Appearance

Common Loons have 10 functional primaries, 23-24 secondaries (including 4-5 tertials or tertial-like feathers), and 18-20 rectrices. Wings are moderately pointed and tail is very short. Geographic variation in appearance slight (see Systematics: geographic variation) and no geographic variation in molt strategies reported.

Molts

Molt sequence of Common Loon is complex, often protracted, and individuals may appear to be molting throughout much of the year. Molt and plumage terminology follows Humphrey and Parkes (1959) as modified by Howell et al. (2003, 2004). Common Loon exhibits a Simple Alternate Strategy (cf. Howell et al. 2003), including complete prebasic molts, a single absent-to-incomplete inserted first-cycle molt, and partial-to-incomplete prealternate molts in definitive cycles (Dement'ev and Gladkov 1951, Palmer 1962, Stresemann and Stresemann 1966, Oberholser 1974, Cramp and Simmons 1977, Godfrey 1986, Baker 1993, Heubeck et al. 1993, Howell and Pyle 2005, Pyle 2008; Fig. 5). The single first-cycle molt has been interpreted as a first prealternate molt (Howell and Pyle 2005, Pyle 2008) but it also may have evolved from a merging of preformative and first prealternate molts of ancestral taxa, given earlier starting time to this molt than to later prealternate molts. Definitive cycle in loons has been interpreted in various ways (cf. Sutton 1943, Palmer 1962, Woolfenden 1967), primarily as involving replacement of primaries following that of body feathers during prebasic molts, while some authors have described this remigial molt as being part of the prealternate molt. Interpretation here based on a careful tracing of molts through predefinitive to definitive cycles (Howell and Pyle 2005, Pyle 2008:218-219) and fact that most birds initiate prebasic molts with primaries rather than body feathers (cf. Pyle 2005 regarding waterfowl). Details of feather replacement based on observations of autopsied specimens and hand-hatched/reared birds (JFB) and by observations while following the molt progressions of adult and juvenile loons banded on their marine wintering and maturation habitats (D. Long, pers. com.).

Definitive plumage aspect often assumed following the third prebasic molt or fourth prealternate molt; thus, the Definitive Cycle equates to the third or fourth cycle, although it may not be reached until the fifth or later cycles in some individuals (study based on marked, known-age birds needed).

Prejuvenal (First Prebasic) Molt

Complete, Jun-Jul, in the nest. Description of Prejuvenal Molt based primarily on findings from 2 sets of hand-hatched/reared loons (n = 7) raised in different years in e. Ontario (JFB). Juvenal plumage largely replaces second down by week 8, but there is some growth, especially in primaries until week 15.

Juvenal feathers appear first on white underparts by week 3, then on head (week 4) where first gray Juvenal feathers replace down on lores and chin, advancing rearward and down throat. Juvenal feathers appear anterior and medial to humerus by week 5, when white belly-feathers are prominent, almost complete. Head-feathers appear by week 6. By day 42, general appearance still sooty brown, down-covered but down on belly almost completely replaced by white feathers at this age. Still no gray white streaks on sides of breast. Down still present on nape and dorsal tract by week 7; long down tips still present on scapulars. Whitish feathers appear around eyes, lores, and chin. Chicks fully feathered by week 8. Remiges begin erupting about day 24; rectrices about day 27. Growth of primaries accelerates in week 4 (longest primaries average 52.3 mm by week 6, n = 7), remains rapid to week 11, when about 80% grown (average 145 mm, n = 7; approximately 7 wk after initial eruption of remiges) and juvenile is capable of flight, then declines gradually. Primaries fully grown (average 185 mm, n = 7) by week 15.

Preformative Molt ("first prebasic" or "Basic I" molt of previous authors)

Single inserted first-cycle molt here interpreted as a First Prealternate Molt (see below), although it may have evolved partially from a preformative molt of ancestral taxa, and thus be interpreted as a merged, "preformative/first prealternate" molt (cf. Pyle in press regarding similar molts in alcids).

