Brown-headed Cowbird
Molothrus ater
Brown-headed Cowbirds are a common sight in Oregon, particularly in open and semi-open habitats. Like Red-winged Blackbirds, they exhibit clear sexual dimorphism, making it relatively easy to distinguish males from females.
Male Brown-headed Cowbirds in Oregon:
Overall Color: Males have a striking appearance, with a body that is a glossy, iridescent black. This black can sometimes show subtle greenish or purplish reflections in good light.
Head Color: The most defining feature is their rich, chocolate brown head. This brown color often extends down to the nape of the neck and upper breast. In certain lighting, or from a distance, the brown head can sometimes appear almost black.
Bill: They have a relatively short, conical, and stout black bill.
Eyes: Dark eyes.
Size: Males are slightly larger than females.
Behavior: During the breeding season, males are often seen displaying, puffing up their feathers, bowing, and making their distinctive gurgling and bubbling calls.
Female Brown-headed Cowbirds in Oregon:
Overall Color: Females are much more subdued in coloration, primarily a plain, dull grayish-brown overall. This makes them less conspicuous, which is important for their brood parasitic lifestyle.
Underparts: Their underparts may have faint streaking, and their throat is often a lighter, whitish color.
Head: The head is generally the same plain brown as the body, lacking the distinct dark brown cap of the male. They often have a plain dark eye.
Bill: Similar to the male, they have a short, stout, conical bill, though it might appear slightly less robust than the male's.
Size: Females are smaller than males.
Behavior: Females are known for their brood parasitic behavior, laying their eggs in the nests of other bird species. They are often more secretive as they search for host nests.
Key Identification Points:
Color Contrast (Males): The stark contrast between the glossy black body and the brown head is the key for male identification.
Plainness (Females): The uniform, unstreaked (or very finely streaked) grayish-brown plumage is characteristic of females.
Bill Shape: Both sexes share a distinctive, relatively short and thick, finch-like bill, which differentiates them from other blackbirds with longer, more pointed bills.
Size and Shape: Cowbirds are generally chunkier and have shorter tails than many other blackbird species.
Juvenile Brown-headed Cowbirds resemble the females, being a dull brown with some streaking, before developing their adult plumage.
Brown-headed Cowbirds are notorious for their unique and highly impactful nesting habits, especially in Oregon and across North America. They are obligate brood parasites, which means they never build their own nests, incubate their own eggs, or raise their own young. Instead, they rely entirely on other bird species, known as "host" species, to do all the work for them.
1. Brood Parasitism:
No Nest Building: This is the defining characteristic. A female Brown-headed Cowbird will not construct any nest whatsoever.
Egg Laying: She will discreetly lay her eggs in the nests of other unsuspecting bird species. She's remarkably stealthy, often laying her egg very early in the morning before the host bird is actively on the nest.
Egg Removal: Before laying her own egg, the female cowbird often removes one or more of the host's eggs. This helps ensure her egg has a better chance of being incubated and reduces competition for food for her future chick.
2. Finding Host Nests:
Female cowbirds are adept at locating active nests. They often watch other birds during their nest-building activities from a hidden perch, or quietly search through vegetation.
They are highly generalized in their host choice, having been documented to parasitize over 220 different species of North American birds, and successfully raised by at least 139.
3. Breeding Season in Oregon:
The egg-laying period for Brown-headed Cowbirds in Oregon generally extends from April to July, with a peak typically in May and June.
A single female can lay a surprising number of eggs in a season, though not all will result in a successfully fledged cowbird. Estimates range from a few dozen to as many as 40 or even 70 eggs in a season, spread across many different host nests.
4. Cowbird Egg Characteristics:
Cowbird eggs are typically whitish to grayish-white with brown or gray spots or streaks, often concentrated at the larger end.
They can vary in size, but are often similar in size to the eggs of their host, especially for larger hosts like cardinals. For smaller hosts, the cowbird egg can be noticeably larger.
5. Advantages for Cowbird Chicks:
Shorter Incubation Period: Cowbird eggs typically have a shorter incubation period (around 10-12 days) compared to most host species. This means the cowbird chick usually hatches first, giving it a head start.
Rapid Growth: Cowbird nestlings grow much faster than their host nestmates. This allows them to quickly become larger and more dominant in the nest.
Begging Behavior: Cowbird chicks are known for their vigorous begging calls and often have a bright red "gape" (the corners of their mouth when open), which can stimulate the foster parents to feed them more frequently.
Competitive Edge: The larger size and aggressive begging often mean the cowbird chick outcompetes the host's own young for food, leading to starvation, crushing, or ejection of the host's offspring. It's common to see a much smaller adult bird tirelessly feeding a much larger cowbird fledgling.
6. Impact on Host Species in Oregon:
Brown-headed Cowbirds are considered a significant conservation concern for some bird species, especially those with already declining populations or those that haven't evolved defenses against cowbird parasitism. Their spread into new areas, often facilitated by habitat fragmentation and the creation of "edge" habitats (forest clearings, agricultural land, suburban areas), has exposed new host species to their parasitic behavior.
Host Responses: Some host species have evolved defenses:
Egg Rejection: Some birds, like Yellow Warblers, can recognize cowbird eggs and will either eject them from the nest or build a new nest on top of the parasitized clutch.
Nest Abandonment: In some cases, hosts may abandon a parasitized nest altogether.
Acceptance: Many species, however, do not recognize the foreign egg and raise the cowbird chick as their own, often at the expense of their own young. There is some evidence that some species might accept the eggs to avoid "punishment" by the cowbird (where the cowbird may destroy the nest if its egg is rejected).
In Oregon, Brown-headed Cowbirds are common breeders in open habitats, woodland edges, agricultural areas, sagebrush, juniper steppe, coastal scrub, riparian zones, and suburban areas. Their adaptability and generalist parasitic strategy have made them a widespread and impactful species in the state's avian ecology.
