Table 2. Results for the GAM with daily foraging mass gain as dependent variable.

Explanatory variables	Df	F	P
Breeding season	1	0.726	0.396
Chick rearing stage	1	0.865	0.355
Wind speed	1	0.692	0.010
Wind direction	1	<0.001	0.597
Wind speed*wind direction	2.2	0.682	0.001

Breeding season (2009/10 or 2010/11) and chick rearing stage (guard or crèche) were included as fixed factors, wind speed (in m/s) and wind direction (in degree, circularity was accounted for by a circular smoother) were included as continuous variables. In addition, we included the interaction between wind speed and wind direction (again accounting for the circularity with a smoother). n = 111 days, the model explained 15.8% of the deviance. Significant results are marked in bold.