  We study a model for the evolutionarily stable strategy (ESS) used by
biological populations for choosing the time of life-history events, such as
migration and breeding. In our model we accounted for both intra-species
competition (early individuals have a competitive advantage) and a disturbance
which strikes at a random time, killing a fraction $1-p$ of the population.
Disturbances include spells of bad weather, such as freezing or heavily raining
days. It has been shown in Iwasa and Levin (1995), that when $p=0$, then the
ESS is a mixed strategy, where individuals wait for a certain time and
afterwards start arriving (or breeding) every day. We remove the constraint
$p=0$ and show that if $0<p<1$ then the ESS still implies a mixed choice of
times, but strong competition may lead to a massive arrival at the earliest
time possible of a fraction of the population, while the rest will arrive
throughout the whole period during which the disturbance may occur. More
precisely, given $p$, there is a threshold for the competition parameter $a$,
above which massive arrivals occur and below which there is a behaviour as in
Iwasa and Levin (1995). We study the behaviour of the ESS and of the average
fitness of the population, depending on the parameters involved. We also
discuss how the population may be affected by climate change, in two respects:
first, how the ESS should change under the new climate and whether this change
implies an increase of the average fitness; second, which is the impact of the
new climate on a population that still follows the old strategy. We show that,
at least under some conditions, extreme weather events imply a temporary
decrease of the average fitness (thus an increasing mortality). If the
population adapts to the new climate, the survivors may have a larger fitness.
