It’s one of the most recognizable symbols in conservation: the flying Great Egret, symbol of the National Audubon Society, devised when the organization was lobbying to curtail the millinery trade. Most of you have probably also seen a live Great Egret as well, but have you stopped to think about how these essentially southern birds persist in the cold environs of the Midwest? It’s not obvious why they among all the egrets should make it here.
When I first moved to central Ohio, I was startled to find Great Egrets here. It’s a quintessential southern bird, especially to someone who grew up in Florida. Great Egrets are abundant in the southeastern US: estimates place 75% of the population there, and they have lots of swamps, shallow bays, and slow rivers to hunt. However, most of the rest of our Great Egrets are scattered in an arc up the Mississippi and Ohio River valleys, up to the Great Lakes. Why do Great Egrets survive here, while Snowy Egrets and other medium-sized herons have difficulty? Great Egrets have devised a series of adaptations to finesse the usual heron problems.
For food, Great Egrets have some unusual restrictions that they’ve overcome. They have an unusual neck anatomy – their esophagus is twisted behind their spinal column to give it extra protection and flexibility, but this forces the birds to restrict their food to smaller fish, crayfish, and amphibians. You’ll never see a Great Egret tussle with big fish as will Great Blue Herons. To compensate, Great Egrets use a variety of stalking and striking techniques, from the statuesque “neck outstretched” to the “crouching S”. They’re also not picky; they’ll take virtually any swimming creature less than five inches.
For foraging, Great Egrets need still, shallow water, something that can be scarce in the Midwest. They increase their odds by nesting and roosting on islands in shallow lakes and bays. If the island can persist, the nearby waters must have some shallow, calm areas. Also, they’re very social about feeding areas – they look for white birds standing in the water when they search for feeding locations. If another Great Egret is already there, it’s likely a decent spot. This “white beacon” strategy is so good that other small herons will gather in the same areas, resulting in the big mobs of white egrets in pools in the Southern US and Mississippi Valley.
Of course, feeding in still, shallow water has some drawbacks. This is the first water to silt up, so last year’s great site is this year’s mediocre site. Drought can also eliminate this shallow water. This means that reproduction is a chancy affair, with unpredictable food levels. Great Egrets confront this problem by laying each of their 3-5 eggs at one- or two-day intervals, guaranteeing a range of nestling sizes. The older ones will get most of the food unless conditions are good. Indeed, the older ones are often aggressive towards their youngest siblings if food levels get low, forcing the smaller birds into starvation and death unless conditions improve.
Shallow water is also the first water to freeze up in fall, so Great Egrets need to move when it gets cold. They’re migrants, flying along rivers, usually early in the morning. This means virtually all of our Midwestern herons move down the Ohio and Mississippi Valleys in fall to the warmer climes of the Gulf States in the winter. The timing of these movements can vary somewhat; I’ve seen migrating groups from August through October, with even a few stragglers into November.
When they migrate back in the spring, Great Egrets are among the earliest wading birds to return, usually in late March or April. The adults look for appropriate islands for rookeries, preferably with Great Blue Herons already nesting there. Breeding Great Egrets develop long lacey plumes down their back, used during ritual mating displays. They also change their lore color (area between the bill and the eyes) to an intense lime-green. The presence of such showy birds in June is often the clue that a rookery is nearby.
Despite all these tricks to extend their range into the upper Midwest, Great Egrets are not common here. There are only a few rookeries in our area that attract Egrets, and most of them are around Lake Erie. Ohio has colonies in Sandusky Bay and on West Sister Island, while neighboring Michigan has colonies at Point Mouille and the Rouge River, as well as several colonies around Saginaw Bay on Lake Huron. Inland colonies are rather rare—we have one in the Campbell Mound Quarry here in Columbus, and Michigan has a few, but Indiana has none. Inland colonies can do well if conditions are right; Great Egret numbers are strong in the large mixed heron colonies along the Mississippi River, especially in the American Bottoms area east of St. Louis and Reelfoot Lake in Tennessee.
What keeps Great Egrets from becoming more common here in Ohio? Three major problems keep cropping up. The major one is simply lack of habitat; shallow, still waters describe many of the wetlands that we’ve destroyed over the years. Another one is pesticides; as top consumers in the marsh food web, Great Egrets tend to accumulate any toxins that get into the system, affecting their survival and fertility. The third whammy is cormorants; these fish-eaters love to nest on the same islands as Great Egrets (and other herons), but their acidic droppings tend to kill the vegetation of the island much quicker than heron excretions. Both the West Sister and Columbus colonies are under cormorant stress, but this wouldn’t be much of an issue if there were more islands and habitat for the egrets. Hopefully, many of the new wetlands near Pickerington Ponds and Darby Creek can help give them some more options locally.
|
|
|
|
