The shiny cowbird (Molothrus bonariensis) and the Brown-headed cowbird (Molothrus ater) are two similar looking species of brood parasitic birds in the family Icteridae. The females of these two species can be difficult to distinguish visually in the field. However, there are some key differences between female shiny cowbirds and female Brown-headed cowbirds in terms of physical appearance, behavior, habitat, and range that can help identify them.
Physical Appearance
In general, female shiny cowbirds tend to be smaller and slimmer than female Brown-headed cowbirds. Female shiny cowbirds reach a body length of about 7.5-8.5 inches compared to around 8-10 inches for female Brown-headed cowbirds.
The plumage coloration is one of the most noticeable differences between the two species. Female shiny cowbirds have mostly dark brown plumage on the head, back, wings and tail. The underside is pale gray-brown. Female Brown-headed cowbirds on the other hand have dark gray-brown plumage on the head, neck, back and breast, while the underside is pale tan to buff.
When perched, the tail shape can also help distinguish the two species. Female shiny cowbirds have longer, more graduated tails compared to the shorter, squared off tails of female Brown-headed cowbirds.
Additionally, female shiny cowbirds have pale pinkish legs and feet, while female Brown-headed cowbirds have darker gray legs and feet. The bill shape also differs slightly – female shiny cowbirds have thinner, more pointed bills than the thicker bills of female Brown-headed cowbirds.
Behavior
In terms of behavior, female shiny cowbirds are generally more active foragers that spend more time walking on the ground searching for food compared to female Brown-headed cowbirds which do more perched waiting to ambush prey.
Female shiny cowbirds have a faster, more direct flight pattern with quicker wing beats compared to female Brown-headed cowbirds which have a slower, bound-and-glide flight style.
When it comes to breeding habits, female shiny cowbirds are more selective in choosing host nests to lay their eggs. They typically parasitize only one species or family of birds in a given area. Female Brown-headed cowbirds on the other hand are less discerning and will parasitize a wider variety of host species.
Additionally, female shiny cowbirds typically lay just 1 egg per host nest, while female Brown-headed cowbirds often lay multiple eggs in a single host nest. Female shiny cowbirds also have a shorter breeding season that peaks earlier in spring compared to Brown-headed cowbirds.
Habitat
The preferred habitats of these two brood parasites also differ. Female shiny cowbirds mainly inhabit open grassland, pasture, and agricultural areas. They particularly thrive in areas with livestock grazing.
Female Brown-headed cowbirds are more of a forest-edge generalist that inhabits semi-open areas like woodland edges, small forest openings, suburban parks, and backyards. They do well in more fragmented, patchy landscapes.
So if you encounter a female cowbird in open ranchlands or grasslands, it is much more likely to be a shiny cowbird than a Brown-headed cowbird which would be rare in those habitats.
Range
The geographic breeding ranges of these species also offer a good clue for identification. Female shiny cowbirds exclusively occur in South America, parts of Central America, Mexico, and the Caribbean islands.
Meanwhile, female Brown-headed cowbirds primarily breed in most of North America from southern Canada to Mexico, apart from the southwestern United States.
So a female cowbird spotted north of Mexico is almost certainly going to be a Brown-headed cowbird, while any sightings south of Mexico indicate a shiny cowbird. The ranges do overlap in a small area of western Mexico where identification can be more challenging.
Distinguishing Features Summary
Feature | Female Shiny Cowbird | Female Brown-headed Cowbird |
---|---|---|
Size | Smaller and slimmer | Larger and bulkier |
Plumage Color | Mostly dark brown | Dark gray-brown head, neck, back and breast with pale tan to buff underside |
Tail Shape | Longer, graduated | Shorter, squared off |
Legs and Feet | Pale pinkish | Dark gray |
Bill Shape | Thinner, pointed | Thicker |
Foraging Behavior | More active, terrestrial | More perch waiting |
Flight Style | Faster, direct with quicker wingbeats | Slower, bound and glide |
Breeding Habits | More selective, lays 1 egg per nest | Less selective, lays multiple eggs per nest |
Peak Breeding | Earlier in spring | Later in spring through summer |
Habitat | Open grasslands and ranchlands | Woodland edges, semi-open areas |
Range | South America, Central America, Mexico, Caribbean | North America except SW United States |
Identification Tips
Here are some helpful tips for identifying whether you’re looking at a female shiny cowbird or Brown-headed cowbird:
Size and Shape
The smaller, slimmer body, long graduated tail, and pointed bill indicate shiny cowbird. The larger, stockier body, short blunt tail, and thick bill point to Brown-headed cowbird.
Plumage
Uniformly dark brown plumage suggests shiny cowbird, while dark head and back with pale tan underside indicates Brown-headed cowbird. Also look at the legs – pale pinkish legs indicate shiny cowbird while gray legs suggest Brown-headed.
Behavior
A cowbird walking on the ground actively foraging is likely a shiny cowbird. A cowbird perched waiting and watching is more likely a Brown-headed. Also look for differences in flight style.
Habitat
If seen in open grassland or ranchland, it’s almost certainly a shiny cowbird. Woodland edges and semi-open areas indicate Brown-headed cowbird.
Range
Any sightings south of Mexico are shiny cowbird. North of Mexico, Brown-headed cowbird becomes much more likely, though ranges can overlap in western Mexico.
Song and Vocalizations
The songs and calls of shiny and Brown-headed cowbirds also differ, so listening for vocalizations can aid identification. But visually, following the plumage, size, shape, behavior, habitat, and range clues above will help distinguish between female shiny and female Brown-headed cowbirds.
Conclusion
In summary, female shiny and Brown-headed cowbirds have several distinct physical, behavioral, habitat, and range differences that allow observant birders to identify them. Factors like size, plumage color patterns, tail and bill shape, foraging behavior, breeding habits, preferred habitat, and geographic distribution provide diagnostic clues on which species you are observing once you are familiar with their features. With a little practice, picking out a female shiny versus a female Brown-headed cowbird can become a straightforward process. The next time you see a female cowbird, be sure to look closely and run through these identification points to determine which remarkable brood parasite you’ve encountered.