Kivonat:

In Latvia, monitoring of small hole-nesting birds that use nestboxes has been carried out during a 20-year period (1981-2000). For data analysis, 20 nestbox plots from different parts of Latvia were used. The total number of nestboxes was 2140. The main species breeding in the nestboxes were Pied Flycatcher Ficedula hypoleuca and Great Tit Parus major. Breeding populations of both species declined during the study period. These negative trends may reflect the actual state of the populations or may have been caused by other factors, such as nestbox aging. That birds tend to avoid old nest boxes has been mentioned in the literature, but many nestbox surveys have not taken this factor into account. To test the importance of nestbox age, the current study investigated nestbox occupancy against nestbox age. It found a significant correlation between declining occupancy rate and nestbox age. When in one study plot 7-year old nestboxes were replaced by new boxes, in the subsequent year the number of the Pied Flycatcher broods substantially increased. However, Pied Flycatcher clutch size increased in older nestboxes.