"Southeast coast" for this book is defined as the U.S. coastline from North Carolina to Texas-Mexico border. This includes beaches, brackish bays and salt marshes, and the Atlantic Ocean and Gulf of Mexico to the edge of the continental shelf. Ecosystems there are influenced by offshore currents, principally the Gulf Loop Current and Gulf Stream. It is also shaped by the many streams and rivers that flow into the Atlantic and the Gulf. These build up barrier islands with sediments and nourish salt marshes with nutrients, which allow such diverse ecosystems to flourish. These estuaries are important conduits for annual migrations – for birds that stop to feed along the way and for saltwater fish that must swim upstream to spawn. Humans are also altering these ecosystems through climate change, pollution, overfishing, and building artificial reefs offshore.
Birds are are the most conspicuous creatures in coastal habitats, and they get extensive treatment in A Field Guide to the Southeast Coast & Gulf of Mexico. Coverage of birds is limited to raptors and waterbirds, and even within those boundaries, it is incomplete. Rails, for instance, are not included. The bird illustrations are generally useful and accurate. I was surprised, though, not to see a depiction of the pale southeastern form of the Red-shouldered Hawk. The depiction of an immature Black-crowned Night-Heron looks misshapen. Fish and sharks also receive extensive coverage, with an emphasis on commercially-important fish. Plants and marine invertebrates get compressed treatment. If you have an interest in botany or collect and identify seashells, you may want more specialized guides for those subjects. (I do not know which to recommend.) Other groups treated in the guide include aquatic reptiles (mainly sea turtles and crocodilians) and marine mammals. As part of their emphasis on conservation issues, the authors highlight the names of endangered and threatened species (according to the IUCN Red List) in red. It is particularly striking to see just how many species are threatened. In some cases whole pages of sharks, fish, or marine mammals are red.
Proctor and Lynch's A Field Guide to the Southeast Coast & Gulf of Mexico covers the most notable organisms of the southeastern coast and offshore areas of the U.S. It would serve as a useful field guide for casual visitors to southeastern beaches, anglers and scuba divers, or for birders looking for help identifying non-avian wildlife. However, it is not sufficient as a stand-alone bird guide, even for waterbirds. For complete coverage of birds, I would suggest turning to the recently-published 6th edition of the National Geographic guide or David Sibley's guide for eastern birds.