Offspring survival after leaving the nest is a critical element of population viability and in the management of game species. We estimated brood survival to 35 days and examined the factors thought to influence individual chick survival during the first two weeks post-hatching in a population of sharp-tailed grouse Tympanuchus phasianellus near Fort St. John, British Columbia, Canada, during 2004–2005. Using program MARK and an informationtheoretic approach, we assessed the importance of female attributes, date of hatching, distance moved from the nest and weather conditions experienced prior to hatching and during brood-rearing for survival of offspring. During 2004–2005, 67% of broods had at least one chick that survived to 35 days of age, but within these broods only 34% of the chicks survived. Later hatching dates were positively related to survival during the 0–14 day age interval, whereas the distance moved from the nest by a brood and inclement weather conditions during the first seven days post-hatching significantly reduced the survival rate of chicks to 14 days of age. Further, inclement weather prior to hatching was positively related to the survival of offspring to 14 days, perhaps because wet weather created favourable foraging environments and habitat characteristics during brood rearing. Cohesive management of nesting and brood-rearing habitats is required to reduce offspring mortality associated with increased travel between suitable habitats, and may minimize mortality during periods when weather is unfavourable.
Success during the nesting period is one of the most important factors affecting reproductive success and population growth in birds (Bergerud 1988, Hudson & Rands 1988). However, offspring survival after leaving the nest is also a critical element of population viability and management of species (Aldridge & Brigham 2003, Panek 2005). Relatively little information exists on the factors affecting survival of avian offspring after nest departure, largely because of the difficulty in accurately measuring survival of chicks and determining sources of mortality in secretive and mobile species (Johnson et al. 1992, Rotella & Ratti 1992). Survival of precocial offspring is, however, often most influenced by chilling from inclement weather or poor female attentiveness, predation or starvation (Bergerud 1988, Johnson et al. 1992).
In precocial species such as grouse, chicks are susceptible to environmental conditions because of their inability to thermoregulate until approximately eight days of age (Erikstad & Spidsø 1982). Poor weather conditions experienced during the hatching period can directly result in mortality of chicks and even complete brood loss (Flanders-Wanner et al. 2004). Chicks may also starve during poor weather, as cool and wet conditions result in increased time spent brooding and, therefore, less time spent foraging (Erikstad & Spidsø 1982, Erikstad 1985). In addition, weather conditions can indirectly affect offspring survival by influencing the abundance and availability of insects, which are the primary food of gallinaceous chicks through their first week of life (Erikstad 1985). Habitat conditions can also affect survival of chicks by creating suitable foraging environments and high-quality cover characteristics that allow them to avoid predation (Bergerud 1988, Hagen et al. 2005), which is likely the ultimate cause of most offspring mortality in gallinaceous and waterfowl species (Riley et al. 1998, Pietz et al. 2003).
Populations of sharp-tailed grouse Tympanuchus phasianellus in the Peace River region of northeast British Columbia, Canada, are thought to have been decreasing over the past several decades, but the cause of this decline is unknown (British Columbia Ministry of Environment, unpubl. data). Our specific objectives were to: 1) measure the success of broods of sharp-tailed grouse from hatching until independence, 2) determine survival rates of individual chicks at various ages during brood rearing, and 3) identify the importance of female body condition and age, weather conditions, hatching date and brood movements for survival of individual offspring.
Material and methods
Our study was conducted in a 320-km2 area located approximately 35 km southeast of Fort St. John, British Columbia, Canada, and encompassed the upland areas and river breaks at the confluence of the Beatton and Peace Rivers (56°11′N, 120°25′W; ca 600 m a.s.l.). The study area was located in the Aspen Parkland Ecoregion (Meidinger & Pojar 1991), although much of the area had been converted to agricultural land use including cereal crops, a variety of hay crops and pasture-land for livestock. Aspen Populus tremuloides forests, mixed aspen-white spruce Picea glauca forests, and black spruce Picea mariana muskeg bogs exist in patches amongst large-scale agricultural developments. Along the south and west-facing river breaks, there was a strong influence of natural shrub and grassland communities. Sharp-tailed grouse were reasonably abundant, and multiple lek sites existed in the study area.
During April-May of 2004 and 2005, we captured sharp-tailed grouse on nine different leks throughout the study area using walk-in traps (Schroeder & Braun 1991). All females captured were fitted with 15-g, necklace-style radio transmitters (Model RI-2BM, Holohil Systems Ltd., Carp, ON). We recorded body mass and length of the wing chord for each bird, and used the residuals from a reduced major axis regression of mass against length of wing chord as an index of female body condition (Green 2001). Females were aged as either in their second year (SY) or after-second year (ASY), based on the shape and degree of fraying of the ninth and tenth primary feathers (Bihrle 1993).
We located nests by flushing each marked female on a weekly basis until a nest was found. To estimate date of hatching, we recorded stage of incubation using field candling techniques (Weller 1956). We considered a nest successful if we observed evidence of detached shell membranes, indicating that a minimum of one egg had hatched (Klett et al. 1986). Once eggs hatched, we estimated the survival of chicks by flushing females and their broods on a weekly basis from age seven to 35 days. Beyond 35 days of age, the likelihood of chick dispersal from the brood increases, and quantifying survival past this age may provide biased information as dispersal could be mistaken as chick mortality. We assumed that chicks not observed during a flush count were dead, but because not all chicks may take flight, our measures of survival of individual chicks represent the most conservative estimate of the number of chicks alive.