DOI: 10.5061/dryad.771p1
title: Data from: Quantifying population size of migrant birds at stopover sites: combining count data with stopover length estimated from stable isotope analysis

description: 1. 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. 2. Here, we developed a new method to
 quantify the total number of migrant birds using stopover areas hosting
 overlapping populations of local (breeding, wintering or resident) 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. 3. 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, i.e., 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 ca. 30000
 passage dunlins using the study site. 4. Our novel approach is not strict
 in its assumptions, and therefore can be customized 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.
description: Dunlin (Calidris alpina)
 countsTotal number of dunlins
 (Calidris alpina) counted in high-tide roost regular counts at the Tagus
 estuary (Portugal) during spring migration.contagens.xlsCarbon and nitrogen stable isotopic ratios of Dunlin (Calidris alpina)Carbon and nitrogen stable isotopic signatures of dunlin (Calidris alpina) toenails, red blood cells (RBC) and plasma collected at the Tagus estuary (Portugal) and Banc d'Arguin (Mauritania) during the winters of 2012-2013 and 2013-2014, and at the Tagus estuary (Portugal) during spring migration of 2013 and 2014. Wintering origins of dunlins sampled during spring migration at the Tagus estuary, as determined by a discriminant analysis based on isotopic signatures of toenails (see Catry et al. 2016, 10.1016/j.baae.2015.10.005), are also included. Birds identified as originating from wintering areas outside Mauritania and Portugal (e.g. Morocco) are included in the dataset but were excluded from further analyses, as well as two birds with low classification scores.Calidris_alpina_SIA.xls