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Common Loon
Gavia immer
Order
GAVIIFORMES
– Family
GAVIIDAE
Authors: Mcintyre, Judith W., and Jack F. Barr
Revisors: Evers, David C., and James D. Paruk

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Breeding

Figure 5. Annual cycle of the Common Loon in Ontario.
Adult Common Loon with chick
Adult Common Loon on nest, ME, May.
Adult Common Loon and chick on artificial nest platform, Presque Isle Co., MI, June.
Adult Common Loon tending egg, ME, June.
Adult Common Loon feeding chick a small crayfish, Presque Isle Co., MI, June.
Common Loon breeding habitat, Alaska.
Common Loon clutch, Minnesota.

Phenology

Pair Formation

Pairs rarely observed together prior to arriving at breeding lakes. After ice out (Apr-May) males and females arrive separately, generally within a day or two and immediately begin socializing, developing and establishing, or re-establishing, the pair bond (McIntyre 1988, JDP, DCE) for up to 6 wk (or more) before the first nest is built and eggs are laid.

Nest-Building

Most nests are constructed in May and June, but on reservoirs with fluctuating water levels, may be built as late as early July (JDP, DCE). Nest construction takes several days; but replacement nests may be built in a day (JWM, DCE).

First/Only Brood Per Season

Initially lay Apr–Jun (Fig. 5); timing is partly latitude-dependent, since 3–4 wk may pass between return to breeding grounds and onset of nesting in New York (43°N; JWM) but no more than a few days in Iceland (60°N; Sjölander and Ågren 1972). Range 21 May–1 Jul in Saskatchewan (Yonge 1981), 21 Apr–23 Jul in British Columbia (Campbell et al. 1990, 2008), 7 May–28 Jun in n.-central Minnesota (McIntyre 1975). Peak dates in Saskatchewan 24–29 May (Yonge 1981); Minnesota peak at the same time, although latitude is 8° farther south (McIntyre 1975). Based on nearly 3,600 nesting attempts, New Hampshire statewide peak is 6 Jun; nest initiation ranged from Apr 30 to Aug 2 from 1975 to 2008 (LPC, unpubl. data).

Normally lays 2 eggs, less common 1, which may be more likely for a second nesting attempt; 2-d laying interval between eggs; <24 to 48 h hatching interval.

Replacement clutches laid 8–19 d after nest loss through June, with longer intervals later in nesting season (Yonge 1981); rarely into August (McIntyre 1988a, Campbell et al 2008, DCE/JDP).

Nest Site

Selection process

Contrary to early reports (Sjölander and Ågren 1972, McIntyre 1975), Piper et al. (2008b) used known individuals on 98 territories over 14 years to document that males control nest site placement. Territories with unchanged mates or unchanged males will reuse a successful nest site 50% of the time; while those with unchanged females or both mates changed will use a new nest site 95% of the time (Piper et al. 2008b). Copulation site may also be nest site (JWM, JDP).

Microhabitat

Ground nest. Prefers sheltered side of islands facing mainland (Yonge 1981), lee side of mainland on small lakes with no islands (McIntyre 1975), or floating bog islets with screened view to open territory (JFB, JDP).

Site Characteristics

Prefers nesting on islands versus mainland (McIntyre 1988a, JDP). Nest adjacent to water near drop-off steep enough for underwater approach; prefers open view of territory (>130°; McIntyre 1983), generally near cover. Opportunistic; readily uses marshes, sedge mats, cranberry bogs, floating bogs, and even tops of muskrat (Ondatra zibethicus) houses or logs (Olson and Marshall 1952, Palmer 1962, Vermeer 1973b, Alvo 1981, McIntyre 1988a). Will use artificial nest sites (see Conservation and Management).

Nest

Construction Process

Diurnal builders. Adults contribute equally (McIntyre 1975, JDP), although (because males control nest site selection) early construction is primarily by males (Piper et al. 2008b); pull or retrieve vegetation adjacent to nest or from lake bottom and throw it sideways onto nest. Nest shaped by adult sitting and contouring materials to body shape and pulling in surrounding materials while sitting on nest. Early in season, nest takes ≥1 wk to build; late in season, nest may be completed in 1 d. Materials added throughout incubation.

Structure And Composition Matter

Variable, majority of nests large oval to circular masses of plant material; will form simple scrapes and depressions in sand or on rock. Nesting material consists of wide variety of plant species (>22 species of vegetation identified; McIntyre 1975, Sutcliffe 1980, Yonge 1981). May not construct a nest under certain conditions, but simply lay an egg on sand or rock (McIntyre 1988, JDP).

Dimensions

In Minnesota, average diameter 55.9–56.9 cm outside, 24.4–33.0 cm inside (n = 113; Olson and Marshall 1952, McIntyre 1975). Weighs up to 18 kg (McIntyre 1975).

Microclimate

Shelters nest from waves by avoiding prevailing winds (McIntyre 1975, Yonge 1981). Untested if moist rotting vegetation provides heat. When adults not incubating, eggs remain uncovered. No lining materials.

Maintenance Or Reuse Of Nests, Alternate Nests

Successful nests sites are often reused from year to year, especially if the male returns (Piper et al. 2008b); failed first nests result in re-nesting at a different location (sometimes not very far from the original nest). If alternate site fails may return to original nest location (McIntyre 1975, DCE). Continues to add materials during incubation and may increase height 15–20 cm to prevent flooding (McIntyre 1988, JDP).

Nonbreeding Nest

“Dishes” (initial nest bowls) are often created and not used for nesting; one or more dishes can be common reservoirs (J. Fair, pers. com.).

