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Christmas Shearwaters have been extirpated from west
Pacific colonies on Ogasawara Island, Minami
Torishima, and Wake Island. There is no fossil
record from the Hawaiian Islands but they probably
occupied the main islands before extirpation there.
Currently, they are still present on offshore islets
and most islands in Northwest Hawaiian Islands,
although numbers are reduced. Rabbits (Oryctolagus
cuniculus) introduced in 1903 destroyed native
vegetation on Laysan and Lisianski Islands
eliminating appropriate nesting habitat and causing
steep population declines that have not recovered in
spite of rabbit eradication.
The
total breeding population in the Hawaiian Islands is
estimated at approximately 2,500 - 3,300 pairs. The
largest breeding colonies in the archipelago occur
at Laysan (1,500 - 2,000 pairs), Lisianski (400 -
600 pairs), Nihoa (200 - 250 pairs), and Midway (200
pairs). The worldwide population is unknown but is
likely less than 10,000 breeding pairs.
Main threats to the species include:
§ Predation
No terrestrial mammalian predators are native to
the Hawaiian Islands, so this species is incapable
of defending itself or its nest from mammals such as
rats (Rattus spp.), cats (Felis sylvestris),
dogs (Canis familiaris), pigs (Sus scrofa),
and mongooses (Herpestes auropunctatus).
Predation therefore is a serious threat to adult
seabirds and their eggs and chicks. Small
Procellariiformes are particularly vulnerable
because of their small size, low annual
productivity, and lack of effective anti-predator
behavior. Although none of these predators occur
where shearwaters nest, care needs to be taken to
avoid reinvasion.
§ Habitat
availability
Dense vegetation provides critical shaded and
protected nest sites. Many nesting islands in Hawaii
have been invaded by non-native vegetation (e.g.,
Golden-crown Beard, Verbesina encelioides),
which typically has short growth periods. When the
plants die, chicks are exposed to temperature
extremes. Larger areas of appropriate nesting
habitat will decrease the amount of competition with
other surface-nesting species, such as Red-tailed
Tropicbird, Wedge-tailed Shearwater, and Brown
Noddy, potentially increasing breeding success.
§ Fisheries
In
Hawaii, overfishing may directly or indirectly harm seabird
populations; harvest of Skipjack and Yellowfin Tuna
(Thunnus albacares) could eliminate predatory
fish needed to drive prey species to surface. Also,
live bait needed for the fishery, Goatfish (Mullidae),
and Mackerel Scad (Decapturs spp.) could
potentially decrease this species primary prey
items.
Development of a squid fishery could also impact
Christmas Shearwaters. |
