Large-scale variation in growth of Black Brant goslings related to food availability

ABSTRACT.-We examined variation in growth of Black Brant (Branta bernicla nigricans) goslings among two colonies on the Yukon-Kuskokwim Delta in southwestern Alaska and the Colville River Delta on Alaska's Arctic coast. We simultaneously measured abundance and quality of a key food plant, Carex subspathacea, and grazing pressure on that plant at the three colonies. Our goal was to measure variation in gosling growth in relation to variation in grazing pressure and food abundance because growth of goslings is directly linked to first-year survival, and consequently is the principal mechanism for density-dependent population regulation. Goslings grew substantially faster on the arctic coast and were nearly 30% larger than those on the YukonKuskokwim Delta at four to five weeks old. Faster growth on the arctic coast was associated with 2X greater standing crop of C. subspathacea during brood rearing than on the Yukon-Kuskokwim Delta. Dispersal rates are high enough (Lindberg et al. 1998) to rule out local adaptation and genetic variation as explanations for observed variation in growth. Our results are consistent with lower survival of goslings from the Yukon-Kuskokwim Delta during their first fall migration and stronger density-dependent regulation on the Yukon-Kuskokwim Delta than on the Arctic coast.

The growth period of long-lived animals is a period when selection acts strongly (Rose 1991), likely because adults have evolved to maintain their survival in variable environments (Charlesworth 1994). Furthermore, growing young require diets of higher quality than those adequate for adult maintenance, because higher dietary concentrations of digestible energy and protein are required for tissue production (O'Conner 1984, Sedinger 1992, 1997). Consequently, growth rates vary considerably in response to environmental conditions (Cooch et al. 1991, Larsson and Forslund 1991, Sedinger and Flint 1991).

Growing geese appear to be especially susceptible to nutrient limitation during growth, probably because they are small bodied herbivores and many plant foods contain inadequate concentrations of nutrients, especially protein, to support maximum rates of growth (Sedinger 1992, 1997). Goslings, therefore, are highly selective foragers (Sedinger and Raveling 1984), and preferred foods that will support rapid growth frequently may be depleted (Cargill and Jefferies 1984, Sedinger and Raveling 1986, Person et al. 1998). As a result, several studies have reported spatial (Aubin et al. 1993, Cooch et al. 1993, Leafloor et al. 1998) or temporal (Cooch et al. 1991, Sedinger and Flint 1991, Sedinger et al. 1998) variation in growth of goslings.

Growth is especially important in geese because size of goslings at the end of their first summer strongly influences their probability of surviving their first year (Owen and Black 1989, Sedinger et al. 1995, van der Jeugd and Larsson 1998), adult body size (Cooch et al. 1991, Sedinger et al. 1995, Leafloor et al. 1998) and fecundity (Sedinger et al. 1995). Environmentally induced variation in adult body size has been observed in other populations of birds (James 1983). Relationship between growth and individual fitness indicates that variation in growth is likely an important mechanism of population regulation in response to changing density (Larsson et al. 1998, Sedinger et al. 1998).

Because of the importance of growth to both individual fitness and population dynamics, we studied growth of Black Brant (Branta bernicla nigricans) (hereafter "brant") breeding in three widely separated colonies: Tutakoke River and Kokechik Bay on the Yukon-Kuskokwim Delta, Alaska, and the Colville River Delta on Alaska's Arctic coast. We analyze variation in growth rates of goslings among those colonies and relate variation in growth to food quality and abundance and grazing intensity at the three colonies.

Methods.-We marked and recaptured brant on the Tutakoke River (61 deg N, 165 deg W) (1994-1996) and Kokechik Bay (62 deg N, 166 deg W) (1994-1995) colonies (Sedinger et al. 1993) and associated brood-rearing areas on the Yukon-Kuskokwim Delta. The colonies were within 1 km of the Bering Sea coast or the coast of Kokechik Bay. Vegetation in the colonies was dominated by graminoids, primarily Carex ramenskii meadows or a mixed sward dominated by Elymus arenarious, C. ramenskii, and forbs (Kincheloe and Stehn 1991). Brant from those colonies took their broods up to 40 km from the colony (Flint 1993). Broods fed primarily in two communities: (1) monospecific saltmarsh swards of either C. subspathacea or Puccinellia phryganodes, where they maintained grazing lawns (Jefferies 1989, Person et al. 1998), or (2) levee communities, which supported mixed stands of graminoids and forbs. Levees supported high densities of Triglochin palustris (arrowgrass), which was an important food for growing geese (Sedinger and Raveling 1984, Mulder et al. 1996). We report data from goslings recaptured on brood-rearing areas used by brant from the Tutakoke River colony. All of those areas were along the Tutakoke or Kashunuk Rivers. We captured brant goslings at a single brood-rearing area used by brant from the Kokechik Bay colony.