First Prealternate Molt

Incomplete, Dec-May, on non-breeding grounds. Can possibly be absent in maladapted individuals, but typically includes up to some or all body feathers of the head and back, perhaps some proximal lesser upperwing coverts, and the rectrices. Reports of complete molt result from different interpretation of first post-juvenal wing-feather and some body-feather replacement as being part of this (or Preformative/Basic 1) molt rather than as part of Second Prebasic Molt (below). In hand-reared juveniles (n = 7; JFB), and in banded juveniles on marine maturation habitats (n = 27, D. Long, pers. com.) by mid-Jan first signs of First Prealternate Molt can be seen in some birds along flank (perhaps an adaptation to increase floatation; see Pyle 2005, in press). By second week of Feb, can gradually begin losing crown feathers and perhaps lesser coverts. By first week of Mar, can begin molting scapulars and molt can extend from crown over nape, neck, and back. By fourth week of Mar, new feathers can predominate on back and are present on breast and belly; new rectrices can erupt. Individuals (n = 11) on winter range, Gulf of Alaska, late Mar/early Apr, showed varying degrees of body molt (JFB). In hand-reared birds rectrix molt complete by mid-Apr; rump-feathers still distinctly brown in mid-May (10–80% replaced) but by second or third week in May, rump-feathers 90–100% replaced and First Prealternate Molt complete.

Second And Third Prebasic Molts

Complete, Apr-Oct, on non-breeding grounds. Primaries, secondaries, and probably upperwing coverts replaced synchronously (see Definitive Prebasic Molt) in May-Jul (second prebasic molt) or Apr-Jun (third prebasic molt) followed by replacement of body feathers in Jul-Oct. Wing molt sometimes interpreted as part of first-cycle molts or second prealternate molt; often overlaps with these molts. During Second Prebasic Molt in hand-reared birds, primaries molted simultaneously, beginning late May or early Jun (25 May–6 Jun). Primary molt reported to continue through Aug (Palmer 1962). Second remigial molt into Third Basic Plumage can occur approximately six months following first remigial molt during the previous summer into Second Basic Plumage (D. Long, pers. com.). Remiges molt later in season during Second Prebasic Molt than in adults; however, remigial molt during Third Prebasic Molt on closer schedule to adult, as determined through tracking banded, known-age individuals in marine maturation areas (D. Long, pers. com.).

Second Prealternate Molt

Incomplete, Apr-Jun, on or near non-breeding grounds. Includes some to most feathers of head, back, and breast, and the rectrices. Typically overlaps wing-feather replacement of Third Prebasic Molt (see above).

Definitive Prebasic Molt

Incomplete to complete, primarily Mar-May and Aug-Nov. Occurs in two stages separated by suspension for breeding. Wing feathers replaced in Mar-May on non-breeding grounds. Primaries and secondaries replaced synchronously (May 1930, Sutton 1943, Woolfenden 1967) during this period resulting in flightlessness for 4-6 weeks. Molt of secondaries more prolonged than for primaries; in hand-reared birds, molt occurred from proximal feathers to distal feathers but may be considered nearly simultaneous. Precise duration, timing of onset, and flightless period for individual wild adults not known, but may be influenced by latitude and age; first birds arrive to breeding grounds in early Apr (45–47°N), before territories are completely ice-free; others are still migrating north into this region throughout May. Healthy adults from winter range in Gulf of Alaska were still in wing molt between fourth week of Mar and first week of Apr 1989. One male had not shed primaries. Average length of P10 was 123.7 mm (range 1–220, n = 159). Renewal of primaries more advanced in males (average 143.7 mm, range 41–220, n = 91) than in females (104.4 mm, range 1–196, n = 68; JFB).

Timing of remigial and wing-covert molt (midwinter through spring), long after body molt completes, unusual in birds (Woolfenden 1967). Common Loons are heavy birds with high wing-loading; loss of even a few flight-feathers could result in flightlessness. Thus selection appears to have favored the simultaneous molting of all primaries, reducing period of flightlessness to a few weeks. Most individuals molt remiges on marine waters, where conditions are generally favorable. Because breeding season extends into fall, until young are fledged, most individuals do not have time to molt flight-feathers before freeze-up and thus this wing-feather molt appears to have been pulled forward, occurring in spring (Howell and Pyle 2005).

