The correct answer is - the competition will increase significantly.
If a large number of birds from the population ''a'' join the birds of the population ''b'' than the competition will increase significantly. Suddenly there will be much more competition for the food sources, for spaces for living and nesting, more competition for mating as well.
This will result in a survival of only of the fittest because the resources are limited and they only can support certain numbers of certain species. The birds that will be most efficient in finding food, be strongest, and maybe develop a certain advantageous trait will be the ones that will survive and produce offspring.