During five years (1992–1996) the effect of age of collared flycatcher males on arrival time, quality of nest site, partial losses, feeding effort and reproduction were examined in the primaeval, mature (170–250 year old) oak-lime-hornbeam forest of Białowieża National Park. The males were divided into two age groups: males with brown primaries and wing covers were aged as young, and males whose wings were uniformly black were aged as old ones. Only the arrival date and the clutch size were significantly different between the groups of males but other breeding features were very similar. These results may be explained by a short life-span of flycatchers and by conditions in the Białowieża National Park. A surplus of good natural cavities reduced competition between young and old males, and a surplus of food could reduce differences in foraging abilities. A high predation rate was the main cause of nesting losses. In such conditions, the strategy to take each opportunity of reproduction may lead to high life time reproduction success and can reduce potential differences in breeding success young and old males.
The aim of this study was to check what decide about breeding success in red- backed shrike Lanius collurio: nest-site concealment or parent’s aggressive behaviour. The study was carried out in eastern Poland in 1999–2003, on study plot consisted of 855 ha of extensively agricultural landscape. The effect of nest site concealment and defence of brood by parents on breeding success was determined. In observations only natural nests were used. During the two of 10 days periods after commence of egg laying, no effect of index nest concealment and index of parents aggressiveness was observed. However, in first seven days of life of nestlings, was noticed that breeding success was depended on aggressiveness of parents, in relation to index nest concealment. The results showed behavior (aggression) as an important factor in breeding success of the red-backed shrike.