Surprises in the Long Key Hammocks
Believe it or not, elevation plays a huge role in the type and occurrence of habitats in southern Florida and the Florida Keys. In the interior of South Florida, for example, the highest points are pine or oak dominated woodlands. Drop three or four feet in elevation, and you’ll be in a cypress dome or wet marl prairie (the latter being true Everglades). The mangrove forests that line the coast are yet another patent habitat associated with the region. Yet there is another important but lesser-known ecosystem in extreme South Florida and the Florida Keys that recalls more of a West Indian influence: tropical hardwood hammocks.
A hammock in this use of the word describes a dense hardwood stand of trees bordered by a wetland. And a tropical hardwood hammock, more specifically, is a forest comprised of tree species such as Gumbo Limbo, Poisonwood, Blolly, Wild Dilly, and Pigeon Plum, among others. In the Keys, it is often the habitat just inland from the mangroves of the coastline and estuaries.
If you visit Cuba, Hispaniola, The Bahamas, and even coastal Yucatan Peninsula, you’ll see that the flora of the tropical hardwood hammocks of extreme peninsular Florida and the Keys are more similar to the Caribbean than to the flora of other parts of temperate Florida. This is also apparent in the relationship between the bird species. For example, “specialties” of the Keys such as the White-crowned Pigeon, Mangrove Cuckoo and Black-whiskered Vireo are best found in the dense hammocks of the Keys. If you were to visit Cuba or the Bahamas, you would see these species breeding and foraging in nearly the same tree species.
The same goes for migrating passerines. During the fall at Long Key State Park, where there are a few parcels of hardwood hammocks, we see some of the highest numbers of migrants using the hammocks to rest and forage. The fruiting trees and shrubs have proven to be extremely important, perhaps even vital to large numbers of migrating thrushes, vireos and flycatchers. American Redstarts, Worm-eating, Black-and-white, and Black-throated Blue Warblers are among several species of warblers that can often be found deep in the interior of a hammock’s middle and upper stories, while Ovenbirds, Swainson’s and Kentucky Warblers comb the ground level where there is often very little sunlight penetration. Common Nighthawks and Chuck-will’s-widows seem to rely heavily on this habitat for roosting during migration.
Along with a menagerie of breeding and migrating species, there have been several that were certainly not expected. Within the state park just in the second half of September of this year, this habitat has accommodated three exceptional vagrants: Key West Quail-Dove (9/27/14), Yellow-green Vireo (9/24/14), and Sulphur-bellied Flycatcher (9/17/14). The quail-dove and the flycatcher frequented the Golden Orb Trail, while the vireo was located in a nearby restricted area where the Florida Keys Hawkwatch team has been granted permission to survey.
As touched upon before, the botanical composition of a hardwood hammock in the Keys is nearly a mirror image of those found elsewhere in the West Indies. If you were to examine and compare the North American bird species that winter in hardwood hammocks in Cuba and the Yucatan, you’d also see that there is a lot in common with extreme southern Florida and the Keys (i.e. the Black-throated Blue Warbler). Similarities in climate between the Keys and the West Indies help explain how this is possible, but here is some literal food for thought: birds are often vehicles for transporting seeds and berries to foreign locations. It is likely that the strong-flying and exploratory White-crowned Pigeons helped aid the colonization of many of the plants found in the Keys. Fruit ingested by a pigeon gorging in a hardwood hammock in Cuba might have come out the other end in the Keys after a hop over the Straits of Florida, an easy flight for a pigeon. Or maybe the secretive and unforthcoming Key West Quail-Dove, which historically may have bred in the Keys, also played a role?
However, there is no denying the threats to such a unique and spatially restricted habitat. In the Keys and coastal South Florida, any dry land is prime real estate for humans. Much of the developed areas of the Keys were historically hammocks, and tracts of this West Indian habitat are now more-or-less restricted to protected areas. Human encroachment has fragmented this highly valuable habitat up and down the Keys, but the hardwoods of Long Key State Park are still very much for the birds.