Saturday, January 3, 2015

Mammal predators primary threat to Hawai`i's endangered petrel

There are several seabirds that nest on the main Hawaiian Islands, but only one of them, the Hawaiian petrel, is endangered.

And it is increasingly understood that mammal predators--dogs, pigs, cats, rats and mongooses--are their main threat.

We hear more about albatrosses and shearwaters, but the Hawaiian petrel or Hawaiian dark-rumped petrel is a special one. Once consumed as a delicacy reserved for chiefs, it was known to nest from the mountains to the sea, on all the islands. Its nesting range has contracted dramatically.

(Image: Hawaiian petrel in flight. Credit: FWS image by Jim Denny.)

The biggest nesting colony is on Haleakalā within the national park. A National Park Service information sheet on the birds says pairs mate for life. The birds begin life in burrows that can be from a few feet to as long as 30 feet in length.

They are remarkably loyal to their nesting islands and nesting colonies, and there was very little genetic mixing between islands, except between Molokai and Lana`i. But the birds may be gone from Molokai. 

This study suggests the petrels of the different islands may even feed in different areas of the Pacific.  It suggests that Big Island Hawaiian petrels may go south to find food for their young, while Kaua`i birds head north.

The Park Service says young birds, on flying for the first time from their mountain burrows in the evening, can be confused by the lights of civilization and crash to the ground. Residents on Maui and Kauai are encouraged to collect fallen birds and take them in for checkups and release.

But the lights of civilization are not considered the main cause for their decline. That is tied to “loss of habitat due to land development, degradation of land by feral goats and pigs, predation by introduced mongooses, feral cats, rats and dogs,” the parks paper says.

Hawaiian petrels, according to a 2013 study from the island of Lana`i, seem to prefer nesting sites that are steep, in native vegetation and under open canopies.

The paper was published in the journal Waterbirds, by University of Hawai’i researcher Marie VanZandt, along with, Donna Delparte, Patrick Hart, Fern Duvall and Jay Penniman. It was titled Nesting Characteristics and Habitat Use of the Endangered Hawaiian Petrel (Pterodroma sandwichensis) on the Island of Lāna'i.

They listened for the birds at night and also visually sought out nesting sites, and located 71 nests. When they studied the characteristics of the nesting sites, they found “the top model in predicting Hawaiian Petrel nest site selection was influenced by increasing slopes, an understory dominated by native vegetation, and open canopy.”

Why those things? The steep slopes seem to make sense, since these gliding birds would have an easier time taking off from an angled surface than a flat one. And the open canopy would suggest an easier time for flying birds to get in and out than a dense overstory. Adults are back and forth regularly from the open ocean feeding grounds to the nesting colonies.

What about native vegetation would be important for seabirds that don’t feed on land? VanZandt and her team felt it might simply be that dense native ferns might protect the birds from attack by predatory mammals.

“Native understory vegetation, dominated by Uluhe ferns (Dicranopteris linearis and Diplopterygium pinnatum), may provide some protection from introduced mammalian predators,” they wrote.

Mammal predators are a known serious threat to the ground-nesting birds of the Hawaiian islands. 

Pet dogs regularly take out nesting albatross on Kaua`i. Cats have been photographed pulling Newell’s shearwaters in their burrows in the Alakai Swamp. Mongooses have taken numerous birds from Maui dark-rumped petrel colonies in a single night. Dogs and cats ravage coastal nesting sites of wedge-tailed shearwaters on several islands.

This isn’t new information. In a 1985 paper in The Condor,  researcher Theodore Simons wrote of Hawaiian petrels, “Predation is the primary threat to the birds’ survival…if it can be controlled, the remaining populations should thrive.”

VanZandt and her team suggested protection of native vegetation in the uplands is also critical, since alien species are fast invading the best seabird nesting habitat.

Hawaiian petrels,`ua`u,  are easily mistaken by non experts for some of their cousins, including the Newell’s shearwater,`ao, and the Wedge tailed shearwater, `ua`u kani. 

© Jan TenBruggencate 2015

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