Orell M. & Ojanen M. (1983) Timing and length of the breeding season of the Great Tit Parus major and the Willow Tit Parus montanus near Oulu, northern Finland. ARDEA 71 (2): 183-198

The date of laying was investigated for 1216 Great Tit and 125 Willow Tit clutches in northern Finland. The average for the first clutches of the Great Tit was 19 May (1969-80), and that for the Willow Tit 17 May (1975-80). The temperature period before laying producing the highest correlation with the date of laying was longer in the former species (50 d) than in the latter (30 d). The date of laying was delayed by 1.6 days per degree of northern latitude in the Great Tit, but less in the Willow Tit, compared with populations Only 9.5% of the Great Tit pairs, and none of the Willow Tit pairs, had second clutches. Those pairs most frequent in years when early breeding was followed by a warm summer. The population density was unimportant in regulating this. The breeding season was longer in the Great Tit (112 d) than in the Willow Tit (65 d).