 It undertakes seasonal movements in response to the availability of water and food . African birds move southwards in the southern summer to breed and return north in the winter , and Asian populations are highly nomadic due to the variability of rainfall . This species has strong colonising tendencies , having expanded its range in Mexico , the US and the West Indies in recent decades . Wandering birds can turn up far beyond the normal range , sometimes staying to nest , as in Morocco , Peru and Hawaii . 
