In the southern parts of its range, the Pacific loon begins to breed in March, whereas in the north the timing is dependent upon the arrival of spring (10). Following courtship displays, which may involve the birds swimming and diving in pairs, usually with ritualised head movements, loon species usually form monogamous breeding pairs. These pairs stay together for life and return to the same lake year after year, sometimes even using the same nesting site. Male loons produce a wide range of fascinating calls in order to advertise ownership of their territory and strengthen the pair-bond with their mate (12).
In all loon species, both the male and female take part in building the nest (12), which in the Pacific loon is typically a simple depression in the ground with minimal vegetation (2), or a heap or bowl of grasses and aquatic plants (2) (10) (12). Loon species tend to build their nests near the water’s edge (2) (10) (12), which enables them to escape easily to the safety of the water if threatened (12).
Pacific loon eggs are variable in colour, generally being buff, brown or olive-green (2), and usually one or two are laid per clutch (2) (12). Both the male and female take part in incubation (12), and the chicks leave the nest just one or two days after hatching (2). Pacific loon chicks are able to swim almost immediately, but may often be seen riding on the back of one of the adults for protection and to save energy. Young of this species take their first flight at about 50 to 55 days old (12).
The Pacific loon feeds mostly on fish (2) (5) (10) (12), but is also known to eat a variety of aquatic invertebrates (2) (10) including insects, molluscs, and crustaceans (10) (12). In addition, this species may supplement its diet with some plant matter (10) (12). Prey is caught underwater (5), with the Pacific loon diving after and pursuing its quarry before seizing the fish with its bill (2) (10) (12) and swallowing it whole, headfirst (12). On average, loon species dive for about 45 seconds at a time, but they are capable of remaining submerged for several minutes (12).
Pacific and Arctic loons are known to be more sociable than other loon species (12), and have been observed forming large groups and feeding cooperatively (2) (12). In addition, these species may migrate in flocks, which is unusual for a loon (5). Despite being agile underwater, the Pacific loon is extremely awkward in terrestrial environments, and is not capable of taking off from land. In order to take flight, this species requires an expanse of about 30 to 50 metres of open water, flapping its wings vigorously and pattering across the surface to gain lift (2).