Tits as bryologists: patterns of bryophyte use in nests of three species cohabiting a primeval forest

Abstract

Numerous birds use bryophytes as nest construction material but the species used and their biology remain largely unknown. Therefore, questions related to the selectivity of birds in their bryophyte use, and why they use/ignore particular species, remain unanswered. We studied the composition of bryophytes, including both mosses (Bryophyta) and liverworts (Marchantiophyta), in nests of Marsh Tit Poecile palustris, Blue Tit Cyanistes caeruleus, and Great Tit Parus major breeding sympatrically in tree cavities in pristine patches of the Białowieża National Park, Poland. We checked whether the bryophyte composition of the nests differed, compared usage of different bryophytes with their availability in the surroundings (10 m radius) of the tree cavities, and looked for characteristics that may have resulted in their selection as nesting material. The birds appeared to be highly selective; of 54 bryophyte species found near cavities, 21 were never used. Individual nests contained 4.0–5.5 bryophyte species on average, and only 2.3–2.7 species exceeded 5% of the total bryophyte volume. In each tit species the three most abundantly used bryophytes belonged to mosses and comprised ca. 80–89% of the total bryophyte volume. All of the tits utilized pleurocarpous bryophytes, growing as wefts, mats or pendants mostly on tree trunks. Simulations of plucking showed that the mosses employed as nest constituents yielded larger bundles with longer shoots when plucked. The tit species differed in the sets of bryophyte species collected. Great Tit nests contained an almost completely different assemblage of mosses from that used by Marsh and Blue Tits. This variation was related to the varying mass of their broods; more robust mosses provided support for heavier broods of Great Tits, while the finest moss species were sufficient to form a cushion for the much lighter Marsh Tit broods.