Appearance

Because of the wide ranging distribution and abundance of Herring Gulls, an enormous amount of basic and applied research has been done on them, including many studies on the molts and plumages of European populations. However, the only studies on molts and plumages of North American birds were conducted by Dwight (1901, 1920, 1925). Poor (1946) described individual variation in plumage and soft-part colors in relation to age in Atlantic Coast populations, but he presented few data relevant to molt. These studies mainly presented brief and general descriptions of the extent of each molt and the appearance of each resulting plumage. They were not quantitative or detailed; nor did Dwight describe the sequence of replacement of feathers in each molt or the relative timing of molts (1) between sexes, (2) among age classes, (3) among different populations, or (4) in relation to ecological factors such as the effect or failed (or unattempted) versus successful breeding. Thus, such studies in North America are warranted.

In European populations, and therefore presumably in North American populations as well, there is tremendous geographic variation in nearly every aspect of the molts and plumages of this species as well as in their morphometrics and bare part colors (e.g., Harris 1971, Verbeek 1977, Walters 1978, Coulson et al. 1983, Ginn and Melville 1983). Therefore, the following discussion is confined to the North American subspecies smithsonianus .

The scenario presented below depicts the sequence of molts and plumages for an average individual. Herring Gulls typically do not achieve Definitive plumage until they are > 3 yr old (i.e., during their fourth Prebasic molt), but there is considerable individual variation in the age at which this plumage is attained, some doing so earlier and some later (Poor 1946, Monaghan and Duncan 1979).

Hatchlings

Chicks hatch semi-precocial and downy. Down pale drab-gray with only slight cast of cream-buff or pinkish-buff, particularly on head, grading to pale gray on belly; underparts are palest on chin and throat and unmarked except for spots on sides of chin and an obscure speckling or clouding on the sides, posterior abdomen and undertail coverts. Finely marked on the head with indistinct fuscous-black to deep gray spots that are stellate to polygonal in shape; back, thigh, and undertail coverts diffusely mottled with drab-gray to light clove-brown; this often gives the back a coarsely lined or spotted appearance; dusky spot on the shoulder and two blurred bars across the forearm; a small median spot or line on forehead; very variable spot on anterior crown which diverges into 2 less defined rows of spots in temporal region; a median spot, transverse group of spots or 2 spots one behind the other on occiput; often 2 pairs of large spots on nape, and often an additional large spot to each side of occiput; spots also on lores, relatively smaller spots around eye, and irregular spots on cheek, throat, and side of neck (Dwight 1925, Fjeldså 1977, Cramp and Simmons 1983).

Juvenal Plumage

Prejuvenal molt complete; down shed progressively over 30–40 d; lost first on wings, followed by back, underside, neck, and head; Juvenal plumage typically complete by 5–6 wk.

Head and underparts streaked gray-brown with paler face and nape, darker ear coverts and blackish eye-crescent. Mantle and scapulars gray-brown with pale edgings forming scaly pattern. Rump streaked gray-brown with same general tone as mantle and tail, thus contrast usually slight. Wing coverts same color as mantle and scapulars but more barred, less scaly, including outer greater primary coverts. Tertials blackish-brown with obvious pale border and obvious pale notches at sides and subterminal pale bar. Secondaries mainly blackish-brown, forming secondary bar. Primaries and outer wing mainly blackish-brown, but inner primaries pale with dark subterminal marks forming pale window, prominent from above and below. Underwing has prominent pale window; rest of underwing rather uniform pale gray-brown. Base of tail whitish with darker bars, generally gray-brown like rump and underparts; broad blackish-brown subterminal tail band (color plates in Cramp and Simmons 1983, photos and line drawings in Grant 1986, Urban et al. 1986).

Basic I Plumage

Partial molt of body plumage, mainly limited to head, neck, many or most mantle feathers, some scapulars, some rump feathers, and the breast and sides. All Juvenal flight feathers retained. Molt begins in Sep, Oct, or Nov (rarely later), typically ending by Dec. Prebasic I and Prealternate I molt overlap in some individuals (Dwight 1925, Cramp and Simmons 1983).

Basic I plumage similar to Juvenal except head (and sometimes underparts) whiter, and new mantle and scapular feathers with more complex pattern of dark bars; scaly Juvenal scapulars often retained (line drawings of primary and rectrix color pattern in Dwight 1920, color plates in Cramp and Simmons 1983, photos and line drawings in Grant 1986, Urban et al. 1986).

Alternate I Plumage

Partial molt of body plumage, usually in Mar, Apr, and May, though some may start earlier; chiefly confined to the head, some to most of back, rump, and underparts (Dwight 1925, Cramp and Simmons 1983).

