Quantifying population size of migrant birds at stopover sites: Combining count data with stopover length estimated from stable isotope analysis
| Autor: | José Pedro Granadeiro, Pedro M. Lourenço, Teresa Catry |
|---|---|
| Rok vydání: | 2017 |
| Předmět: |
0106 biological sciences
education.field_of_study geography food.ingredient geography.geographical_feature_category Ecology Phenology Ecological Modeling Population size Population Estuary Biology 010603 evolutionary biology 01 natural sciences 010605 ornithology Fishery Calidris food Animal migration education Ecology Evolution Behavior and Systematics Isotope analysis Count data |
| Zdroj: | Methods in Ecology and Evolution. 9:502-512 |
| ISSN: | 2041-210X |
| DOI: | 10.1111/2041-210x.12913 |
| Popis: | Regular counts of migrating animals at stopover sites have been used as a measure of site importance at the global scale as well as for monitoring long-term population changes. However, migratory passage can last for several weeks and the turnover rate of individuals is often high, preventing the use of peak counts to estimate the total number of migrants. This estimate can be achieved, however, by combining count data with information on stopover length. Here, we developed a new method to quantify the total number of migrant birds using stopover areas hosting overlapping populations of local (breeding or wintering) and passage individuals of a given species. We illustrate the application of this method by estimating the number of spring migrant dunlins Calidris alpina stopping over at the Tagus estuary (Portugal). We used carbon and nitrogen isotopic signatures in toenails to identify migrants and to estimate their proportion in total counts. We then employed isotopic clock models to determine time since arrival (TSA) of individuals from signatures in red blood cells and plasma and, using a simulation approach, we derived the relationship between TSA and stopover length. Finally, by dividing the number of migrants.day (obtained from counts) by the stopover length, we assessed the size of the migrant population. We demonstrated that stopover length can be directly predicted from mean TSA values of birds sampled during migration, regardless of variations in (1) migratory rate, that is, the phenology of arrivals throughout the migratory period and (2) the number of birds involved in the migratory event. Migrant dunlins stayed on average 7.5 days at the Tagus estuary during spring migration, which combined with count data resulted in an estimate of c. 30,000 passage dunlins, using the study site. Our novel approach is not strict in its assumptions, and therefore can be customised to the specificities of different taxa and study areas. Estimates of total number of migrants along migratory flyways are critical to identify priority sites for conservation action. In fact, single stopover sites may host a very significant proportion of a global population albeit for a short period. |
| Databáze: | OpenAIRE |
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