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Home arrow Dentist Articles arrow What happens to old nests in natural cavities? | Auk, The | Find ...
What happens to old nests in natural cavities? | Auk, The | Find ...

It is well known that old nest material accumulates in nest boxes and should be removed to keep the boxes usable. Perrins (1979) and Moller (1989) proposed that the removal of old material, via reduction of ectoparasites, could improve conditions in the boxes. This in turn could have profound effects on nest-site choice, mating success, and reproductive efficiency of the individuals that use the boxes.

Natural cavities are not cleaned by humans. Thus, by implication, conditions in them should deteriorate owing to the accumulation of old nest material. Indeed, Perrins (1979) stated that ". . . the nest material would slowly decompose within the chamber and presumably over a series of years the site might become filled with old nests." I have tried to find support for this statement in the literature, but so far I have failed to find any information on this issue. Therefore, it seems that the information presented below, which indicates that old nest material disappears rapidly from natural cavities, constitutes the first data on this subject.

Study Area and Methods.-Data were gathered from 1992 to 1998 in the Bialowieza National Park in eastern Poland, within which the last surviving fragments of European primeval lowland temperate forest are preserved. The tree stands of the park have never been cut, and the entire area has been strictly protected as a reserve since 1921. Hence, one can still observe cavities and cavity nesters in conditions free of direct anthropogenic disturbance. The forest consists of several types of old-growth stands (see Tomialojc and Wesolowski 1990, Tomialojc 1991, Wesolowski and Tomialojc 1995), but most of the data were gathered in two types of chiefly deciduous stands. One was a stand of riparian trees composed mostly of alder (Alnus glutinosa), ash (Fraxinus excelsior), and Norway spruce (Picea excelsa); the other was a stand of upland deciduous forest composed of more than 12 species of trees, mainly hornbeam (Carpinus betulus), small-leafed linden (Tilia cordata), continental maple (Acer platanoides), pedunculate oak (Quercus robur), and spruce.

Since 1992, all cavities used by breeding birds within four large study plots" (33 to 55 ha each; see Wesolowski 1998) and accessible from a ladder (up to 5 m above ground, in living trees) were marked and checked the following year (in the second half of April) to see whether old nest material was still present. For checking cavity contents, I used a small light bulb on a flexible wire and a small mirror.

The April inspections showed that cavities from previous years could be impossible for birds to use for several reasons (e.g. flooding, or being filled with rotten wood up to the cavity entrance). I have omitted these instances from the present analysis because they are irrelevant to the question at hand. If the cavity contained remnants of old material (e.g. moss and hair), it was classified as an "old nest." If the cavity contained new material (e.g. fresh pieces of moss), it was classified as a "new nest" (the timing of the cavity inspections coincided with the nest-building phase of earliest breeding species in the study area; Wesolowski and Stawarczyk 1991, Wesolowski 1998). When a cavity contained no nest material and the bottom was covered with decayed material and rotten wood, or occasionally with a single fragment of leaf or piece of moss, it was classified as containing "no nest."

Because the type of nest material could have influenced the rate of nest disappearance, I divided nests into two categories: (1) "tit"nests, which were constructed mostly of moss, wool, hair, or feathers and were made by Parus major, P. caeruleus, P palustris, P. ater, and Certhia familiaris; and (2) "flycatcher" nests, which were composed mostly of dry leaves and other plant material and were made by Ficedula albicollis, E hypoleuca, and Erithacus rubecula.

Results and Discussion.-No trace of the previous year's nest was visible in two-thirds of the cavities (Table 1), nor did cavities with new nests (ca. 20%) contain remains from the previous year's nest. As a rule, new nests were in the initial stages of construction, so any remains of old material would have been apparent. I found remnants of old nest material in only 6% of the cavities that originally had contained "tit" nests and in 20% of those that had contained "flycatcher" nests (Table 1 ). "Tit" nests vanished significantly more frequently from one year to the next than did "flycatcher" nests (X^ sup 2^ = 8.4, df = 2, P

Causes of the disappearance of nest material are unknown. Nest material could have been removed by a non-human animal, or it could have decomposed in situ. In cavities that contained new nests, the remains could have been removed by the birds themselves; e.g. I commonly observed tits remove debris from nest cavities (Wesolowski 1998,1999). Nest boxes in the managed part of the Bialowieza Forest often contained nearly intact nests from the previous season. It is difficult to envisage why old material would be selectively removed from natural cavities but not from nest boxes. Therefore, I propose the alternative explanation that conditions in the cavities themselves, i.e. a favorable microclimate and a rich assemblage of decomposing organisms, result in high decay rates of old nest material.