Alternate I plumage similar to Juvenal except head and underparts often extensively whitish. Dark areas of wings and tail often faded to pale brown, and pale areas faded/worn to whitish, giving generally very pale appearance by summer. A few clear gray scapulars may be acquired from Apr onwards (color plates in Cramp and Simmons 1983, photos and line drawings in Grant 1986, Urban et al. 1986).

Basic II Plumage

Prebasic II molt complete. Dwight (1925) states that molt occurs from Aug to Oct, i.e., at the same time or later than adults. However, this molt occurs much earlier than in adults, at least in European populations (Coulson et al. 1983, Cramp and Simmons 1983). Whatever the exact timing may be, it cannot be confined to Aug–Oct as molt takes a minimum of 4 mo and usually as long as 6 mo (see below). Flight feather molt begins with the innermost primary (P1, not P10 as stated by Grant [1986] who followed Dwight's long-abandoned numbering system) and its associated greater covert, and progresses outward to the outermost primary (P10). At any one time, only 2 primaries typically are growing. Secondary molt starts simultaneously from both the outermost (S1) inwards, and from the innermost secondary outwards beginning at about the time P6 and P7 are growing. Secondary molt proceeds much more rapidly than primary molt, many feathers often missing at one time, typically finishing before completion of primary molt. Rectrix molt begins shortly after onset of secondary molt, usually beginning with the central pair and proceeding outward. Some birds replace R6 before R5, and some individuals may lose all rectrices synchronously. Wing coverts other than greater primary coverts are molted just before secondary molt, beginning with the median coverts and progressing to the greater and lesser coverts. Molt of underwing coverts is prolonged and spans the entire molt period. Body molt usually begins on the back (but occasionally on underparts [Harris 1971]), followed shortly thereafter by the humeral and ventral tracts, anterior part of head. Thereafter, body molt progresses outwards in all directions. All subsequent Prebasic molts follow same sequence. At the population level, molt takes about 6 to 7 mo, but individuals complete molt in about 4 to 6 mo (e.g., Harris 1971, Walters 1978). (line drawings of primary and rectrix color pattern in Dwight 1920, color plates in Cramp and Simmons 1983, photos and line drawings in Grant 1986, Urban et al. 1986).

Head white, usually with extensive dusky streaking; underparts and rump mainly white with variable amount of dark streaking; mantle and scapulars sometimes similar to Basic I but usually with extensive gray (contrary to Cramp and Simmons 1983). Outer wing mainly blackish (P10 sometimes with a small white terminal spot), but P1–P4 and their greater primary coverts mainly clear gray; tertials barred black and white or with extensive white internal markings, not mainly dark-centered as in previous plumages. Coverts of inner wing paler, sometimes with much gray color, more uniform than in previous plumages and with variable amount of brown barring; greater coverts vermiculated or finely barred, but not strongly barred as in previous plumages. Underwing generally whiter. Tail extensively whitish at base; broad, mainly solid blackish subterminal band of variable pattern.

Alternate II Plumage

Extent and timing of molt same as Prealternate I molt. Plumage same as Basic II except head and underparts mainly white. Mantle and scapulars clear gray, sometimes with a few brown-barred feathers. Dark areas on wings and tail faded, and pale areas faded to whitish (often contrasting with gray mantle and scapulars to give saddle effect) by summer.

Basic III Plumage

Extent of molt same as Prebasic II molt; molt probably begins at same time, but finishes about a month earlier (e.g., Dwight 1925, Cramp and Simmons 1983).

Head and body white with extensive dusky streaking especially around eye and on crown, nape, and hindneck; a few streaks on breast-sides and flanks. Mantle and scapulars uniform pale gray. Wings uniform pale gray except for black and white on outer primaries. Extent of black on distal portion of primaries decreases toward more proximal primaries, but is more extensive than in Definitive Basic plumage, i.e., black tips extend to primaries more proximal than P5 (typically innermost primary with a black tip in Definitive Basic plumage). Greater primary coverts and alula also black-tipped. Outer primaries with black also have white tips (mirrors), though smaller, less numerous, and on fewer primaries than in Definitive Basic plumage. Variable amount of brown markings on inner wing, especially on median and lesser coverts, secondaries and their greater coverts, and tertials (which are mainly gray); these brown markings are less than in Basic II plumage. Underwing white (sometimes with some brown marks on coverts) with gray subterminal trailing edge and black tips to outer primaries. Tail white with subterminal markings of highly variable extent and pattern; these markings typically finely barred or freckled (not mainly solidly dark as in Basic II plumage) or often confined to feather centers giving tail dark-and-white striped pattern (line drawings of primary and rectrix color pattern in Dwight 1920, color plates in Cramp and Simmons 1983, photos and line drawings in Grant 1986, Urban et al. 1986).

