Captive breeding and cross fostering experiment
We separated 16 breeding pairs and placed them into breeding cages with material to build the nest and ad libitum mixture of food. We used pairs that were spontaneously formed in the aviaries, but we removed eggs for a few weeks to ensure male paternity. Because pigeons lay almost invariably two eggs (Johnston & Janiga 1995), we randomly cross-fostered one of the two chicks. Nestlings were cross-fostered when two days old, ensuring that age difference with their new brother was less than two days. Because we did not have a pedigree for all individuals, we matched each offspring with its biological parents and assumed that pairs were distantly related to each other.