Black-throated sparrows (Amphispiza bilineata) are a species of sparrow found in the southwestern United States and northern Mexico. They are medium-sized sparrows with distinct black and white striping on their heads. Black-throated sparrows build nests on or near the ground, often at the base of shrubs or clumps of grass. Their nesting habits are adapted to the arid environments they inhabit.
Nesting Habitat
Black-throated sparrows nest in a variety of arid habitats across their range, including:
- Desert grasslands
- Chaparral
- Sagebrush
- Pinyon-juniper woodlands
- Oak savannas
They are found at elevations ranging from sea level to over 8000 feet. Black-throated sparrows tend to avoid dense forests and woodlands. They prefer open habitats with scattered shrubs and bunch grasses. Suitable nesting habitat provides cover from predators and weather as well as access to insects and seeds for feeding nestlings.
Some key features of good black-throated sparrow nesting habitat include:
- Presence of shrubs like sagebrush, mesquite, acacia, creosote bush
- Availability of bunch grasses and yucca plants
- Areas of bare ground for foraging
- Ample supply of insects such as grasshoppers, beetles, caterpillars
- Access to water sources
Nest Placement
Black-throated sparrows typically nest on the ground or very low in shrubs. Favorite nest sites include:
- At the base of shrubs, especially sagebrush or mesquite
- Under clumps of grass
- In rock crevices
- Among yucca and cactus plants
- Under fallen branches
The nest is a cup made of grasses, plant fibers, animal hair, and feathers. It is loosely constructed and has a lining of finer materials. The nests are typically 3-5 inches across and 1-2 inches tall.
Black-throated sparrows may nest as solitary pairs or in loose colonies with nests clustered within a few feet of each other. Nest height above ground ranges from directly on the ground up to about 3 feet high. Placing the nest close to the ground helps conceal it from predators.
Nest Construction
Nest building is done primarily by the female black-throated sparrow over a period of 2-6 days. Materials used include:
- Grasses
- Stems
- Roots
- Bark strips
- Spider webs
- Hair
- Feathers
The outer part of the nest is made with coarser materials like grass, twigs, and bark woven together. This provides the structural support. The inner cup where the eggs rest has a softer lining of fine grasses, hair, and feathers.
The male may accompany the female while she builds the nest and may contribute some material, but the majority of construction is done by the female. She weaves the materials together to form the nest cup shape while turning in a circle.
Nest building occurs in late winter through spring, with nesting season varying across the black-throated sparrow’s range:
- January-June in Arizona
- February-August in Texas
- March-July in California
Several nests may be built before one is selected for use. Nests are reused for subsequent broods or in following years.
Clutch Size
Once the nest is completed, the female black-throated sparrow lays a clutch of 3-6 eggs, with 4 being typical. The eggs are pale blue or greenish-white with brown spotting.
The female lays one egg per day over a span of several days. She does not start incubating the eggs until the clutch is complete, so all the eggs hatch around the same time. This helps ensure that all the nestlings develop at the same rate.
Some key facts about black-throated sparrow clutches:
- Clutch size ranges from 3-6 eggs
- The average clutch size is 4 eggs
- Eggs are laid at a rate of 1 per day
- Incubation starts when the last egg is laid
- Incubation period is 11-14 days
Both parents take turns incubating the eggs. They develop a brood patch on their breast and belly feathers where blood vessels increase blood flow to provide warmth to the eggs. The parents rotate duties so each one takes a shift warming the eggs while the other finds food.
Nestling Period
The eggs hatch asynchronously over a period of several hours. Both parents feed and care for the hatchlings, who are born naked and helpless. For the first week, the nestlings are brooded by the parents to help regulate their body temperature.
The nestling period (time in the nest) lasts about 9-12 days. The chicks grow quickly, developing feathers and gaining mass. Around day 8, their eyes open. At the end of the nestling period, the young are ready to leave the nest and follow their parents to forage on their own.
Key facts about the black-throated sparrow nestling stage:
- Hatchlings are altricial (helpless) and rely on parents
- Eyes open by day 8
- Brooded frequently the first week
- Fledge from nest at 9-12 days old
- Fledglings follow parents and are fed for several more weeks
The young sparrows leave the nest before they can fly well. They hop out of the nest and follow their parents on foot. The parents continue to feed and care for the fledglings for 2-3 more weeks as they gain independence.