Male Brown-headed Cowbirds in Oregon:
Overall Color: Males have a striking appearance, with a body that is a glossy, iridescent black. This black can sometimes show subtle greenish or purplish reflections in good light.
Head Color: The most defining feature is their rich, chocolate brown head. This brown color often extends down to the nape of the neck and upper breast. In certain lighting, or from a distance, the brown head can sometimes appear almost black.
Bill: They have a relatively short, conical, and stout black bill.
Eyes: Dark eyes.
Size: Males are slightly larger than females.
Behavior: During the breeding season, males are often seen displaying, puffing up their feathers, bowing, and making their distinctive gurgling and bubbling calls.
Female Brown-headed Cowbirds in Oregon:
Overall Color: Females are much more subdued in coloration, primarily a plain, dull grayish-brown overall. This makes them less conspicuous, which is important for their brood parasitic lifestyle.
Underparts: Their underparts may have faint streaking, and their throat is often a lighter, whitish color.
Head: The head is generally the same plain brown as the body, lacking the distinct dark brown cap of the male. They often have a plain dark eye.
Bill: Similar to the male, they have a short, stout, conical bill, though it might appear slightly less robust than the male's.
Size: Females are smaller than males.
Behavior: Females are known for their brood parasitic behavior, laying their eggs in the nests of other bird species. They are often more secretive as they search for host nests.
Key Identification Points:
Color Contrast (Males): The stark contrast between the glossy black body and the brown head is the key for male identification.
Plainness (Females): The uniform, unstreaked (or very finely streaked) grayish-brown plumage is characteristic of females.
Bill Shape: Both sexes share a distinctive, relatively short and thick, finch-like bill, which differentiates them from other blackbirds with longer, more pointed bills.
Size and Shape: Cowbirds are generally chunkier and have shorter tails than many other blackbird species.
Juvenile Brown-headed Cowbirds resemble the females, being a dull brown with some streaking, before developing their adult plumage.
Brown-headed Cowbirds are notorious for their unique and highly impactful nesting habits, especially in Oregon and across North America. They are obligate brood parasites, which means they never build their own nests, incubate their own eggs, or raise their own young. Instead, they rely entirely on other bird species, known as "host" species, to do all the work for them.
1. Brood Parasitism:
No Nest Building: This is the defining characteristic. A female Brown-headed Cowbird will not construct any nest whatsoever.
Egg Laying: She will discreetly lay her eggs in the nests of other unsuspecting bird species. She's remarkably stealthy, often laying her egg very early in the morning before the host bird is actively on the nest.
Egg Removal: Before laying her own egg, the female cowbird often removes one or more of the host's eggs. This helps ensure her egg has a better chance of being incubated and reduces competition for food for her future chick.
2. Finding Host Nests:
Female cowbirds are adept at locating active nests. They often watch other birds during their nest-building activities from a hidden perch, or quietly search through vegetation.
They are highly generalized in their host choice, having been documented to parasitize over 220 different species of North American birds, and successfully raised by at least 139.
3. Breeding Season in Oregon:
The egg-laying period for Brown-headed Cowbirds in Oregon generally extends from April to July, with a peak typically in May and June.
A single female can lay a surprising number of eggs in a season, though not all will result in a successfully fledged cowbird. Estimates range from a few dozen to as many as 40 or even 70 eggs in a season, spread across many different host nests.
4. Cowbird Egg Characteristics:
Cowbird eggs are typically whitish to grayish-white with brown or gray spots or streaks, often concentrated at the larger end.
They can vary in size, but are often similar in size to the eggs of their host, especially for larger hosts like cardinals. For smaller hosts, the cowbird egg can be noticeably larger.
5. Advantages for Cowbird Chicks:
Shorter Incubation Period: Cowbird eggs typically have a shorter incubation period (around 10-12 days) compared to most host species. This means the cowbird chick usually hatches first, giving it a head start.
Rapid Growth: Cowbird nestlings grow much faster than their host nestmates. This allows them to quickly become larger and more dominant in the nest.
Begging Behavior: Cowbird chicks are known for their vigorous begging calls and often have a bright red "gape" (the corners of their mouth when open), which can stimulate the foster parents to feed them more frequently.
Competitive Edge: The larger size and aggressive begging often mean the cowbird chick outcompetes the host's own young for food, leading to starvation, crushing, or ejection of the host's offspring. It's common to see a much smaller adult bird tirelessly feeding a much larger cowbird fledgling.
6. Impact on Host Species in Oregon:
Brown-headed Cowbirds are considered a significant conservation concern for some bird species, especially those with already declining populations or those that haven't evolved defenses against cowbird parasitism. Their spread into new areas, often facilitated by habitat fragmentation and the creation of "edge" habitats (forest clearings, agricultural land, suburban areas), has exposed new host species to their parasitic behavior.
Host Responses: Some host species have evolved defenses:
Egg Rejection: Some birds, like Yellow Warblers, can recognize cowbird eggs and will either eject them from the nest or build a new nest on top of the parasitized clutch.
Nest Abandonment: In some cases, hosts may abandon a parasitized nest altogether.
Acceptance: Many species, however, do not recognize the foreign egg and raise the cowbird chick as their own, often at the expense of their own young. There is some evidence that some species might accept the eggs to avoid "punishment" by the cowbird (where the cowbird may destroy the nest if its egg is rejected).
In Oregon, Brown-headed Cowbirds are common breeders in open habitats, woodland edges, agricultural areas, sagebrush, juniper steppe, coastal scrub, riparian zones, and suburban areas. Their adaptability and generalist parasitic strategy have made them a widespread and impactful species in the state's avian ecology.