Identification of all large gulls, especially in immature plumages, is complex and very difficult and will be covered only superficially here. The presence of large amounts of individual variation, complex and prolonged periods of immaturity, and extensive hybridization (especially in the Pacific Northwest) render gull identification among the most challenging of all bird identifications. Those wishing to study these identification challenges further are directed towards excellent publications such as Olsen and Larsson (2004) and Howell and Dunn (2007).
Glaucous-winged Gulls of all ages are relatively easily distinguished from most other large white-headed gulls in the Pacific Northwest by their mid-grey (adults) or brown (immatures) wingtips that contrast little with the colour of the upperparts. The relatively thick bill with its pronounced gonydeal angle, the long neck, and the overall bulky proportions give this species a somewhat distinctive posture, especially when perched. Adults have mid-grey upperparts that further help distinguish them from dark-mantled species such as Western Gull and Slaty-backed Gull as well as very pale-mantled species such as Herring Gull. These species, as well as Thayer’s Gull, are all further distinguished by their black or blackish (rather than grey) wingtips. Based on wingtip pattern, adults are most likely to be confused with Glaucous Gull or Iceland Gull, although both species have either white (Glaucous Gull, some Iceland Gulls) or irregularly patterned pale to dark grey wingtips (some Iceland Gulls). Iceland Gulls with grey wingtips can typically be distinguished from Glaucous-winged Gull by the more extensive white in the wingtips, paler mantle, smaller size, more slender proportions, more rounded head, much smaller bill, and (usually) pale or amber-coloured eye.
First-year immatures are very uniformly coloured brown or grey-brown, including the wingtips, and are thus relatively easy to distinguish from immatures of other large gulls such as Herring Gull and California Gull, which have blackish wingtips. First-year Glaucous Gull is often parimarily brown, but the wingtips are whitish or pale brownish-white and are paler than (rather than a similar shade to) the rest of the underparts. The sharply bicolored bill (pink at the base, black at the tip) of Glaucous Gull is also a distinctive characteristic that helps distinguish it from most Glaucous-winged Gulls (although some Glaucous-winged Gulls can have a bill pattern approaching that of Glaucous Gull). First-year Thayer’s Gull is the most likely species to be confused with first-year Glaucous-winged Gull in B.C., and given the abundance of both species in coastal B.C. this is a common identification pitfall. Thayer’s Gull tends to have wingtips that are slightly darker than the rest of the upperparts (dark brown) and, when fresh (e.g., in the fall or early winter) usually shows distinctive pale crescents at the tips of the primary feathers that, due to the darker primaries, are more contrasting than in Glaucous-winged Gull. The upperwings and upperparts of Thayer’s Gull also tend to be more prominently mottled and speckled with pale whitish or buffy-white markings, and it is a slightly smaller, more slender species with relatively shorter legs, a smaller and more domed head, and a smaller and straighter bill.
Second-year and third-year immature Glaucous-winged Gulls remain relatively easy to differentiate from other gulls due to their grey or brown wingtips (vs. blackish or whitish) as well as the increasing amount of mid-grey (rather than dark grey or pale grey) feathers on the mantle.
Source: Howell and Dunn (2007)
| Produces a very wide array of calls throughout the year. The most commonly-heard calls include a series of loud sounds delivered in succession ( au-au-khau-khau-khau) (known as the “Long Call”), a shorter kjau, and a rapidly-repeated, guttural hoh-hoh-hoh. Also produces a high, repeated, yelping kea, a rapid ka-ka-ka alarm call, a a drawn-out ma-ah. The calls are similar to those of Western Gull but average flatter, slower-paced, lower-pitched, and more hollow-sounding. Source: Sibley (2000); Howell and Jaramillo (2006); Hayward and Verbeek (2008) |
Courtship
This species is monogamous, with pairs forming on the breeding grounds in the late winter or early spring (February to April) and remaining together throughout the breeding season. Courtship includes a wide variety of displays such as head tossing (in which the head is jerked back), allopreening (in which one individual preens the feathers of its mate’s head), and courtship feeding. In one courtship display, known as the “Mew Display”, the displaying bird holds the body horizontal or slightly oblique, arches the neck, points the head downwards, and producesa mewing call; this display is also associated with non-courtship functions (territoriality, etc.).
Nest
The nest is constructed in late April and May, although some birds may begin the first nest scrapes as early as March. The nest is situated on the ground or other flat structure (pier, building roof, etc.); nests are rarely placed in large tree. This species is generally colonial, although isolated pairs regularly occur as well, and colonies are regularly associated with other colonial seabirds such as cormorants and alcids. The nest is typically placed in a shallow scrape or on a low mound and is enclosed by a bulky ring of loose grass, moss, dead twigs, string, garbage, bones, seaweed, feathers, and other debris. Nests average 38-39 cm in diameter (21-22 cm inside diameter) and 10-11 cm in depth.
Eggs
A single clutch of (2) 3 (4) eggs is laid between late April and late July, with most clutches laid in May or June. The eggs are incubated by both sexes for 26-29 days before hatching. The smooth or slightly granular, non-glossy eggs range in colour from pale greenish or olive-green to greyish-olive or buff and are variably marked with dark brown, grey, and olive scrawls and blotches. This species does not lay a second clutch, but replacement clutches are laid if the first clutch is lost. Eggs have been recorded in B.C. between late April and mid-August.
Young
The young are semi-altricial upon hatching, with a complete covering of down. The down is buffy, with black spots on the head and throat, blackish-grey patches on the back and flanks, and an area of blackish-based and light-tipped feathers across the breast; the upper surface of the wings and occasionally the belly also have some dark spotting. The legs and feet are dull flesh-coloured at hatching but quickly turn blackish and the bill is black with a dusky-pink outer third. The young are able to leave the nest almost immediately after hatching, and remain in the vicinity of the nest for the first few days after hatching. The chicks gradually range farther from the nest, and are tended by both adults throughout the fledgling stage. The fledglings are capable of flight at 35-54 days of age, at which point they leave the colony and are fully independent shortly thereafter. Dependent fledglings have been observed in B.C. between early June and mid-September.
Source: Campbell et al. (1990b); Baicich and Harrison (1997); Hayward and Verbeek (2008)
|
Like many large gulls, the Glaucous-winged Gull consumes an enormous array of food items throughout the year and is considered an opportunistic and omnivorous species. Major food items include fish, marine invertebrates, carrion, and (in areas near communities) garbage. In the marine environment, this species can be found along virtually any coastline or estuary in search of fish (both dead and alive) and marine invertebrates such as crabs, shellfish, sea stars, marine worms, etc. It commonly carries shellfish and crabs into the air and drops them onto hard shoreline rocks to break the shells, while other large invertebrates (such as sea stars) are often swallowed whole. It also pursues small surface-feeding fish, and large aggregations of gulls often swarm schools of fish and attempt to capture individuals by dipping their heads under the water or engaging in shallow dives from a few metres in the air. Particularly large concentrations of Glaucous-winged Gulls are associated with spawning events of herring and salmon, where they prey on both live and dead fish as well as the abundance of eggs that they produce.
Where it ocurs near cities and towns, this species feeds extensively on garbage, both at landfills and throughout urban areas. It also consumes carrion, and will wait near feeding carnivores such as bears and eagles in order to steal scraps as they are left unguarded. The eggs and nestlings of a wide variety of birds, especially colonial marine species (cormorants, other gulls, etc.), are also a common food item, and it regularly engages in kleptoparasitism or piracy (stealing food items from other birds).
Source: Hayward and Verbeek (2008)
|
|
