The American robin (Turdis migratorius) is a common songbird found throughoutNorth America. It feeds mostly on insects, fruits, and berries. Its average lifespan isabout 2 years. Only 25 percent of robins survive after one year. Despite this seeminglylow survival rate, the American robin is among the most abundant species in NorthAmerica. To help maintain stability, there are a number of interrelated nonliving(or abiotic) factors and living (or biotic) factors that act as checks and balances on the growth of a population.

Which four factors are living (biotic) factors that could affect a robin population?