Migration Info The spring migration of this species occurs in distinct waves. The first northern arrivals are second-year and older males, all of whom arrive within a 5-day period. Females appear next, about 5 to 7 days after the adult males. First-year males, with their intermediate plumage, arrive 12 to 16 days after the first males arrive. This staggered arrival is probably an adaptation to reduce competition between males; first-year males usually do not mate but will sometimes do so if an unmated female is available. During the winter in the Mexican highlands, Black-headed Grosbeaks frequent pine-oak forests. In the spruce forests above Mexico City, they are known to feed on Monarch butterflies. The birds feed heavily on the butterflies about every eight days -- the time interval between feeding bouts is believed to allow their system to recover from the toxic effects of the Monarchs.