Dynamique de la population camarguaise de Mouettes rieuses Larus ridibundus L. : un modèle mathématique

La dynamique de la population camarguaise de Mouette rieuse Larus ridibundus L. a été étudiée à l’aide d’un modèle matri ciel de Leslie (1945), d’estimation au maximum de vraisemblance des survies à partir des reprises de bagues, et d’études de sensi bilité du modèle aux valeurs des paramètres démographiques. Nous avons pu mettre en évidence le facteur déterminant de l’explosion démographique de la Mouette rieuse en Camargue : la diminution de la mortalité hivernale des adultes. Les autres paramètres n’ont joué qu’un rôle secondaire. La réduction de la mortalité hivernale est mise en relation avec l’utilisation inten sive, en période internuptiale, des sources de nourriture artifi cielles, comme les décharges publiques. La périodicité liée aux saisons des sources de nourriture naturelles de l’espèce a ainsi été supprimée. Une brève comparaison est tentée avec les opinions émises sur le même sujet par différents auteurs. Il est probable que les résultats obtenus à partir de ce modèle expliquent d’autres explosions démographiques chez la Mouette rieuse en particulier, et chez d’autres espèces de Laridés en général.The population dynamics of the Black-headed gulls of the Camargue has been studied with a Leslie (1945) matrix model, using maximum likelihood methods to estimate survival rates from band returns. A sensitivity analysis of the model with various values of the demographic parameters was also made. A decrease in adult mortality is shown to be the major factor responsible for the large and recent increase in numbers of the Black-headed gulls in the area. The other parameters played only a secondary role. The reduction of winter mortality is related to the intensive use of new food sources, such as garbage dumps, during the internuptial season. This extra supply of food counterbalances the effects of the seasonal variation of natural foods

Technique et fiabilité des dénombrements aériens de canards et de foulques hivernant en Camargue

The changes in abundance of the ducks and coots wintering in the Camargue have been estimated by monthly air counts since 1964. A series of control trials (n = 63) was conducted in 1976 and 1977 to estimate the reliability of the air counts and to calculate the relationship between the numbers estimated from the air by an observer (E) and the actual numbers of individuals counted on color slides (R) of the same bird flocks. The degree of relationship between the two figures is expressed by a correction coefficient (R/E) expressing the accuracy of the me asurement, and by a margin of error based upon a statistical model, which indicates the reliability of the measurement. The proposed model is based upon the numerous data available for coots, and it bas been extended to populations of diving- and surface-feeding ducks. This model makes the best possible use of the various estimates made during a same census ; it also integrates the spatial distribution of the birds and their numbers. For coots, the R/E ratio is 1.42 from September to February, and 1.13 in March. Working models derived from the statistical one had to be used for ducks. For diving ducks the correction coefficient is 1.11 early and late in season when ducks are few and scattered ; it reaches 1.28 from November to February when they live in large flocks. For surface-feeding ducks, the R/E ratio is 1.18 early and late in season, and 1.34 from November to January. In all cases the margin of error is about 20

11-2001 Newsletter

Minnesota State University, Mankato, Library Services Newsletter for November 2001

False-positive diagnosis of ascending aorta dissection by single-plane transesophageal echocardiography

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