Since the early 1990s you could mark the event on your calendar: when mid-May rolls around, the cliffs of Champlain Mountain in Acadia National Park become a temporary home for the aeries of peregrine falcons.

Indeed, across the 88 million acres of the National Park System the landscapes are vital to birds, whether they arrive to nest, stop for some rest during their migrations, or spend the entire summer -- or winter -- in the forests, grasslands, wetlands, or coastal areas of the parkscapes.

Birders are well-aware of this. They know they can find horned and tufted puffins in Kenai Fjords National Park, sandhill cranes at Padre Island National Seashore during the winter months, migrating raptors over Glacier National Park or the Blue Ridge Parkway in the fall, or hundreds of species that winter in Everglades National Park.

And, along with the return of peregrines to Acadia two decades ago, there have been some notable species conservation events in the parks:

* Beginning in 2006, bald eagles made a stunning return to Cuyahoga Valley National Park, the first time in more than seven decades that eagles had nested in the park.

* In Yellowstone National Park, the 167 trumpeter swans counted this past winter represented a 600 percent increase from the 23 tallied during the winter of 2009-10 and marked the highest count in the park since 2007, according to U.S. Fish and Wildlife Service records.

* Big Bend National Park is a well-known birding area, with more than 450 species having been counted, among them "such unique birds as Mexican mallard, Lucifer hummingbird, Mexican jay, black-capped and gray vireos, Colima warbler, and varied bunting," notes the National Park Service.

Driving home the value of national parks to birds is the Interior Department's 2011 State of the Birds report, which, while noting declines in many species, documented the importance of national parks and other public lands for the future of birds.

Today, more than 850 million acres of land and 3.5 million square miles of ocean are publicly owned, including more than 245 million acres managed by the Bureau of Land Management, 6,000 State Park units, 1,600 Marine Protected Areas, 550 National Wildlife Refuges, 350 military installations, 150 National Forests, and nearly 400 National Park Service units. These areas support our native bird species, many of which are declining, as described in the 2009 and 2010 State of the Birds reports.

               
The report points out that public lands contain "vital" habitat for more than 800 bird species, of which 251 are federally threatened, endangered, or listed as a species of concern.

Among the report's findings specific to national parks:

* Public lands in the East are often the largest blocks of remaining forest in rapidly developing urban landscapes. Expanding the network of protected lands is important for bird populations. National Parks, National Forests, and state-owned forests support core populations of eastern birds. Improved management is key for declining species that require young forests.

* In the Eastern United States, (M)ore than 2 million acres of forest are protected in Great Smoky Mountains and other National Parks.

* Spanning roughly three million acres, the pineoak forests of the “sky island” mountains of southeastern Arizona, southern New Mexico, and west Texas are an extension of the forests in Mexico’s Sierra Madre ranges. Sixty-one percent is on public lands, more than half of which is on several large National Forests in Arizona and New Mexico. Other significant public lands include Big Bend and Guadalupe Mountains national parks in Texas and Fort Huachuca in southeastern Arizona.

* About 40% of U.S. subtropical forest is protected on public land, mostly in Florida, with more than 200,000 acres of hardwood hammocks in Everglades National Park and Big Cypress National Preserve.

* Alaskan National Parks provide more boreal forest habitat for Great Gray Owl, Northern Hawk Owl, Common Loon, Lesser Yellowlegs, and Common Goldeneye, than any other public agency lands.

* In American Samoa, 73% of the land is territorial and the NPS is the most significant federal land manager, with 27% under lease as the National Park of American Samoa. NPS controls invasive species and monitors bird populations there. An average of 86% of bird distributions is on territorial land. The Fiji Shrikebill and the Bluecrowned Lorikeet have more than 27% of their distribution on NPS managed land.

* The strengths of the NPS for bird conservation efforts include conservation mandates for more than 99% of NPS holdings, well-established avian inventory, monitoring and research programs, ecosystem restoration projects, invasive species management, educational programs highlighting bird conservation, and protection of coastal habitat. In National Parks within the U.S. and its territories, 732 regularly occurring native bird species and up to 44 native vagrant species can be observed.

Despite this great habitat across the National Park System, there are threats to these areas and the birds they support.

