Data from: Quantifying population size of migrant birds at stopover sites: combining count data with stopover length estimated from stable isotope analysis

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.