The Case of the Continent-Hopping Cattle Egret

As I write this, Eurasian Collared-Doves call outside the window, Rock Pigeons flap overhead, and European Starlings lurk around a nest box I intended for screech-owls.  Not too far away, though thankfully not in my yard, noisy flocks of House Sparrows bully native birds, driving them out of potential nest sites.  And so the story goes across nearly every city and town in North America, a tale of competition and strife as ecosystems attempt to assimilate non-native birds introduced to these shores from distant lands.

At the edge of town, Cattle Egrets roost together on top of houses situated along a grassy park.

Some estimates put the number of non-native species of birds that have established self-sustaining populations in North America near one hundred.  These introduced species run the gamut from fairly benign species with localized populations, like the beautiful and beloved Rosy-faced Lovebirds of Phoenix, Arizona, to widespread, much-despised and downright destructive species, like the aggressive and invasive European Starling.

Invasive species are classified as those that have a drastic detrimental effect on native species.  Non-native birds may become invasive when they out-compete native species for food resources and nesting sites, some invasive species even going as far as to kill native species.  For example, the aggressive House Sparrow, introduced to North America from Europe in the 1800's, not only drives out but also kills native bluebirds to take over nesting cavities.  Species that are considered invasive also may become pests, destroying crops (starlings), making messes in cities (pigeons) and transmitting disease.

Invasive species often become so because of a disruption in the ecosystem's natural predator-prey balance.  A species with few or no natural predators in the ecosystem to which it was introduced lacks the natural check on its population.  When this is coupled with favorable environmental conditions and a species' natural tendency to be a strong competitor, through opportunistic feeding preferences and an aggressive nature, the populations of certain introduced species have the potential to skyrocket out of control in a surprisingly short period of time.

Not all introduced species become invasive, however.  The Cattle Egret (Bubulcus ibis), originally an African bird, is one such example.

Cattle Egrets are frequently found in grasslands - and grassy neighborhood parks - far from water.

While most of the species mentioned above - and indeed most invasive species in general - were introduced by humans, either intentionally (in the case of House Sparrows) or accidentally (in the case of escaped pet doves), Cattle Egrets arrived in North America by their own power.  In one of the great tales of avian range expansion in modern times, Cattle Egrets made their way from the grasslands of Africa to northeastern South America in the late 1800's, a distance of about 2,000 miles.  From South America, they reached Florida in the mid-1900's and were first recorded breeding there in 1953.  (Read Roger Tory Peterson's wonderful book, Wild America, for his description of Florida's breeding egrets and herons in the 1950's and predictions for the future of the Cattle Egret!)  By the 1960's, Cattle Egrets reached the West Coast of the United States, and were breeding in California by the end of the 1970's.

While it is likely that this stocky member of the heron family (Ardeidae) does compete with native species for nesting sites in some areas, the negative effects do not appear to be very great.  Cattle Egrets may even be a beneficial addition to North American avifauna, as they consume large quantities of flies, ticks and other insect pests that otherwise plague cattle and other grazing animals.

An adult Cattle Egret in March, showing its lovely breeding plumage.

Unlike other herons and egrets, which tend to favor areas of habitat near water, Cattle Egrets are commonly found in grasslands, both wet and dry.  Contributing to their success in new lands, Cattle Egrets are not too particular about what they eat.  Along with insects and other invertebrates disturbed by the feet of cattle, tractors and even wildfires, Cattle Egrets also prey on small fish, amphibians, birds and mammals.  One account tells of Cattle Egrets picking off exhausted warblers that have made landfall on the Dry Tortugas during their arduous migration.

Common earthworms found in the lawns of city parks do quite well for Cattle Egrets.

Considered to be the most gregarious members of the heron family (a characteristic which has no doubt contributed to the species success), Cattle Egrets nest colonially in trees, shrubs or marsh vegetation, in wetlands as well as uplands.  And a nesting colony of egrets is a sight to be seen - and a sound to be heard!  After the breeding season and throughout the rest of the year, Cattle Egrets stay together in large flocks, roosting communally at night and flying out to forage together during the day.

A small part of the neighborhood flock, roosting communally on an obliging rooftop.

Cattle Egrets have proven themselves to be strong fliers, and as such are highly migratory.  Many of the Cattle Egrets that breed in North America spend the winter months in Mexico and Central America, but those in Florida, along the Gulf Coast, and in parts of California stick around throughout the year.  The San Joaquin Valley is home to Cattle Egrets year-round, and for at least the last few winters a flock of over one hundred has been spending the winter months in a grassy strip of parkland along the edge of my town.

An adult Cattle Egret shows its buff-colored breeding plumage.

The Cattle Egret may be easily distinguished from our other white egrets - namely, the Great Egret and Snowy Egret - by its stockier overall appearance and thick yellow bill.  During the breeding season, the Cattle Egret's black legs turn yellow, and the feathers on its head, chest and back develop a lovely buff color.  Its preference for dry habitat and tendency to gather in large flocks at all times of the year are additional clues to its identity.