952).
Gunnison sage-grouse have been documented moving as far as 17 miles to their winter habitat, but movements depend on severity of the winter, topography, and cover (GUSG Range-wide Conservation Plan 2005). Winter habitat requires tall sagebrush, such as that found on ridges, drainages, or southwest aspects.