Body feathers replaced Jul or Aug-Nov, often beginning on breeding grounds but usually occurring primarily or entirely on non-breeding grounds. Increased preening, loss of belly-feathers, and subtle changes in plumage aspect first noticed in late Aug–Sep in some hand-reared birds. Adults on breeding grounds without chicks start molt earliest, some by late Jul (but may include those undergoing Third Prebasic Molt); transition appears slow until Sep. Molt begins with feathers at base of bill, on lores, and on cheeks, and progresses rearward; feathers of belly and back found on water with greater frequency by late Aug. Many birds are in advanced molt by Oct but some are still show mostly breeding plumage aspect in late Nov (these possibly birds in basic plumage with breeding-like patterns to feathers). Scattered alternate back feathers, scapulars, and/or basic wing coverts from previous generation are usually retained during this molt.

Definitive Prealternate Molt

Incomplete, Jan-Apr, on or near non-breeding grounds. Includes most to all feathers of head, back, breast, flanks, and undertail coverts, and the rectrices. Typically overlaps wing-feather replacement of Definitive Prebasic Molt (see above).

Although adults are found in all stages of Definitive Prealternate Molt along California coast in early Mar (JFB, JWM, D. Long, pers. com.), in general, Definitive Prealternate Molt may occur later during winter/spring at higher latitudes (see also Definitive Prebasic Molt). Healthy adults from winter range in Gulf of Alaska were still in molt between fourth week of Mar and first week of Apr 1989; only 2 of 159 (1.26%) had completed molt to Definitive Alternate Plumage. All others were in active molt, many with numerous pin-feathers, especially on breast and belly.

Plumages

Following based primarily on detailed plumage descriptions in Dement'ev and Galdkov (1951), Roberts (1955), Oberholser (1974), Cramp and Simmons (1977), Godfrey (1986), and McIntyre (1988), and examination of specimens and live birds by JFB; see Baker (1993) and Pyle (2008) for specific age-related criteria. Sexes show similar aspects in all plumages.

Several predefinitive plumages have been ascribed by various authors, including Palmer (1962), before acquisition of definitive plumages (in third or fourth calendar year), birds obtaining increasing number of white spotted feathers on upperparts and wing coverts. Because younger individuals often remain in similar habitats used by adults as wintering grounds for up to three years before commencing migrations to potential breeding areas, “summer” plumages worn during the period before they begin to breed are poorly known, and have been described primarily from specimens. However, Palmer's conclusions appear to be drawn from mis-aged birds (see below). Examination of 100s of specimens by Pyle (2008) results in the following interpretation of plumage maturation in this species.

Natal Down

(Jun). Chicks semiprecocial and completely downy at birth. Down dries within 1 h of hatching; down is covered with fine flaky gray dusting, which quickly sloughs off. There are 2 generations of down. First down (protoptile) is very dense, black or sooty, with white breast and belly and transitional gray margin between. Second down (mesoptile) displaces first down in week 3. Second down is sooty brown, longer, and less dense than first down (JFB). Chicks continue to have gray-brown appearance by week 7 as down wears off feather-tips of Juvenal Plumage (see also below). Brown down still covers nape and lower neck; long loose down still present on legs and rump, lateral to uropygial gland; crown, lores, and cheeks with down present on tips of feathers; down absent from some back and scapular feathers. Down nearly or completely gone by d 60, when slight traces of down can still form a single large “V” on nape.

Juvenal (First Basic) Plumage

(Aug-Jan or Jun). Aspect grayish above, white below; no spots. Forehead, crown, side of head, nape, and side of neck medium neutral gray to gray brown; feathers small, dense, velvety, and downlike. Proximal portion of head and neck feathers paler gray than tips, giving mottled or speckled appearance to lower side of neck when feathers are disrupted or wet. Pale gray white feathers fringe gape, lower cheek, and throat and extend farther around side of neck in same place that necklace of Definitive Alternate plumage later appears (see below). Pale brownish white eye ring (less conspicuous than in adults). Dark feathers of hindneck abruptly meet lighter gray of back and side of breast. Sides of breast finely streaked brownish gray on white. Back and scapular feathers brownish gray, with gray white tips (most prominent on scapulars), forming distinctive, regular pattern of transverse scalloping. Remaining upperparts and wing coverts brownish gray with diffuse pale tips resulting in scalloped appearance; wing coverts rounded. Remiges and rectrices brownish black with distinct “V” at tip of rectrices where down has broken off; this character is diagnostic of juvenal feathers. Outer primaries narrow at tip. Center of breast and remaining underparts whitish, except upper flank-feathers dark brownish gray with paler margins, and underbelly has black band across it through vent. Underwing coverts white; axillaries have prominent black stripe along quill; stripe on underwing tertial coverts is less prominent gray, becomes narrower toward outer secondaries and inner primaries where limited to distal half of outer web, then diffuse to tip (see illustration and photos in Appleby et al. 1986).

