The species has been threatened by extremely intense grazing by goats.
The Guadalupe junco (Junco insularis) is a bird endemic to Guadalupe Island off Pacific Mexico. Many taxonomic authorities classified it in 2008 as asubspecies of the dark-eyed junco. In 2016, it was classified as full species. This American sparrow has a dull grayish head with a gray bill and brownish upperparts. Its wings and tail are blackish, though the tail has white edges. Its underparts are white with a rufous fringe at the bottom of the wings. It makes a high, sharp sik and a long series of chipping notes.
This bird is today found mainly in the Cupressus guadalupensis cypress grove on the island of Guadalupe, with a few birds in the remaining Guadelupe pine stands. Around 1900, it was known to utilize almost any habitat for breeding. It ranged over the whole island for feeding then, and indeed still does theoretically, but actually only a handful of flocks exist. A testimony to the adaptability of this junco is the fact that today a few birds breed at the seashore in non-native Nicotiana glauca tobacco shrub since this is dense enough to provide some protection from cats.
The breeding season is from February to June. Three to four eggs are laid in a bulky cup nest of dried grass stems, which is either in a depression in the ground or in the lower branches of a tree. The eggs are greenish white with reddish brown spots. If food is plentiful, the birds apparently breed twice a year.