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

Abstract

Knowledge about the population size and trends of common bird species is crucial for setting conservation priorities and management actions. Multi-species large-scale monitoring schemes have often provided such estimates relying on extrapolation of relative abundances in particular habitats to large-scale areas. Here we show an alternative to inference-rich predictive models, proposing methods to deal with caveats of population size estimations in habitat-specialist species, reed warblers (Acrocephalus scirpaceus and Acrocephalus arundinaceus). Reed warblers were only found in pure reedbeds within riparian woodlands or in riparian vegetation scattered within or around reedbed patches, as expected according to their habitat specialization. The proportion of individuals located in reedbed associated with lotic and lentic waters differed between species, and no reed warbler was recorded in reedbed located along dry streams. This indicates that microhabitat features or their effects on reedbed structure and other factors made a proportion of the apparently available habitat unsuitable for both warbler species. Most warblers detected were males performing territorial singing (females seldom sing and do not perform elaborate territorial song, and are undistinguishable from males by plumage). The regional population sizes of the warbler species (~4000 individuals of A. scirpaceus and ~ 1000 individuals of A. arundinaceus) 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. These results highlight the importance of considering the habitat actually used and its suitability, the manner of sex-related detection, population sex-ratio and their interactions in population estimates. Ideally, the value of predictive methods to estimate population size of common species should be tested before conducting large-scale monitoring, rather than a posteriori. Although logistically challenging, this can be achieved by designing monitoring programs including an intensive sampling of abundance in ad hoc reference areas of variable size.

The Decline of Common Birds Exemplified by the Western Jackdaw Warns on Strong Environmental Degradation

“…As a consequence, the resulting data may be inappropriate to estimate population size. In fact, attempts to estimate population sizes based on citizen science surveys can greatly overestimate actual population sizes, due to the aforementioned sampling biases as well as analytical inconsistencies associated with habitat extrapolation and other deficiencies and difficulties [27,54,56,[92][93][94]. Overestimating population sizes in the case of species with such a marked decline can be perniciously considered in hunting management plans by the competent administrations, with dramatic consequences for target and non-target species and populations [44,95,96].…”

“…In Spain, the jackdaw was widely distributed and considered very abundant in the past [42,52], although the actual size of the Spanish population is unknown and has been only estimated after extrapolating densities by habitat. However, huge differences have been found between these estimates and data obtained by direct counting of individuals in communal roosts during winter in areas such as Madrid [54], suggesting that estimates may also be substantially inflated for other regions and species [54][55][56]. Indeed, there has been a clear reduction of its distribution range during the last decades, and jackdaws have disappeared as breeders from large areas of the Castilian Plateaus, Ebro Valley, Central System, Sierra Morena, and the Betic Systems, while their presence is very scarce and fragmented in other regions [55].…”