Greenish Warbler (Phylloscopus trochiloides viridanus): An overlooked indicator of old-growth forest?

Abstract

Effective bioindicators of old-growth forest are important for conservation. The Greenish Warbler (Phylloscopus trochiloides viridanus), a forest-dwelling passerine, has been recorded in old-growth forests; however, its precise habitat preferences are poorly studied. We used opportunistic observations collected by citizen scientists and stand descriptions from a forestry database to analyse its habitat preferences in Estonia, with a focus on the characteristics of old-growth forests. The Greenish Warbler preferred productive spruce and black alder stands but also favoured rare broad-leaved stands. Forest stands were older, less drained and contained more standing and fallen dead trees in warbler sites than in control sites. Positive effects of stand age and soil fertility exhibited the highest average relative importance in generalized linear models, drainage was of intermediate importance and the occurrence of dead wood was least important. The terrain at warbler sites was also more often uneven than that at control sites. The preference for old-growth forests observed in our study makes the Greenish Warbler, despite occupying various forest stands suggestive of plasticity, a good candidate for inclusion in a suite of old-growth forest indicators.