Maternal androgens in avian brood parasites and their hosts: responses to parasitism and competition?

Highlights • In the arms race, two discoveries: both brood parasite & hosts use maternal effects. • Host bird eggs have elevated yolk T in parasitized nests. • Cowbirds use elevated yolk T when competition is high for host nests. • First evidence that songbirds use a tolerance defense against brood parasitism. • Small host size may determine use of tolerance rather than resistance defense.