Seasonal Survival of Greater Snow Geese and Effect of Hunting Under Dependence in Sighting Probability

Abstract

Although much of life-history theory assumes increased mortality at certain stages (e.g., migration), survival rates are rarely estimated on a seasonal basis within the annual cycle of migratory species. We estimated variations in seasonal survival rates in a long-lived, hunted species in the presence of short-term (between consecutive seasons) and long-term (between years in the same season) dependence in sighting probabilities. We also tested the two contrasting hypotheses that hunting mortality is either compensatory or additive to natural mortality. This study was conducted on adult female Greater Snow Geese (Chen caerulescens atlantica) from 1990 to 1998, and is based on 3890 neck-banded birds and 13 657 resightings on the northern breeding grounds in summer, and southern staging areas in spring and autumn. Birds were 10-20% more likely to be seen in autumn and spring if they were seen on the previous occasion (summer and autumn, respectively). Birds were 30-40% more likely to be seen in autumn and spring if they were last seen in the same season in the previous year. Differences in behavior according to family status (presence or absence of young) and heterogeneity in site fidelity may explain the dependence in sighting probabilities; failure to account for this may lead to biased survival estimates. Monthly survival rates from spring to summer (3-mo period) and summer to autumn (2.5mo period) were equal (0.989 Ϯ 0.003, mean Ϯ 1 SE) and showed little variation over the years, even though the two lengthy migratory flights (3000 km each) and breeding occurred during these periods. In contrast, monthly winter survival (from autumn to spring, 6.5 mo) was lower than during the other seasons and varied significantly over the years (range 0.936 Ϯ 0.021 to 0.993 Ϯ 0.008). Mean survival of adult females (corrected for neck band loss) was 0.96 from spring to autumn, 0.86 during winter, and 0.83 Ϯ 0.05 for the whole year. Natural mortality (i.e., excluding hunting) was equal among seasons and did not vary over the years, which suggests that mortality risk is not increased during migration or reproduction. There was a significant inverse relationship between winter survival and the kill rate (b ϭ Ϫ1.21 Ϯ 0.56), which suggests that hunting mortality was additive to natural mortality. This is probably a general feature of long-lived species because their low and relatively constant natural mortality rate does not allow them to compensate for an additional source of mortality such as hunting. Contrary to life-history theory, we did not find evidence that migration or reproduction entailed a survival cost in this long-distance migrant bird.

Partitioning prediction uncertainty in climate-dependent population models

“…In our well-studied population [23,[25][26][27], we were able to work with a single model that received strong support from the data. However, this may not always be the case in less intensively studied species, in which several models could represent the available data equally well.…”

Predation as a probable mechanism relating winter weather to population dynamics in a North American porcupine population

“…Seasonal survival rates are crucial to understanding how mortality risks faced by animals vary during their annual cycle, or with changing weather conditions. Reliable estimates of seasonal survival rates are infrequent in mammals (e.g., Crespin et al 2002;Lima et al 2002), and factors affecting survival on a seasonal basis have been poorly explored (Gauthier et al 2001). Calculating seasonal survival rates of porcupines allowed us to establish that summer survival was constant, and to identify winter as the most critical phase of the annual cycle for porcupine survival during our study period.…”

A new exploratory approach to the study of the spatio-temporal distribution of ring recoveries: the example of Teal (Anas crecca) ringed in Camargue, Southern France

“…Although we need to assume that the first probability is adequately reflected by the data to draw reliable conclusions on the migration corridors of Teal in Europe, the other three are likely to vary in both space and time, a problem inherent to the use of ring recoveries in general, including in demographic studies (Gauthier et al 2001). The probability that a Teal dies at a given place depends on many factors that vary in both space and time (greater mortality of the Teal during the hunting period as opposed to the breeding season because of the apparent higher effect of hunting versus natural mortality causes, Devineau 2010).…”