Wesołowski T., Czapulak A. 1986. Biologia rozrodu kosa (Turdus merula) i drozda śpiewaka (Turdus philomelos) w Polsce - wstępna analiza kart gniazdowych. (Breeding biology of Blackbird and Song Thrush in Poland - a preliminary analysis of nest cards). Notatki Ornitologiczne 27: 31-60.
This is the first paper based on data gathered by the Polish Nest Record Scheme (established in 1978). Poland has been divided there to west-east along the 19°E meridian, to north-south along the 52° parallel. Criteria of division into habitats: synanthropic habitats - parks, (urban and suburban), cemeteries, garden plots, greenery among buildings; deciduous forest - without or with scanty of conifers; mixed forest - of nearly equal share of deciduous and coniferous trees, or with slight dominance of one type; coniferous forest - without or with slight admixture of deciduous trees; others - other habitats. Dates of egg-laying commencement were in majority of cases calculated by back-counting. For calculations of clutch sizes data on all clutches known to be completed were used, in spite of the fact that mean values calculated from such data were underestimated by 0,3-0.4 egg. For calculation of hatching succes only nests found during building and egg-laying and for calculation of the fledging only nests found before hatching were used. Both species situated their nests mainly in trees and bushes but T. merula regularly used other places as well. T. philomelos in synanthropic habitats as well as in deciduous and mixed forests situated its nests significantly more often in Picea excelsa than T. merula. Both species, when breeding inPicea excelsa, situated most of the nest close to tree trunks, but position of nests in deciduous trees differed significantly between them. T. philomelos nested close to tree trunks more often than T. merula, which placed its nests more frequently in forks of tree-trunks and in semi-holes. In both species the proportion of nests in evergreens was much higher early in the season than later on. The majority of T. merula nests were 0-2 m above the ground, while T. philomelos bred slightly higher, most often at 1.6-2 m level. Both species situated their nests in lower places in deciduous stands than in other types of forests or synanthropic habitats. The first T. merula commenced egg laying on 10 March in S Poland, the start of breeding in N Poland was slightly retarded. The breeding in T. philomelos started later than in T. merula, the first birds begun to lay eggs in SW Poland at the beginning of April. Commencement of breeding in other parts of Poland took place still later. Clutch size in both species varied between two and six eggs, full clutches contained as a rule 4-5 eggs. Clutch size in both species varied over a season, the largest clutches were laid in the middle of the season. There was no difference between mean clutch size of T. merula and T. philomelos in Poland, nor, within the species, between different habitats or regions. However, a clear-cut geographical variation in mean clutch size is visible when data from different parts of Europe are compared. The hatching success was similar in both species (about 41%). The fledging success was almost twice as high as the hatching success and amounted to 81%. in T. philomelos and 73% in T. merula though the latter difference was not significant. The average number of hatched eggs was by 0.3-0.4 egg lower than the number of eggs laid. Predation was responsible for majority of total nesting failures at the egg as well as the nestling-stage.