First Alternate Plumage

(Feb-Aug). Aspect similar to that of Juvenal Plumage except replaced feathers darker above; more sharply defined wedge of gray/black apparent on lower neck. Replaced upperpart feathers steel blue gray; generally less rounded and have darker tips, thus upperparts appear more uniform and less scalloped. New flank-feathers similar to those of Juvenal Plumage. Rectrices have white tips. Juvenal outer primaries retained through May-Jun, worn, frayed, and bleached; this, basic-like plumage aspect, and lack of distinct spots on replaced feathers enables identification of first-cycle individuals.

Palmer (1962) stated that birds obtain white spots on some feathers of upperparts and lesser wing coverts during summer of second calendar year but his description apparently was based on individuals or specimens that Palmer considered to be 1 yr old (in First Alternate Plumage) but likely were older birds, including those still molting from basic to alternate plumages. No white spotted feathers ever appeared in feather coats of 2 different sets of hand-reared birds (n = 7) as result of molt following Juvenal Plumage (JFB). See further information under Second Alternate Plumage (below).

Second Basic Plumage

(Jun-Feb). Aspect similar to that of Definitive Basic Plumage but diffuse spots on back feathers more rounded; distinct spots on wing coverts smaller and oval-shaped or rounded (Pyle 2008).

Second Alternate Plumage

(May-Aug). Aspect similar to that of Definitive Alternate Plumage but head and chin variably mottled dark greenish black and white; aspect varies from primarily white to primarily blackish green. Some basic back feathers and wing coverts sometimes retained and brownish or brownish with pale spots. Aspect of some individuals may resemble that of Definitive Alternate Plumage (see below).

Through following loons that were banded in juvenal plumage during their first winter at Morro Bay, CA, 2004-2008, the first white-spotted coverts and scapular feathers are determined to appear at age 20 months, commencing with the appearance of the bird’s Second Alternate Plumage (D. Long, pers. com.). The degree of fullness of white spotting in Second Alternate Plumage varied dramatically at age 24 months, ranging from nearly complete breeding aspect with just a few streaks of gray on the face, to only a few white spots throughout the scapulars and coverts, or in many cases, incomplete (cream-colored) spots (D. Long, pers. com.).

Timing, duration, extent, and number of molts before first Definitive Alternate Plumage previously debated (see above). Palmer (1962) stated that Definitive Alternate Plumage is first acquired during spring of fourth calendar year, and that during summer of second and third calendar years, First and Second Alternate plumages, respectively, were acquired, both of which were intermediate in appearance between Definitive Basic and Definitive Alternate plumages. However, Palmer’s description of Second Alternate Plumage was based on 4 specimens collected Jul–early Aug. These specimens were described as having an appearance similar to that of Definitive Alternate Plumage, “except dark areas are duller; head and neck less glossy, black of throat more or less mixed with white, collar streaks finer, much more brownish” (Palmer 1962: 24). These characteristics also describe the appearance of adult loons during Jul and Aug, at which time their plumage has become faded, and, at least in failed breeders or nonbreeders, definitive basic feathering has already begun to appear on head and neck (see Definitive Basic Plumage, below). Since age of birds when collected as specimens cannot always be determined conclusively, it remains possible that specimens described by Palmer as being 2 yr old (in Second Alternate Plumage) were actually faded adults that had begun their Definitive Prebasic molt. One individual raised in captivity for 4 yr at the Minnesota Zoo also acquired Definitive Alternate plumage during the spring of its third calendar year (J. Pichner pers. comm.) For these reasons, although hard evidence is limited, we feel that the existence of a Second Alternate Plumage (similar to Definitive Alternate but with some characteristics of Definitive Basic plumage) in all birds remains unproven. On the other hand, Pyle (2008) concluded based on specimens that some to most individuals in Second Alternate Plumage show white mottling to the head and throat, and that some non-breeding individuals in Third Alternate or later plumage may also have this appearance.

