About 240 species of birds have been recorded in the park and preserve. The high diversity and abundance of birdlife is due to the variety and extent of favorable breeding habitats available within the park, many of which contain ample food resources and low numbers of land predators.
Thousands of seabirds nest on cliffs and rocky shores both within the bay and on the park's outer coast, where they prey on small fish and other sea life. Small to medium-sized colonies of gulls, guillemots, puffins, and cormorants disperse along park shores, especially in the northern half of Glacier Bay, Cross Sound and the southern portion of the outer coast. Relatively major colonies occur on the Marble Islands, Boussoule Head, Cenotaph Island, beside Margerie and Johns Hopkins Glaciers, and at the mouth of Hugh Miller Inlet. Large flocks of phalaropes, molting sea ducks, and foraging gulls are prominent summer residents, while flocks of loons, gulls, murrelets, and sea ducks dominate the winter scene. Bald eagles are conspicuous inhabitants on shorelines throughout the park.
Considered at-risk elsewhere in their range, a world-class population of marbled murrelets occurs within and just outside of the park's waters. This extraordinary small diving bird is believed to breed extensively in the old growth forests along Icy Strait, though finding evidence of breeding activity can be difficult. Marbled murrelets do not assemble nests. A pair of birds will lay their lone egg on a stout moss-covered branch high up in a spruce or hemlock tree. The parents will fly to the sea at dawn and return at dusk daily to feed the chick, which sits on the branch camouflaged and quiet. When it is time for the chick to fledge, the parents depart one morning and never return. The chick, realizing that it is on its own, leaps off the branch on its first flight and finds its own way to the ocean which can be up to 30 miles away.
Glacier Bay also hosts a large portion as much as 20 percent of the world's population of Kittlitz's murrelets . It is believed that perhaps 95 percent of the Kittlitz's murrelet worldwide population breeds in Alaska with the remainder breeding in the Russian Far East. Unfortunately, surveys indicate that their numbers in Alaska have declined 80 to 90 percent in the past decade and this bird may soon be listed as a threatened or endangered species on the federal endangered species list. Unlike the marbled with which it can easily be confused, Kittlitz's murrelets nest in recently deglaciated mountain areas. A multi-year study is underway to more fully understand this bird, including its ecology and use of habitats in Glacier Bay, and impacts (if any) of vessels on these birds as they forage for food. Researchers are hoping to discover what is causing and how to reverse this population decline.
In the winter, the number of land birds present in the park dwindles. Ravens, crows, magpies, winter wrens and woodpeckers are not uncommon. Large mixed flocks of pine siskins, redpolls and chickadees feed on alder cones and grass seeds.
But come summer, substantial populations of several neotropical migrant warblers, thrushes and other songbirds come to the bay to nest. Ruby-crowned kinglets, fox sparrows, yellow-rumped warblers and varied thrushes are among the many birds that can be easily seen in the Bartlett Cove area. In terms of variety and abundance, landbirds are most common at lower elevations in shrub and deciduous plant communities. They are less common in high alpine regions, glacial barrens, spruce and hemlock forests, and boggy peatlands. After the long quiet winter, human residents look forward to their return in the spring when every beach thicket and mountainside in the park comes alive with song.