Answer:Survival.
Explanation
Birds migration is the periodic movement of birds from location to another this is done to secure a good environment condition. Not all birds migrate and their migration is subject to a particular time.
For latitudinal migration, birds moves away from tropic during raining season to a dry place and then returns when the environment is dry.
October is a dry season and migration will favor birds who can do well in the absence of rain hence the increase in population. This is done for Survival.
