Update from the Everglades: March 17, 2014

This year has the potential to be a great nesting season for wading birds. That's because the Everglades has remained wet over large areas for quite some time now and wetland fish, the main prey for most wading bird species, clearly grow and breed best when the marsh doesn't dry too much.

By mid January I began to see the first signs that conditions were indeed favorable. Water depths had dropped low enough for birds to start feeding (about 6-12"), and I recorded exceptional numbers (about 8000) of storks, ibises and egrets foraging in the shallower areas of Loxahatchee National Wildlife Refuge and Water Conservation Area 3A (WCA-3A).  

A small mixed-species foraging flock in the A. R. M. Loxahatchee National Wildlife Refuge in February 2014

A bountiful supply of fish is needed to get the birds in to breeding condition and by early February hundreds of wood storks were nesting in the cypress domes bordering Big Cypress National Preserve and in the pond apple along Tamiami Trail. Similarly, great egrets, all decked out with their showy aigrettes, were nesting at the large Alley North colony in WCA-3A. All in all, the birds were off to a pretty promising start.

Wood storks and great egrets incubating eggs in WCA-3A

But then it started to rain, which was not good. And then it rained again and again.  Now why would rain be bad, you might ask, if water is clearly needed for growing fish? The issue here is that wading bird nesting is constrained not only by the amount of fish in the marsh, but also by how easy those fish are to catch. In short, birds need wet/deep conditions during the wet season to grow fish, but they also need the marsh to drydown during the dry season so they can easily find and catch those fish. As the dry season progresses and water levels drop, fish move from drying habitats into deeper areas, until eventually they become concentrated and highly abundant in any remaining pools of shallow water. This creates easy fishing for the birds, but such conditions are essential if parent birds are to provide sufficient food to support three or four rapidly growing nestlings. Unfortunately, the aforementioned rains caused water levels to rise rapidly and re-flood the marsh (we call this a reversal), and any fish that were previously concentrated were able to disperse back into the landscape - good for the fish, but bad for nestling birds.

I don't know yet how the recent reversals are affecting the nesting storks and egrets. A survey last week revealed that few birds are feeding in the marsh, suggesting that foraging conditions are currently rather poor; however, it is possible that birds are managing to get by  with food resources from outside the Everglades. Also, while reversals are generally bad news for nesting birds, they are not guaranteed to have a negative impact and we have yet to fully understand the mechanisms by which they affect nesting. In a couple of days I will conduct my monthly survey of the nesting colonies when I should be able to tell you the full effects of the reversals.