Individual turnover in common pochards wintering in western France

International audienceStudying movements and population turnover is a prerequisite of management and conservation policies. Understanding the relative importance of known wintering sites requires estimating total number of birds using a wintering site with robust statistical methods because counts alone do not account for turnover. To study movements of common pochards (Aythya ferina) during winter (Oct to Mar) in western France, we used a combination of band-recoveries and capture-recapture data. We constructed a multi-state model to estimate monthly local survival and probability of movements between the banding site and other wintering sites, accounting for individual sex and age. We observed significant movements between sites and high population turnover even during winter. We did not detect any effect of age or sex on movement probabilities, except at the beginning of the winter when first-year individuals had a lower emigration probability. This result suggests that these inexperienced birds did not explore the wintering area like adults, at least early in the season. Combining our estimates (survival and emigration) and winter counts data, we computed the number of individuals using our study area throughout winter. This total was twice the maximum number of individuals counted in winter. Unexpectedly, low estimates for apparent survival suggest that permanent emigration occurred; although, recaptures and resightings revealed some birds traveled long distances from the capture site. We hypothesize that this permanent emigration was due to birds moving to areas where observation and hunting pressures were reduced compared to those in the French territory. If this hypothesis is true, high turnover rate would prevail not only at the scale of the wetland but also at the scale of the country. \textcopyright The Wildlife Society, 2013

Monthly variations in numbers of pochards on Grand-Lieu lake.

<p>Values shown are the averages monthly numbers of individuals (± SD) over the period 2004-2011.</p

Seasonal probabilities of presence on Grand-Lieu (Ψ_breeding, Ψ_wintering) by pochards according to type (“local breeder” <i>versus</i> “winter visitor”), age (young <i>versus</i> adult) or sex, derived form the selected model.

<p>95% confidence intervals are in parentheses. Note that while the model objectively retained two types of individuals according to their patterns of presence on Grand-Lieu, the denomination of these types is our own “arbitrary” interpretation of these patterns.</p

Study area, recapture/recovery locations and species' wintering and breeding ranges.

<p>The black star shows the ringing area on Grand-Lieu lake (black star) and the black dots the recapture and recovery locations. Note that the study area is located in the overlapping zone of the breeding (///) and wintering (\\\) ranges of the species.</p

Monthly survival probabilities of pochards ringed on Grand-Lieu in both the wintering and the breeding seasons according to age (young <i>versus</i> adult) and season (15 December–31 May <i>versus</i> 31 May–15 December).

<p>Values derived form the selected model with 95% confidence intervals in parentheses.</p

Model selection for multi-event CMR models of probability of presence on Grand-Lieu (Ψ) and survival (Φ) of pochards ringed on Grand-Lieu (France).

<p>Only four intermediate models and the selected one (in bold), among 37 tested, are presented. <i>P</i> = recapture probability (including physical recaptures and resightings). <i>R</i> = dead recovery probability. Abbreviations include: <i>age</i> (adult <i>versus</i> young), <i>season</i> (breeding <i>versus</i> wintering), <i>s</i> = site (Grand-Lieu <i>versus</i> Elsewhere), <i>type</i> (type 1 <i>versus</i> type 2), “<i>*</i>” means that the terms acts in interaction while “<i>+</i>” that they are additive, B = breeding season, W = wintering season and GL = Grand-Lieu. For example, <i>sex(B GL) is</i> a sex effect on Grand-Lieu during the breeding season only; <i>age(type 2, B) is</i> an age effect during the breeding season on individuals of type 2 only and <i>type*site (B-W)</i> indicates an interaction between site and type between the breeding and the wintering season. K = number of parameters in the model.</p

Movement patterns in a partial migrant: A multi-event capture-recapture approach

International audiencePartial migration is a pervasive albeit poorly studied phenomenon by which some individuals of a population migrate while others are residents. It has tremendous consequences on seasonal variations of population size/structure and therefore management. Using a multi-event capture-mark-recapture/recovery (CMR) approach, we assessed seasonal site occupancy, survival and site fidelity of a partially migratory diving duck, the Common pochard (Aythya ferina), in an area potentially including both local breeders and winter visitors. The modelling exercise indeed discriminated two different categories of individuals. First, locally breeding females which had a probability of being present in our study area during winter of 0.41. Females of this category were found to be more faithful to their breeding site than males (breeding site fidelity probabilities of 1 and 0.11, respectively). The second category of birds were winter visitors, which included adults of both sexes, whose probability of being present in the study area during the breeding season was nil, and young of both sexes with a 0.11 probability of being present in the area during the breeding season. All wintering individuals, among which there was virtually no locally breeding male, displayed a high fidelity to our study area from one winter to the next (0.41-0.43). Estimated annual survival rates differed according to age (adults 0.69, young 0.56). For both age classes mortality was higher during late winter/early spring than during summer/early winter. Our study is among the first to show how and under which conditions the multi-event approach can be employed for investigating complex movement patterns encountered in partial migrants, providing a convenient tool for overcoming state uncertainty. It also shows why studying patterns of probability of individual presence/movements in partial migrants is a key towards understanding seasonal variations in numbers. \textcopyright 2014 Gourlay-Larour et al