Growth and size of Willow Tit Parus montanus under different environmental conditions

This six years study focuses on the effect of environmental factors on Willow Tit (Parus montanus) nestling growth . For each brood, nestling growth during the growth period (day 1-12) was related to the amount of food and temperature du ring this peri od as well as to the degree of synchronisa tion (timing) of breeding to the caterpillar peak. Food abundance was estimated from the amount of caterpillar excrements (frass) fallen from birch (Betula sp.) . I also compared how growth was influenced by clutch size and hatchday each year. The only two variables that contributed to the environmental component of tarsal growth was temperature that explained 33 .2%, and relative hatch-day, which explained a further 6.0%. Temperature was the only environmental variable that explained variance in body mass, but frass explained most when temperature was excluded from the multiple regression. Temperature may influence nestling growth both directly through thermo-regulation and indirectly by e.g . effects on insect abundance. Growth rate was shown to be higher during years with a caterpillar peak than in years lacking such a peak .

Thessing A . 2000, Ardea 88(2) : 215-224