The secondary hole-nesting birds in mature deciduous forests: the effect of nestbox installation

The study was conducted in the mature deciduous forests in north-western Croatia. The aim was to investigate whether there were enough natural nesting cavities in the aforementioned deciduous stands. In 2018, I monitored the nesting pairs of the secondary cavity nesters. In 2019, nestboxes were offered to the birds. The assumption was that if the number of breeding pairs did not increase, it meant that there were enough natural cavities for breeding and that it was not necessary to install nestboxes in mature deciduous stands. However, if there were an increase in population, there probably existed a lack of natural nesting holes and enough prey. Out of the 60 offered nestboxes in total, 47 were inhabited (78.3%). The nestboxes were dominantly occupied by the Great Tit Parus major, which inhabited 32 nestboxes (68.1% of the occupied nestboxes). Out of the other secondary cavity nesters, 7 nestboxes (14.8%) were inhabited by the Blue Tit Cyanistes caeruleus, 6 (12.8%) by Nuthatches Sitta europaea, and 2 (4.3%) by the Marsh Tit Poecile palustris. In 2018, the year before, there were 22 pairs of the secondary cavity nesters recorded in natural cavities in the same area: 14 (63.6%) pairs of the Great Tit, 3 (13.6%) of the Blue Tit, 2 (9.1%) of Nuthatches, 2 (9.1%) of the Marsh Tit, and 1 (4.6%) pair of the Short-toed Treecreeper Certhia brachydactyla. Only the Short-toed Treecreeper did not inhabit any of the nestboxes. In 2019, 9 pairs remain nesting in natural cavities: 4 pairs of the Great Tit, 2 of the Marsh Tit, 1 of the Blue Tit, 1 of the Nuthatches, and 1 pair of the Short-toed Treecreeper. These results indicate that there was more than a double increase in the number of the nesting pairs in 2019, suggesting that the number of suitable natural cavities limits the number of the secondary cavity nesters. Installation of nestboxes can be an effective measure in reducing the populations of defoliator species in mature deciduous forest stands in north-western Croatia, and thus, in reducing the defoliation