A Neotropical Migrant

May 1st, 2024

It is a gorgeous spring day. The weather is breezy but sunny, so it is that great combination of warmth and coolness that I love. Out on the mitigation project along Davis Creek, the many songs tell me that more of our neotropical migrants are back. One of the outstanding migrants that comes from Mexico is the Black-headed Grosbeak (Pheucticus melanocephalus). This bird is part of the Cardinalidae family, which is apparent by its thick, light-colored conical bill. The adult male grosbeak is a rich orange-cinnamon bird with a black head and black-and-white wing bars. If you put up a sunflower seed bird feeder at your house, you will probably see these birds up close. They are spectacular! The females and juveniles are a much softer colored version of the male, with a light buffy coloring all over and a dark eye stripe. The underwing coverts are always yellow below. The male birds do not get their breeding plumage until they are two years old. The juveniles can resemble the females or pale adult males.

Black-headed Grosbeaks are not just visually stunning, they also have a unique and captivating song that is somewhat reminiscent of an American Robin’s. What’s truly intriguing is that the females, unlike many other bird species, can also sing loudly. While the female’s song is often more straightforward than the male’s, there have been documented cases where she has been recorded singing the exact song of another male bird. This behavior, possibly a tactic to keep her highly territorial mate near the nest, adds another layer of fascination to these birds. The male also plays an active role in parenting, assisting with egg-sitting and feeding the young. 

Their nests are typically placed in the outer branches of a small deciduous tree or bush near a stream up to twenty-five feet high. They are generally well concealed by branches and leaves and are bulky and loosely constructed with no mud or solidifying. This loose construction may help to keep the nest cool. The incubation and fledging process is about twenty-eight days long. Their eggs are greenish-blue and brown with reddish-brown spotting.

Another fascinating trait of this grosbeak is that it has learned to follow the migrating monarchs to their winter roosting grounds in Mexico. They can eat the toxic monarch butterflies by discarding their wings, reducing the amount of toxins they ingest. They also eat the butterflies in eight-day cycles to give themselves time to eliminate these toxins. Along with another neotropical bird that does not migrate here, the Black-headed Oriole, these two species are the primary avian predators of the winter populations of roosting monarchs found in Mexico. They are responsible for more than sixty percent of the monarch mortality. The two species feed daily on the butterfly roosts in mixed flocks of five to sixty birds and consume an estimated several million butterflies annually.

It is a complicated world with evolutionary pressures we are not yet aware of. These Black-headed Grosbeaks live different lives between their tropical wintering homes and their North American nesting sites. We are lucky to have them around us from April to September. After that, they must travel back down to their wintering grounds in southern Mexico. This is the time to enjoy their beautiful songs and colors.