What counts as an invasive species might surprise you. Mosquitoes, plants, and house mice and other mammals — including those super-predators, free-roaming cats — have invaded ecosystems they don't naturally inhabit, putting pressure on native birds and other wildlife. During National Invasive Species Awareness Week, we're exploring how these non-natives affect a particularly vulnerable group of birds: seabirds.

Like humans, seabirds are long-lived creatures that invest heavily in their offspring. They evolved to raise relatively few young, because they usually nest in areas with few predators and could expect that most of those young would survive. For millions of years this strategy worked.

Havens No Longer

Today, even the islands and remote headlands where seabirds breed bear the evidence of human activity. Many are overrun by non-native predators, leading to a startling statistic: Nearly 80 percent of the highest-risk seabirds in the Americas have plummeted in population and are now of serious conservation concern, according to IUCN data analyzed by ABC staff. These species include Newell's Shearwater, Hawaiian Petrel, and Townsend's Shearwater.

Newell's Shearwater is one of many seabird species at risk from invasive species. Photo by Brenda Zaun/USFWS

But there is good news. The very trait that makes seabirds vulnerable — their tendency to nest in isolated, closely packed colonies — also provides the greatest opportunity to save them. It allows us to focus conservation action and to restore colonies at a fraction of the cost of delivering large-scale, landscape-based efforts.

Building Fences, Creating Colonies

Cats, rats, and other non-native mammalian predators that came with humans to these remote sites are the most pervasive threat to seabirds on islands worldwide. Cats and rodents kill both adults and chicks, while free-roaming cattle, sheep, and goats trample nest burrows or eat sheltering vegetation.

Conservation fencing is proving to be an especially effective tool for responding to the harm these non-native species cause. These fences can be used to create safe nesting habitat that serve as “islands within islands,” once predators inside the fence have been removed.

Special fencing at Kilauea Point National Wildlife Refuge creates predator-free zones for nesting seabirds. Photo by Ann Bell/USFWS

Also called predator-proof fences, these devices include fine mesh barriers that thwart feral cats, rats, mongooses, and mice. Overhangs on these fences prevent climbing, while underground skirts  prevent digging. More basic fencing effectively excludes larger grazing animals such as sheep, goats, and pigs.

At Kilauea Point National Wildlife Refuge on Kauai, ABC and our partners installed fencing almost half-a-mile long to keep out introduced cats, rats, and dogs. That work helped create a safe zone in which Laysan Albatross and other native species can flourish.

Once invasive species have been removed, conservationists can consider re-introducing species into these habitats. For example, in 2015, with ABC support, scientists undertook a historic translocation of 10 Hawaiian Petrel chicks from their mountain colony to the newly protected area at Kilauea Point, called Nihoku.

Relocating Hawaiian Petrel chicks by helicopter to a new, predator-proof breeding colony. Photo by Lindsay Young/Pacific Rim Conservation

In 2016, we and our partners — including the Kauai Endangered Seabird Recovery Project, Pacific Rim Conservation, the Hawaii Department of Land and Natural Resources' Division of Forestry and Wildlife, and the U.S. Fish and Wildlife Service — relocated eight Newell's Shearwater chicks to the Nihoku protected area. In 2017, a third cohort of both species was brought to the site, bringing the total to 78 chicks translocated. The hope is that the chicks will imprint on the site and establish a new breeding colony there.

Armies of Ants

It's easy to understand how invasive cats or rodents could wreak havoc on breeding seabird populations. But other non-native threats to these birds are less obvious.

In Hawaii, one of the most isolated groups of islands in the world, there were no ants before humans arrived. Following people on boats or in cargo, roughly 57 ant species have found their way onto the islands and have become established in the last two centuries. At least four of these — the big-headed ant, tropical fire ant, Argentine ant, and yellow crazy ant — have had particularly profound effects on native insects, spiders, and the larger ecological communities.

These ants are so successful because they eat a wide variety of plants and animals. They form multi-queen nests, and have multiple nests that form a single “supercolony,” resulting in very large numbers.

Seabirds in the Hawaiian Islands did not evolve with ants, so they have no defenses against the invaders. The injuries to nesting seabirds can be extreme.

Newell's Shearwater chick with severe ant damage to eye and bill. Photo by Sheldon Pentovich/USFWS

Adult birds will sometimes fly away to avoid the ants or abandon their nest sites completely. If somehow they do persevere during incubation and hatch their eggs, the chicks are unable to escape. The injuries they sustain translate into reduced fledgling success and in some cases cause abandonment and even permanent loss of the breeding colony.

Conservationists have mobilized to remove the ants and protect the birds. The U.S. Fish & Wildlife Service's Yellow Crazy Ant Strike Team, for instance, has set out to eradicate these invasive ants from Johnson Atoll and other islands.

Attack of the Killer Plants

When invasive plant species like Kahili ginger, blackberry, and strawberry guava — all problems in Hawaii — take hold in seabird breeding territory, they can choke nesting sites and degrade habitat. Often these are garden plants that escape from cultivation and out-compete and overgrow native vegetation, resulting in the loss of prime nesting areas.

These invaders find their way into remote areas as invasive mammals spread their seeds, sometimes growing so dense that a total loss of nesting habitat can result for burrow-nesting seabirds like Hawaiian Petrel.

Laysan Albatross trying to nest in invasive verbesina, Midway National Wildlife Refuge. Photo by Michael Lusk

On Ecuador's Isla de la Plata, where ABC and partner Equilbrio Azul and Machalilla National Park have been studying threats to critically endangered Waved Albatross, biologists found that rains stimulated overgrowth by non-native morning glory. The plant has taken over the birds' nesting area to an extreme degree, posing an entanglement hazard for chicks. Conservationists are now working with park biologists to contain the spread of these invasive plants and improve management at this site.

What You Can Do

Here are three ways you can help: Help stop the spread of invasive species in your area by gardening with native plants, taking our Cats Indoors pledge, or donating to help American Bird Conservancy achieve conservation results for birds throughout the Americas.

Learn more about ABC's work on behalf of seabirds and what we're doing to address the threat of free-roaming cats and other invasive species in Hawaii.