The optimal arrival date as a function of the standard deviation of the food distribution (σ) when the pre-breeding mortality risk is (a) high (b=0.05), (b) medium (b=0.07) or (c) low (b=0.1). The solid, dashed and dotted lines correspond to z=0.01, 0.05 and 0.1, respectively. The other parameter values are μ=150, x1=10, x2=50 and λmax=170.