Commonness of not-so-common birds: the need for baseline knowledge of actual population size for the validation of population size predictions

Influence of habitat suitability and sex-related detectability on density and population size estimates of habitat-specialist warblers

“…As predicted, the regional population sizes of two species of warblers estimated in this study (~4000 RWs and ~ 1000 GRWs) were much smaller than those estimated for the same area by transforming relative abundance obtained at a national scale to population size through extrapolation by habitat at a regional scale (~23000 and ~17000 individuals respectively, [ 30 ]). Such large estimates based on extrapolation by habitat are likely positively biased by non-random selection of sampling units inflating density, coupled with density extrapolation to unavailable and unsuitable habitat (see also [ 4 – 5 ]). The alternative stating that these huge differences in population size could be due to extreme environmental conditions during the study breeding season was not probable according to the values of long time series of meteorological data from the study area consulted in http://www.aemet.es/ .…”

“…linked to lentic or lotic waters) had a considerable influence on population abundance of each species of reed warblers. However, such microhabitat requirements are frequently disregarded in population size inferences based on extrapolation over large scales [ 4 – 5 ], which often assume that average density obtained by sampling the available habitat mirrors that of the suitable occupied habitat [ 30 ]. Such estimates can be further influenced by the suitability and quality of the sampled units, as determined by their spatial distribution according to the available habitat in large-scale monitoring studies.…”

“…Population size estimates based on statistical inference can greatly benefit from the prior validation of the estimation methods by comparison with population size obtained through targeted counts of particular species in selected reference areas [ 4 , 5 , 39 – 41 ]. In practice, this validation is rarely attained because performing a complete census aimed to record all individuals is often very challenging or logistically demanding, if not altogether unfeasible in very large reference areas hosting large numbers of individuals of the target species [ 9 ].…”

“…In practice, this validation is rarely attained because performing a complete census aimed to record all individuals is often very challenging or logistically demanding, if not altogether unfeasible in very large reference areas hosting large numbers of individuals of the target species [ 9 ]. As a consequence, the accuracy of hypothesized estimates of population size of birds based on statistical inference has been often open to question [ 4 , 5 , 41 ]. An approach based on an intensive and focused search of individuals of the target species in relatively small areas and particular habitats can be, however, assumed for many species in reference areas whose intensive monitoring can be logistically affordable [ 8 ].…”

“…Multi-species large-scale monitoring schemes have often provided these estimates relying on extrapolation of relative abundances in particular habitats to large-scale areas [ 1 – 3 ]. These estimates have been generally proposed without comparison to directly censused population size of particular species in reference areas, which often hampers an objective assessment of their accuracy [ 4 – 5 ]. While these shortcomings have been recognized for endangered species with comparatively small populations (e.g.…”