thumb|In Alaska

The Pacific loon or Pacific diver (Gavia pacifica) is a medium-sized member of the loon, or diver, family.

Taxonomy and etymology

The Pacific loon, previously considered conspecific with the similar black-throated loon, was classified as a separate species in 1985. The genus name Gavia comes from the Latin for "sea mew", as used by ancient Roman naturalist Pliny the Elder. The specific epithet pacifica is Latin for the Pacific Ocean, the term meaning "peaceful".

The phylogeny of this species is debated, with the black-throated loon and the Pacific loon traditionally being considered sister species, whereas a study using mitochondrial and nuclear intron DNA supported placing the black-throated loon sister to a clade consisting of the Pacific loon and the two sister species that are the common loon and the yellow-billed loon. In the former phylogeny, the split between the Pacific loon and the black-throated loon is proposed to have happened about 6.5 million years ago.

Description

thumb|Pacific loon

Breeding adults are like a smaller sleeker version of common loon. They measure in length, in wingspan and weigh . They have a grey head, black throat, white underparts and chequered black-and-white mantle. The black throat has purple reflections. Non-breeding plumage is drabber with the chin and foreneck white. Its bill is grey or whitish and dagger-shaped.

In all plumages, lack of a white flank patch distinguishes this species from the otherwise very similar black-throated loon. The Pacific loon can be differentiated from the red-throated loon in winter by the latter's paler look, the fact that the red-throated loon has less of a contrast between both the crown and hindneck and the throat, and the bill that looks to be upturned. This distance requirement for takeoff limits the number of lakes the Pacific loon can realistically breed and forage on.

Calls

The Pacific loon most actively calls in the spring and summer and has a wide range of calls. When feeding, the Pacific loon may produce an "ark"-like vocalization, a sharp, short call. Additionally, the Pacific loon has a call similar to that of its relative the common loon, it is a loud, eerie, oo-loo-lee wail or yodel that can travel for miles and is typically heard during the loon's breeding season. The loons can also make short and harsh "kok-kok-kok-kok" calls along with a range of other smaller cackles, growls, barks, and clucking noises.

Behaviour

Breeding

Pacific loons begin mating behavior in spring and summer, displaying beak-dipping and splash-diving to attract mates, with males fighting using their beaks, potentially fatally.

The Pacific loon constructs its nest on the ground near deep lakes. This nest is made out of piled-up vegetation and mud.

This loon lays a clutch of one to two light buff or green eggs with brown spots of various sizes. These eggs typically measure . Although the eggs are laid a few days a part, they can usually be seen to hatch at intervals not more than a day apart. Incubation lasts 23 to 25 days.

Pacific loons prospecting for a new lake frequently visited lakes that were already occupied, this along with high retention rates of lakes indicates that the loon population in Northern Alaska may be saturated. The Pacific loon has also been known to be very aggressive, engaging in interspecific killing of both hatch year and after hatch year individuals who intentionally or accidentally approach the loon's nesting area.

A pair of Pacific loons will defend young as a parental unit in which one parent protects the nest or fledglings while the other parent aggressively attacks the perceived threat. If the threat is another animal on the water, the attacking loon will extend its head and neck in an aggressive manner and dive under the threat in an attempt to stab it with its beak as it resurfaces. Current efforts of conservation are focused on loon population monitoring and maintaining accurate counts of populations and breeding behaviors. A recent in-depth study of the Pacific loon population in the Western Alaskan Arctic found that the population of the birds there is actually ~1.5-2.0 times larger than previous aerial surveillance had indicated. The study indicated potential encroachment upon the National Petroleum Reserve in Alaska (NPRA), one of the Pacific loons' primary breeding grounds, by natural gas and oil companies as a potential threat to Pacific loons.

One harmful environmental pollutant to the Pacific loon population is heavy metals like mercury, which can be observed in their habitat as a result of legacy mining in the areas where loons migrate and hunt. It has been shown that elevated levels of mercury in the environment pose a significant health risk to avian piscivores like the Pacific loon. Mercury is absorbed by small fish, which are a primary food source for loons, possibly causing mercury poisoning.

Climate change poses a threat to Pacific loon habitat, primarily due to water loss in the lakes where they breed.

Further reading

The following articles deal with the separation of Pacific diver/Pacific loon from black-throated diver/Arctic loon:

  • Birch, A. and Lee, C-T, 1997, Field identification of Arctic and Pacific Loons, Birding 29: 106–115.
  • Birch, A and Lee, C-T, 1995, Identification of the Pacific Diver - a potential vagrant to Europe, Birding World 8: 458–466.
  • Harrison, Peter (1988). Seabirds: An Identification Guide. London: Christopher Helm.
  • (for Canada)
  • Pacific loon photo gallery - VIREO