Testing for the effect of meteorological conditions on transient dynamics of a reed warbler Acrocephalus scirpaceus population breeding in northern Iberia

Transients can have a severe impact on demographic parameter estimates. For instance, the use of visual counts or number of captures at a ringing station to assess indices of abundance may result in biased over-estimates due to the presence of transients. With the aim of contributing to understand transient dynamics within the Eurasian breeding passerines, we used data collected at a ringing station (2010–2018) in a reed bed area of northern Spain designed to sample breeding reed warblers Acrocephalus scirpaceus. Specifically, we tested for the effect of season on the proportion of transients and explored for correlations of rainfall regimens at both the winter and breeding quarters on annual fluctuations of the proportion of transients. The proportion of transients was not constant across the season; it showed relatively small values until mid-June, coinciding with arrivals from Africa and the peak of the breeding period, and then increased very steeply (means > 60%) already in July, with even ca. 90% of transients captured during the second half of July, coinciding with an influx of birds that would be already passing through the area en route to winter quarters in Africa. Analyses to estimate population trends of breeding reed warblers should take this circumstance into account; otherwise, models may show odd patterns, due to a mixture of local and non-local population that might show dissimilar demographic trends. Limiting data analyses to the breeding sub-period having a lesser amount of transients is recommended. Annual fluctuations in the proportion of transients did not correlate with any of the rainfall values in Africa (winter quarters) or meteorological conditions in Europe (NAO index; rainfall values at a local level), though it might be that we did not choose the appropriate variable/period/location combination. Our data set was relatively small; hence, this may hamper us to detect weak linear trends. Future research should contribute to answer this question and deep into the factors driving transience dynamics in bird populations.