Definitive Basic Plumage

(Apr-Feb). Aspect similar to Juvenal Plumage, but head and neck darker steel gray; dorsal contour feathers terminally more squared, and pale fringe not so pronounced; pattern appears less scaled. Wing coverts (replaced in winter-spring) black with distinct white spots; these are kept throughout year, although difficult to see when bird is at rest and wings are folded. Some alternate back feathers and scapulars retained, blackish with distinct squared white spots. Primaries black, brown black proximally; slightly lighter on ventral side and toward outer edge of posterior inner web. Shafts black on distal end of dorsal side; brown black to lighter horn proximally; ventral side has similar gradient but is lighter. Secondaries glossy black dorsally, brownish ventrally and proximally; inner web paler; inner secondaries have 1 or 2 subterminal white spots. Shafts black dorsally, black brown ventrally, paler proximally. Wing-lining white; coverts have narrow black medial stripe. Axillaries white with black medial stripe more pronounced than on coverts. Rectrices black. Primaries broader, more truncate, and fresher than juvenal primaries; squarer and less-marked back feathers and large squarish white spots on wing coverts best means to separate birds in Definitive Basic Plumage from those in predefinitive plumages.

Definitive Alternate Plumage

(Mar-Aug). Head and neck velvety black; chin, throat, foreneck, and part of side of neck glossed with green; hindneck glossed with purple; sharp line of demarcation between black neck and white breast; a transverse line of small, white spots (6–12) laterally, and very short vertical stripes medially high on foreneck below head; an oblong patch of vertical, parallel, white stripes (these sometimes forked and may change with head and neck movements), on each side of neck; each patch widens toward rear, tapering front and back, patches not quite joining in front or back on either side of neck. Upperparts black with slight green to green-blue sheen. Small white spots on anterior mantle, back, and rump; increase in size rearward on mantle and become largest, and rectangular in shape, on scapulars, forming transverse rows. Each feather of mantle has 2 subterminal, rectangular spots, largest on scapulars. Sides of upper breast have curved black and white stripes, the latter continuing along black flank as undulating rows of white dots. Central breast and belly white with transverse black band through vent. Undertail coverts blackish brown with terminal white fringe. Black feathers lose sheen as season progresses; feathers of back, rump, remiges, and rectrices acquire brownish cast.

Bare Parts

Bill And Gape

In hatchlings bill blackish gray, lighter (bone) around egg tooth. Pale region of bill-tip more extensive in some individuals. By week 7 lower mandible changing color from black gray to bone white. By day 65, upper mandible begins same color change, starting at base and ventral edge, advancing toward nares. In Juvenile, bill pale gray with black gray culmen and tip; bill becomes darker in first spring. In Second and Definitive cycles bill color varies by season. In Oct-Feb bill light gray to horn; culmen dark, blackish at base and with tip color ranging from blackish to bone. In Mar-Sep bill black to slate gray, frequently lighter toward tip.

Iris

Eyes cinnamon to walnut brown as hatchling, starting to turn red in first winter and spring. In later cycles iris red in Oct-Feb, becoming blood red in Mar-Aug of Definitive cycles.

Legs And Feet

In hatchlings legs and feet (including webs) blackish gray; medial side of tarsus, digits, and webs begin changing color from blackish gray to milky white by week 8. In Juvenile, upper side of tarsus and toes dark gray black; undersides and webs pale gray to milk white. Blood vessels may lend pinkish cast to webs. In adults in Oct-Feb legs and feet gray black dorsally; toes usually lighter, light gray to milk white or with blue cast ventrally and on web, which may also have pink blush. In Mar-Sep legs and feet black dorsally, gray black ventrally and on web; underside varies with individual birds, becomes paler as season progresses.

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