Flocking Together

Birds Hanging Around Mammals for an Easy Meal

This bird looks a lot like the one from my very first story, the snowy egret, Egretta thula. The two species are similar in both size and coloration, with some variations. The bird above has an orange bill, while the snowy egret bill is black. Leg color can vary, but only the snowy egret has the distinctive yellow feet. You can also see the orange-pink breeding plumage in the above picture, which is white in snowy egrets. It can be easy to confuse these two, but only the above bird has made a specific symbiotic relationship a large part of its existence.

This is a cattle egret, Bubulcus ibis, and despite its name and similar appearance, it is actually more closely related to herons in the genus Ardea (this includes one of my favorite birds, the great blue heron, Ardea herodias) than to egrets such as the snowy. It is also a relatively recent arrival in North America.

I have previously written about non-native species, especially those in Florida. However, the cattle egret is not a typical example, as its migration and expansion has been entirely natural and not aided by people either intentionally or otherwise (though human activity may have increased the overall niche space available to them). Cattle egrets established themselves permanently in the Americas in the 1930s, eventually expanding northward and establishing breeding populations in Florida in the 1950s.

Previously, these birds were native to Spain, Portugal, and subtropical Africa and Asia. However, they were able to cross the Atlantic Ocean via flight and establish themselves in the Americas. As you might guess from their name, cattle egrets tend to associate with livestock. However, this behavior is not specialized and can be seen with many species of grazing mammals, both wild and domestic.

That egret is with a Cape buffalo, Syncerus caffer, an African species that has never been domesticated and is only distantly related to domestic cattle. As more areas of the world were farmed or cleared for livestock, cattle egrets found more places in which they could thrive.

The nature of their relationship with grazing animals is reflected in the cattle egret diet. The majority of its heron and egret relatives rely on aquatic food sources (usually small freshwater fish, but occasionally also in tidal areas like the snowy egret in my first story linked above). However, cattle egrets instead tend to rely on a diet of terrestrial insects and other arthropods, like the centipede in the picture below.

This prey preference means that cattle egrets are often found in fields or dry grassy areas, the same areas that large mammals use for grazing. There is still some debate as to what type of symbiotic relationship exists between cattle and cattle egrets. The movements of grazing cattle can stir up insect prey. This makes foraging easier on the egret, with no effect on the cattle. This would be another example of a commensal relationship. However, there is some evidence of mutualism as well. Like other species of birds, the egrets can directly remove ticks and other parasites from the cattle, giving the cattle a benefit in exchange for its meal.

Commensalism will likely always be the most debated form of symbiosis. Several of the examples in the last four weeks have shown some potential positive or negative effect on the other organism in the relationship. However, we’re moving on from commensalism now and into a much more concrete type of relationship. Next week will be the first of four stories on parasitism and parasites. I hope you all enjoy them.