The white-breasted nuthatch (Sitta carolinensis) is a small gray, white and black bird with a distinguishing feature: It hops down tree trunks headfirst like a squirrel. By hunting from above, the nuthatch finds insects and their eggs that other birds miss.
Although we see white-breasted nuthatches year-round, ornithologists consider them to be hidden migrants, because approximately every other year, these birds migrate.
When they migrate, it’s not necessarily far and not necessarily south. Sometimes they migrate east, west, or even north! Nutty birds!
Scientists think their migration has been largely overlooked because the birds are widely dispersed and remain within their breeding range. You won’t notice more birds in one year than another because the birds spread out so much.
Such irregular migrations are called irruptions. Many birds do this, mostly in response to the quantity of mast — seeds, nuts, and fruit — that trees produce. Since the birds’ winter survival depends on these foods, when mast is scarce, the birds move elsewhere.
Producing a mast year
Different kinds of trees have different cycles for food production. When a tree species produces a bumper crop, we call it a mast year, and these can vary by species. Pine trees produce bumper crops of seeds some years, cbut in other years they produce few seeds. (The last pine mast was in 2023.) Many fruit trees produce larger crops every other year. White oak trees produce acorns every year, and once every three to five years they produce a bumper crop. Red oaks, on the other hand, take two years to produce acorns and have mast years about every four years. (The last red oak mast year was 2024.) Lack of rain or extreme temperatures can diminish trees’ mast production, as well.
Despite these variations in food supply, before 2011, northeastern White-breasted nuthatch migrations generally took place in odd-numbered years. But both 2010 and 2011 were low migration years, because both saw high levels of mast production. Now nuthatch migration generally takes place in even-numbered years. This winter, which began in 2025, does not seem to be an irruption year.
By the way, birds seem to decide whether to migrate long before there is a seed shortage. Scientists think birds either assess tree health early in the season or they respond to low numbers of insects, who also depend on mast production.
How the nuthatch got its name
During the winter, white-breasted nuthatches cache seeds by wedging them into or under tree bark. To hide the cache, they sometimes cover their bounty with lichen or snow. These birds aren’t picky. They will cache seeds produced by trees but also seeds they find in bird feeders. Although nuthatches do not eat seeds in the summer, seeds make up the majority (68 percent) of their winter diet.
White-breasted nuthatches in the east, along the Pacific Coast, and in the Rocky Mountains have similar appearances, but they have different calls. Evolutionary biologists wondered if the geographic isolation of each group meant they had evolved into separate species.
To find out, in 2023, scientists sampled the genomes of more than 350 white-breasted nuthatches across North America (45 from the Pacific, 23 from the east, 138 from the northern Rockies, 150 from the southern Rockies). They examined more than 300,000 genes or genetic markers, and concluded that there are four highly differentiated populations, but that there was some transfer of genetic material from an unknown north Rockies ancestor into the lineage of the south Rockies population. The researchers concluded, therefore, that white-breasted nuthatches should be considered at least three separate species.
These birds are called nuthatches because they crack, or hack, a nut by wedging it into tree bark and hammering at it until the shell splits opens. But scientists have discovered that given a choice, these birds will select seeds without shells over seeds with shells. That saves them both time and energy. Plus, the birds can cache more seeds without shells than seeds with shells.
(To me, shelled seeds are seeds without the outer covering, but sometimes people understand shelled to mean “with the outer covering.” To avoid confusion, I have not used the word shelled. What do you call seeds with the shells on and off?)
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