 The size difference between sexes has led to the suggestion that the Rodrigues solitaire was not monogamous as stated by Leguat , and that this deeply religious man attributed the trait to the bird for moral reasons . It has been proposed that it was instead polygynous , and the wing @-@ rattling behaviour described for males suggests lek @-@ mating , where males gather for competitive mating display . However , size dimorphism does occur in some monogamous birds ; most other pigeons are monogamous as well . A 2015 article proposed that males invited females into their territories as secondary mates , which would result in the resident female acting aggressively towards the newcomer . Similar behaviour is seen in species that practice resource @-@ defence polygyny . The territories probably provided all the food the birds needed in adittion to acting as breeding @-@ areas , and there was probably intense competition for favourable territories . The fact that Rodrigues island shrank by 90 % at the end of the Pleistocene may also have contributed to such competition over territories , and thereby furthered sexual dimorphism . 
