The breeding phenology of Collared Flycatchers was monitored for 7 years (1993 and 1995–2000) in oldgrowth, oak-lime-hornbeam stands in the primeval Białowieża Forest. In most years, egg laying commenced in the second half of May. The earliest first-egg date (179 broods) was 1 May, the latest such date was 13 May; these dates differed from year to year. The associations between the first-egg date and the mean temperature of the last ten days of April indicated that birds were breeding earlier as a consequence of warmer April temperatures. The earliest hatching date was 21 May, the latest 1 June. Fledging usually started in the first half of June. The length of the breeding season varied from 40 to 53 days and the breeding cycle was negatively related to mean temperature in the first ten days of May.