The black-headed grosbeak is able to consume the toxic monarch butterfly without suffering ill effects. This tolerance is shared only with a few other birds such as the black-backed oriole. Monarchs carry cardiac glycosides from milkweed, which poison most predators, but the grosbeak's physiology neutralises them. This ability lets the bird exploit a food source that many others avoid.
In its Mexican wintering grounds the black-headed grosbeak consumes enormous numbers of monarch butterflies, with estimates exceeding one million individuals each year. This massive intake occurs while the birds gather in overwintering colonies where monarchs are abundant. Their unique resistance to the butterflies' cardiac glycosides allows them to feed on this otherwise lethal prey throughout the season.
Genomic studies have revealed that the black-headed grosbeak possesses the same heart-poison-resistance mutations found in certain insects and even in crows, illustrating convergent evolution. Two of its four sodium-pump gene copies carry changes at positions 111 and 119 identical to those in Euploea butterflies and the common crow, while another copy mirrors a mutation seen in Largus bugs.
Female black-headed grosbeaks construct an open-saucer nest from fine grass, rootlets, twigs, bark and conifer needles, often lining it with hair and soft plant material. The nest is placed on an outer branch of a tall broad-leaved tree or in dense shrub such as blackberry, typically 0.91-10.67 m above ground. A clutch contains two to five pale green, blue or gray eggs speckled with reddish and dark brown.
Both male and female black-headed grosbeaks are vocal, each producing a distinct song that is richer and longer than a robin's warble. They often sing from prominent perches in the canopy, and the pair may also sing while perched on the nest during incubation, with the male displaying his wings and tail spread when courting.
Sources
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In the sun-drenched woodlands of North America, the Black-Headed Grosbeak flits about with an air of nonchalant elegance, its sleek, grey-brown plumage glistening like the gentle sheen on a river stone. As it moves through the trees, its distinctive black hood and lemon-yellow underbelly flash like a burst of warm sunlight, a vibrant splash of color that belies the bird's otherwise subdued demeanor. With a gentle, lilting song that fills the air, the Black-Headed Grosbeak navigates its world with a quiet confidence, a true master of its domain.
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