Parental Care
Male and female black-throated sparrows share parental duties of incubating the eggs and feeding the young. The female alone builds the nest and incubates at night, but the male assists with incubation and all other care.
Key parental care behaviors include:
- Nest building (female only)
- Incubating eggs
- Brooding hatchlings
- Feeding nestlings
- Defending the nest
- Leading fledglings to food
Both parents feed the nestlings a diet of insects, seeds, and berries. Nestlings may be fed over 100 times per day. Even after leaving the nest, fledglings still beg from their parents and follow them to learn where to forage.
Constant vigilance is required to keep predators like snakes, coyotes, and hawks from raiding nests. Parents may perform distraction displays to draw predators away. They continue to protect and feed the young for several weeks until they are independent.
Nesting Success
Only about 30-60% of black-throated sparrow nests successfully produce young that leave the nest. Nest failure can occur due to:
- Predation
- Weather events
- Infertile eggs
- Death of parents
- Competition for resources
- Parasitism by brown-headed cowbirds
The hot, arid environment makes nesting challenging. Drought conditions reduce food and water availability which impacts breeding success. Sparrows may abandon nests or young during periods of extreme heat or drought.
Predation is a major cause of nest failure, even though the low nests are well hidden. Common nest predators include:
- Snakes – gopher snakes, kingsnakes
- Coyotes
- Rodents
- Small mammals
- Cowbirds (nest parasitism)
- Hawks
- Ravens
Nesting success improves when nests are closer together in a colony, as the birds can cooperate to mob predators. Ideal habitat with plenty of cover and food also boosts nest productivity.
Adaptations for Nesting
Over generations, black-throated sparrows have evolved several behavioral and physical adaptations that improve their nesting and breeding in desert environments, including:
- Placing nests close to ground to conceal from predators
- Building nests quickly with available materials
- Nesting in protected nooks in shrubs or rocks
- Colonial nesting for cooperative defense
- Mottled plumage to blend in to surroundings while brooding
- Asynchronous hatching to optimize chick survival
- Efficient metabolic and water regulation systems
- Ability to enter torpor during extreme heat or cold
- Brood patch for transferring heat to eggs
These adaptations allow black-throated sparrows to conserve energy, hide from predators, regulate temperature, and maximize reproductive success in challenging desert conditions.
Range and Habitat
The breeding habitat and nesting locations used by black-throated sparrows depends somewhat on where they are within their range:
Arizona
Black-throated sparrows are common summer residents of southeastern Arizona. There they nest in desert grasslands and scrub with scattered yucca and mesquite trees. Nest sites are typically located at the base of mesquite or desert willow trees.
California
In California, black-throated sparrows nest in desert washes and canyons with sagebrush and other arid shrubs. Their nests are built on the ground beneath shrubs or in rock crevices. Yucca plants provide cover for nests in the Mojave Desert.
Nevada
Black-throated sparrows are rare summer residents in southwest Nevada. There they nest solitarily or in loose colonies in sagebrush flats adjacent to springs or creeks. The nests are placed under clumps of grass or at the base of sagebrush.
New Mexico
In New Mexico, these sparrows are locally common nesters in the southern and western parts of the state. They nest in arroyos and dry canyons within pinyon-juniper woodlands, as well as desert scrublands. Nests are built low in shrubs along the canyon walls.
Texas
Black-throated sparrows breed throughout much of west and south Texas. There they nest in desert grasslands, oak savannas, and chaparral scrub. Favored nest sites are at the base of mesquite, acacia, or yucca plants. Some nest colonially along fencelines.
Mexico
In Mexico, black-throated sparrows nest in very arid desert scrub and thorn forests. Nests are placed on or near the ground in trees like mesquite, acacias, and yucca. They are found breeding in northwestern Mexico south to San Luis Potosi.
Conclusion
Black-throated sparrows are well adapted to nesting and breeding in the harsh, arid climates of the southwestern U.S. and northern Mexico. Their nesting habits and behaviors reflect adaptations for maximizing reproductive success in desert environments. Key features include concealed ground nests, colonial nesting, efficient temperature regulation, and reliance on desert plants for cover. Their resilience and adaptations allow black-throated sparrows to thrive as year-round residents across the arid southwest.