While there once were more than 140 million acres of tallgrass prairie in the country's midsection, today only a fraction remains. At Tallgrass Prairie National Preserve in Kansas, not quite 11,000 acres of tallgrass prairie are preserved in much their original condition. Nearly 130 bird species, including Upland Sandpiper, Prairie Chicken, Eastern Meadowlark, Northern Harrier, Horned Lark, Grasshopper Sparrow, Scissor-tailed Flycatcher, Northern Bobwhite, Killdeer, and Dickcissel, benefit from this unit of the park system.

But this valuable habitat is an exception, according to the DOI report, and grassland bird species "are among our nation's fastest declining species."

Grassland has always been undervalued as wildlife habitat. The percentage of grassland birds on public lands is low because such a small amount of U.S. grassland (less than 2%) is both publicly owned and managed primarily for conservation. More public land specifically protected for grassland birds is needed, and a higher proportion of multiple-use lands should be managed with grassland birds in mind.

        
When it comes to wetlands habitat, Everglades, Big Cypress and surrounding public lands "protect the nation's largest freshwater marsh system," the report points out. However, non-native plants and climate change that is expected to affect freshwater supplies could jeopardize these landscapes and the birds they support, it adds.  In terms of subtropical forests at Everglades and Big Cypress, long-term droughts and fluctuating water levels stand as threats, according to the report, which notes that efforts to restore natural water flows through the Everglades could benefit both the freshwater marshes and the forests.

And while Alaska's national parks protect vast stretches of boreal forest, climate change that is producing warmer winters and summers is enabling attacks of bark beetles and spawning wildfires that are consuming and altering bird habitat.

Coastal areas that support bird species also are under threat, both from climate change that is leading to rising sea levels and more vigorous storms that assault birding grounds as well as from development, off-road vehicle use, and dogs and cats, the report said. Managing these landscapes is not without conflict, as decisions on how to protect nesting birds and bird habitat can run counter to public use of those landscapes, as evidenced at Cape Hatteras National Seashore, where seasonal restrictions on off-road vehicles, primarily, and pedestrians to a certain degree, have been particularly contentious.

Despite these challenges, the State of the Birds report notes there have been significant strides made in recent years in the protection of some shorebird species.

About three-quarters of threatened U.S. Atlantic Coast Piping Plovers nest on publicly managed beaches. Labor-intensive management by a network of cooperators minimizes threats from habitat loss, beach recreation, and predation.

With improved nesting success and habitat protection, the U.S. Atlantic population of Piping Plovers has more than doubled in the last 20 years. Examples on federal lands include growth from 15 to 85 pairs at the Cape Cod National Seashore, from 5 to 32 pairs at Monomoy National Wildlife Refuge, and from 19 to 45 pairs at the Sandy Hook Unit of the Gateway National Recreation Area.

        
National park settings also are crucial to the success of ocean-going bird species, and in some settings the removal of non-native species that prey on the birds when they nest have resulted in population boosts.

Invasive species are a major threat to island-nesting ocean birds. Active management, particularly complete eradication of invasive species, can yield stunning results. For example, the nesting success of Xantus’s Murrelet increased by 81% on Anacapa Island in Channel Islands National Park after rats were eradicated. The Alaska Maritime National Wildlife Refuge removed introduced foxes from many of its islands, resulting in increases of more than 200,000 breeding seabirds of at least 15 species. In Haleakala National Park, an endangered Hawaiian Petrel colony had only 400 known nests in the 1980s. Intensive management and predator control beginning in the 1980s have led to an increase to more than 1,500 known nests.

“Although we have made enormous progress in conserving habitat on public lands, we clearly have much more work to do," Interior Secretary Ken Salazar said in summing up this year's report. "The good news is that because birds so extensively use public lands and waters as habitat, effective management and conservation efforts can make a significant difference in whether these species recover or slide towards extinction.”

The 2011 State of the Birds report is a collaborative effort as part of the U.S. North American Bird Conservation Initiative, involving federal and state wildlife agencies, and scientific and conservation organizations. These include the American Bird Conservancy, the Association of Fish and Wildlife Agencies, the Bureau of Land Management, the Cornell Lab of Ornithology, the Department of Defense, the National Audubon Society, The Nature Conservancy, the National Park Service, the U.S.D.A. Forest Service, the U.S. Fish and Wildlife Service, and the U.S. Geological Survey.