Alternate III Plumage

Extent and timing of molt same as Prealternate I molt. Plumage same as Basic III except head and underparts usually white; freckled brown areas on inner wings worn or faded to whitish; white primary tips reduced or lacking due to wear (Grant 1986).

Definitive Basic Plumage

Extent and timing of molt same as Prebasic III molt. Molt typically begins during egg-laying, incubation, or shortly after hatching (e.g., Verbeek 1977, Walters 1978).

Plumage same as Basic III except black primary markings are less extensive and usually terminate with a subterminal spot on P5, forming a clear-cut black wing tip above and below; some individuals also have small blackish marks on outer greater primary coverts. White mirrors larger, more numerous and on more primaries. Mirrors on P9–P10 obvious at long distance. No brown markings on inner wing or underwing; underwing white with gray subterminal trailing edge and black tips to outer primaries. Scapular-crescent small or lacking; tertial crescent prominent when perched. White leading edge to inner wing indistinct, but trailing edge prominent (line drawings of primary and rectrix color pattern in Dwight 1920, color plates in Cramp and Simmons 1983, photos and line drawings in Grant 1986, Urban et al. 1986).

Definitive Alternate Plumage

Extent and timing of molt same as Prealternate I molt.

Plumage same as Definitive Basic except head and underparts white; white primary tips reduced or lacking through wear (line drawings of primary and rectrix color pattern in Dwight 1920, color plates in Cramp and Simmons 1983, photos and line drawings in Grant 1986, Urban et al. 1986).

Aberrant plumage coloration. Ross (1963) mentions records of 2 albinistic chicks, one albinistic adult and three other unspecified records of albinistic birds. Albinistic individuals have been reported widely outside of North America. No melanistic birds have been reported in North America, but, again, there are many reports outside of North America.

Bare part colors of known age smithsonianus and their individual and age-related variation are discussed by Poor (1946).

Hatchlings

Bill described as "horn color with a pink tip" (Dwight 1925) to aniline black, often pinkish at base (especially on lower mandible), and with the distal third pinkish buff in youngest nestlings (Fjeldså 1977, Grant 1986); legs and feet grayish pink becoming brownish drab during nestling period; iris dark brown (Dwight 1925, Fjeldså 1977).

Juvenal

Bill dark, pale at base; gape pale vinaceous or fawn. Iris sepia or dark brown. Legs and feet dark gray with flesh overtones.

Basic I Plumage

Bill black diminishing toward base. Iris and feet unchanged. First spring, same.

Basic II Plumage

Bill olive-buff, drab, or flesh at base, whitish at extreme tip; gape pinkish white or buff. Iris pale straw, buff, or brown. Legs and feet pale pinkish buff or pinkish white.

Alternate II Plumage

Bill more yellowish basally. Iris, legs, and feet unchanged.

Basic III Plumage

Bill yellowish olive-buff with black or brown bar or spots behind nostrils; gape pinkish buff. Iris straw yellow or buff. Legs and feet unchanged.

Alternate III Plumage

Bar on bill disappears, some of adult red spot appears. Iris brighter yellow. Legs and feet brighter pink.

Definitive Basic Plumage

Bill lemon, cream, or other yellows with scarlet or orange-vermilion spot on each side of terminal portion of lower mandible; gape yellow or cream. Iris straw or pale orange-yellow. Legs and feet pale pinkish cinnamon or flesh color.

Definitive Alternate Plumage

Bill and gape brighter colored. Iris brighter yellow or orange. Legs and feet unchanged.

Norstrom et al. (1986) discuss the energetics of molt in Herring Gulls. The authors conclude that the daily cost of molt is small (about 6 Kcal/d) and may be mostly or entirely offset by reducing other energetic expenditures (e.g., cost of foraging). These conclusions should be viewed critically in light of more recent research on the cost of molting, especially that of Mary Murphy and the late James R. King.

Lustick et al. (1980) showed that as the angle of incidence of solar radiation onto the plumage of Herring Gulls increases, the difference in heat load between light and dark plumages disappears. Thus, by postural adjustment, birds with dark plumage may become thermally white with regard to radiative heat load.

Boss (1943) experimentally induced males and females to acquire Definitive plumage at the age of 1 yr by a series of injections of testosterone beginning at the third day of incubation and continuing periodically for 12 mo. Estradiol, stilbesterol, gonadotropic hormones, and thyroxin had no effect on plumage development. This is noteworthy because it indicates that development of Definitive plumage in both males and females is regulated by testosterone levels.