The Lark Bunting winters in Mexico, summers on the western grasslands of Canada and the U.S., and occupies the minds evolutionary biologists year-round.
By Rex Graham
The bunting is one of 8 grassland species expected to benefit from global warming as their optimal nesting territories edge northward. In the meantime, biologists are intrigued by the oscillating preferences of female Lark Buntings for mates with various appearances.
It is an opportunistic, reproductively mobile species. They capitalize on local boom-and-bust cycles of grasshoppers. They also eat beetles and other insects, as well as seeds of grasses, oats, sunflowers and other annuals and perennials. Their food sources often are dependent on precipitation, agricultural practices and other ecological factors.
Lark Bunting browns and blacks
The Lark Bunting is in the family of Buntings and New World Sparrows (Emberizidae). The worldwide group of birds has conical bills and plumage streaked in browns, black, grays, yellows and white.
In some years, Lark Bunting males with dark, almost black feathers and bright-white wing patches are hot nuptial commodities on the short-grass prairies of Colorado. However, females soon change their minds. In following seasons, they may seek males with brown feathers and dull-white wing patches. Females’ preferences also include various sizes of bills, various overall body sizes, wing-patch sizes and colors, and overall proportion of black versus brown feathers.
What’s a male Lark Bunting to do?
Evolutionary biologists say males are shaped genetically by “adaptive plasticity in female mate choice.” Researchers who described the female fickleness in the journal Science said the shifting tastes could be responsible for fostering genetic diversity in the species. Such diversity could be a huge advantage in adapting to changing ecological conditions. Female mating preferences may also somehow support the needs of their chicks.
Sexual selection ‘plasticity’
Plasticity of sexual selection is not the norm. Usually, sexual selection is more stable, leading to permanently exaggerated secondary sexual traits: brightly colored crests, extra-long tail feathers or bright ornaments. In such “sexually dimorphic species,” other than Lark Buntings, female preferences for extreme traits are typically shared by all females over extended periods of time.
The boisterous spring mating rituals of Lark Buntings are famous. Females chose a mate after watching male suitors singing and performing classic stiff-winged display flights. When a pair forms, other males begin to display in the vacated space and the mate couple move to peripheral areas where females build nests in shrubs.
Males themselves that have adapted to the plasticity of female choice may benefit. “The expression of more than one quality by a male would give him a mating advantage across breeding seasons by providing broad appeal under unpredictable breeding conditions,” scientists from the University of California, Santa Cruz said in Science magazine.
Such reproductive plasticity will likely help Lark Buntings in the future. The plowing of virgin grasslands has devastated most grassland birds, including that of the Lark Bunting.
Birds nesting farther north
A warmer climate is steadily pushing grassland birds northward. By mid-century, inhospitable boreal and parkland vegetation will shift to hospitable grassland, shrub-steppe habitats and pastures according to the Alberta Biodiversity Monitoring Institute.
“Competition with sown non-native grasses along roadsides, and other disturbances will likely limit native grassland expansion,” institute scientists said.
Overall, 8 of 15 grassland birds will find more suitable breeding grounds in Alberta, Canada: Lark Bunting, Horned Lark, Lark Sparrow, Bobolink, Western Meadowlark, Chestnut-collared Longspur, Baird’s Sparrow, and Grasshopper Sparrow. That’s good news for Swainson’s and Ferruginous Hawks, which feed on bunting fledglings.
Seven grassland bird species will decline by the end of the century in Alberta: Vesper Sparrow, McCown’s Longspur, Brewer’s Sparrow, Savannah Sparrow, LeConte’s Sparrow, Sprague’s Pipit, Clay-colored Sparrow,