The Invader

Florida has the distinction of being the epicenter of invasion of non-native reptiles, introduced by collectors who intentionally or unintentionally let non-natives escape, or by accident when eggs or small hatchling reptiles are carried into this country on imported plants.

The Cuban Brown Anole is a 5-9 inch slim lizard, marked with a diamond-shaped pattern on its back. It typically rests in low vegetation, waiting for unsuspecting insects — or other, smaller lizards — to walk by, and then quickly gobbles them up.

Successive invasions of Brown Anoles from Cuba and the Bahamas since the late 1800s have resulted in well established populations throughout Florida that have since moved north and west to Georgia, Louisiana, and Texas, and have also resulted in an apparent coincident decrease in the population of native Green Anoles.

Brown Anoles now occupy the tree trunk-ground niche in their hunt for prey, and occur in very high density in residential shrubbery (sometimes 4-6 to a single bush).
Female Brown Anoles (recognized by the brown stripe down the middle of their back) lay 1-2 eggs in leaf litter or potted plants every few days. Eggs hatch 4-6 weeks later, and the 1-inch long hatchlings move into the vegetation to hide from larger individuals that might eat them.

However, rather than disappearing altogether as a result of the Brown Anole invasion, it may be that Green Anoles simply move out of the low vegetation and up into the tree tops where they can more favorably compete for food and avoid being eaten by the more aggressive Brown Anole.

A model of coexistence?

As a general rule, complete competitors in the natural world cannot coexist, unless they divide up the resources somehow, by hunting in different areas or at different times, or specializing on slightly different food items in their diet.

On the wet prairies and marshes at Cheyenne Bottoms and Quivira National al Wildlife Refuge near Great Bend, Kansas, we saw several Short-eared Owls and and as many Northern Harriers, not only hunting in the same areas and habitats, but at the same time of day!

A Short-eared Owl landed in the grass after an unsuccessful prey capture. Some studies show that owls are successful capturing mice in open grasslands about 44% of the time.
Short-eared Owls traverse the fields in short, coursing flights, searching the same area intensively and listening for mouse activity. Long wings make gliding over the habitat seem effortless.
Northern Harriers make rapid flights across longer distances than the owls, turning infrequently and flying quickly. If they detect movement, they may hover over a spot, honing in on the prey.
Short-eared Owls hunt by sound, even though they are usually hunting in daylight. Their facial disks collect the infra-sounds that active animals make like a parabolic reflector and relay it to their ears. The owls consume mostly rodent prey, especially while feeding young.
Head down is the way we usually see Harriers hunting. Unlike other hawks, Harriers use hearing as well as vision to find their prey. Rodents make up a big part of their diet as well, but they also eat large insects, amphibians, and reptiles, as well as birds.

One study of niche overlap of Short-eared Owls and Northern Harriers in Utah found that their home ranges overlapped as much as 70%, but within their home range the two raptors hunted and nested in different parts. In addition, in this particular study, the owls tended to hunt most intensively in the mid-afternoon, while the hawks concentrated their hunting in the evening hours before dusk. So, even though the two raptors seem like complete competitors, they seem to have found a way to share the resources cooperatively…usually.

However, Sean Crane (500 pix) photographed a Harrier and a Short-eared Owl fighting over a vole in Washington — which is evidence that the two don’t always get along.

odd couples

When they’re not competing for food or space (nest holes, etc.), different species of birds sometimes pair up in odd couples, seemingly coexisting without much ado.  It’s as if they either don’t recognize their differences or don’t care about them.  Hmm…wonder if there is a lesson there for us?

Northern Pintail-American Wigeon

A couple of Northern Pintail and American Wigeon males swam around together in a shallow lake near Brownsville, Texas last January.  Ducks often form rafts of mixed species when they flock up on their wintering grounds.

lazuli bunting-black-headed grosbeak

A Lazuli Bunting and a Black-headed Grosbeak are perfectly happy to share the bird feeder.  These distinctly different looking species (both brightly colored males) are both members of the Cardinal family and they overlap in both their breeding ranges and their wintering areas.

But where there is competition for food or nesting areas, aggressive threats or attacks often ensue, even between individuals of the same species.  Close proximity is not tolerated, and everyone gets hyperactive and flighty.  It’s not a lot different than what happens in human societies living in crowded conditions.

Red-winged blackbirds

Red-winged blackbirds engage in all-out battles with each other over monopoly of a food source.

harris hawk- crested caracara interaction

The postures exhibited in this interaction between an adult Harris hawk and a juvenile Crested Caracara tell the story: the hawk owns this meat, and open-beak threats make the caracara back off in submission.

Will it be competition or coexistence between two species or two individuals — odd couple or fierce combatants?