Determining how many eggs to lay is key to many birds' reproductive strategies, and at least one species seems to base its decision on the choices of another species. Before laying, the pied flycatcher (Ficedula hypoleuca) spies on competing bird species that have already laid their clutches.
Using simulated nests of great tits (Parus major), Jukka Forsman and his colleagues at the University of Oulu in Finland show that flycatchers nesting near neighbours with larger broods themselves invest in laying more eggs. The age of the female birds had the greatest influence on clutch size, with older females laying significantly more eggs in areas where tits looked to have produced 13 eggs than where they looked to have produced only four. That wasn't the only difference: where simulated nests contained more eggs, the average mass of eggs laid by the flycatchers was also higher.