The Homer Spit marine IBA extends from Bluff Point at the mouth of Kachemak Bay to the outer shore of Homer Spit, a long, narrow natural jetty extending 4.5 miles into Kachemak Bay.
Mud Bay/Coal Bay is a sheltered tidal mudflat located at the base of the Spit where it adjoins the mainland.
A small but persistent population of Steller's Eiders, distinct from the Clam Gulch population, spends most of it's time near the off shore end of Homer Spit .
Kachemak Bay is one of the most critical sites for migratory birds in the world; thousands of migrating waterfowl and millions of shorebirds pause along mud flats here each year. It is recognized as the second most important shorebird staging area in Alaska (following the Copper River Delta), and was designated as an International Shorebird Reserve because it supports greater than 100,000 shorebirds and/or 15 percent of a specie's flyway population on migration. The Kachemak Bay WHSRN site is formed by three main areas: Mud Bay /Mariner Park Lagoon, Fox River Flats, and the Kachemak Critical Zone.
Total numbers of shorebirds counted in the Mud Bay and along the Spit average 100,000 during spring migration .Western sandpipers (Calidris mauri), followed by the surfbird (Aphriza virgata), and the rock sandpiper (Calidris ptilocnemis) predominate. Very significant proportions of the world population of both western sandpiper and surfbird are estimated to utilize Kachemak Bay every spring . It has been hypothesized that small shorebirds cannot store enough energy to travel the vast distance from the Copper River Delta to their arctic breeding grounds; therefore, an intermediary stop on the mudflats of Kachemak Bay can be considerably important . A flock of about 1000 rock sandpipers (5% of the global population) over-winters on Homer Spit.
In fall the most abundant bird species in Mud Bay are dabbling ducks (primarily mallards and pintails), surf scoters and gulls .
Species of conservation concern that occur but do not meet criteria thresholds include the Whimbrel, Wandering Tattler, Black Oystercatcher, Pacific Golden Plover, Bristle-thighed Curlew; Hudsonian Godwit, Marbled Godwit, Bar-tailed godwit; Black Turnstone and Trumpeter Swans.
Shorebirds: Passage; 1977; 25,000+; 4biv.
Disturbance to birds, recreation/tourism, industrialization/urbanization, natural resource extraction industry, filling-in of wetlands, and off road vehicle use on the beach.