Eggs

Shape

Sub-elliptical to oval; highly variable among individuals but similar in shape and size, even from year to year (McIntyre 1975).

Size

Varies geographically with body size, but geographic cline is not as distinctive. Average size for 9 states, including Alaska and those in the west, central and east for 1,346 eggs is: length = 89.8 +/- 2.0 mm, breadth = 56.8 +/- 1.5 mm, volume 146.2 +/- 10.8 cm3 (BRI unpubl. data). Average size for eggs not included within the above geographic coverage are Saskatchewan (87.1 mm x 54.7 mm, n = 148; Yonge, 1981) and Alberta (86.5 mm x 53.9 mm, n = 40; Vermeer 1973b). Average volume in Iceland is 150.3 mm (n = 34, Anderson et al. 1970). Variation in egg size within a state/region may exceed 30% around the average (DCE).

In Saskatchewan, second egg in clutch averaged 2% smaller than first (Yonge 1981), 3.5% smaller in New York (McIntyre 1988a). Replacement eggs average 4% smaller than those of initial clutch (Yonge 1981). Size unrelated to hatchability, unless related to mercury concentrations, where smaller eggs are produced as mercury concentrations increase (Evers et al. 2003).

Mass

Fresh whole egg average weight (in grams) is 143.3 +/- 9.6 for 1,346 eggs from a collection effort representing 9 states (see Eggs – size); some eggs can exceed 200 g (BRI unpubl. data).

Color

Ground color olive to brown, with dark brown or black splotches. Variable among individuals and within clutches.

Surface Texture

Slightly granular to glossy (Palmer 1962).

Eggshell Thickness

Post-DDT eggshell-thinning statistically but not biologically significant across range (Vermeer 1973a, Sutcliffe 1978, Fox et al. 1980, Frank et al. 1983). In Minnesota, eggs laid 1970–1974 (n = 55; McIntyre 1975) averaged <10% thinner than those laid before 1946, although 20% were as thick or thicker than pre-1946 specimens from same geographic region (Anderson et al. 1970). New York shells (1978–1986) averaged 8% thinner than pre-DDT shells from same population (Anderson et al. 1970, McIntyre et al. 1993). Eggshells were 3-4% thinner on Wisconsin lakes with pH ≤ 6.3 than on neutral-pH lakes (Pollentier et al. 2007).

Clutch Size

Generally 2 eggs, occasionally 1 egg; production of ≥3 eggs from one known female is yet to be confirmed. Genetic analysis is needed. Three and four egg clutches noted in the literature (Zicus et al. 1983, Nelson 1983) are likely from either two females or represent two clutches laid at separate times by one or two females (DCE).

Egg-Laying

Females begin laying just before, or shortly after, nest is completed (JFB, JWM). Eggs are laid 1–3 d apart; nutrition may influence intervals, which are longest on less productive lakes in the northern part of the range. If part of the clutch is lost, it is not replaced. If entire clutch is lost, re-nesting occurs 8–18 days later; longer intervals later in nesting season, independent of stage of incubation when clutch is lost (Yonge 1981). In New Hampshire (from 1975-2008), 39% of nests failed and 10.4% of failed nesting pairs re-nested (n = 344 of 3,579 nesting attempts) (LPC unpubl. data). May repeat laying 3 times (McIntyre 1975), but a third laying likely indicates a switch in females (DCE). Egg-dumping not documented, but suspected (Nelson 1983, Zicus et al. 1983).

Incubation

Onset Of Broodiness And Incubation In Relation To Laying

Incubation is sporadic until clutch is complete.

Incubation patch

None, but there is extensive subcutaneous vascularization in posterior abdominal region of both sexes during incubation (JFB). Eggs are incubated beneath this region, and between feet, which may also contribute heat.

Incubation Period

Period from laying of first egg to hatching of second egg lasts 27.5 +/- 1.5 d (n = 1,984; LPC unpubl. data). In Saskatchewan, 28 d with a range of 27.4–30.6, n = 17 (Yonge 1981). Over-incubation may happen for non-viable eggs (up to 74 d; Sutcliffe 1982) and in New Hampshire occurs 2.8% of the time (n = 3,949 nests) or accounts for 6.8% of the nest failures (LPC, unpubl. data).

Both parents incubate. Although sexes do not differ statistically in total time spent incubating (McIntyre 1975, Evers 1994, Mager 1995, Paruk 1999a), females incubated more than males in the majority of the pairs (71.4% of total time; Mager 1995, Paruk 1999a). Shift in gender incubation periods from 50–60% by males during week 1 to >60% by females during week 4 (Mager 1995, Paruk 1999a), suggesting females may need additional foraging time to meet energetic demands associated with egg-laying. Time spent incubating by gender may be a function of territory quality and type (JDP).

Nest attentiveness ranges from 96-99% on lakes with a single pair of loons to 78%-96% on lakes with more than one pair (Paruk 1999a, 2000, Cummings 2003). Pairs with nest coverage < 91% are typically less successful (Cummings 2003, JDP); this may be related to environmental stressors such as elevated mercury body burdens (Evers et al. 2008). Reasons for leaving the nest include human disturbance, territorial defense, thermoregulation (presumably) and foraging (Paruk 1999a, Meyer 2002, McCann et al 2004). Incubation bouts vary from 0.5 to 9 h. Average bouts vary with study and location, from 2-4 h (Gromme 1929, McIntyre 1988a) to 5-6 h (Paruk 1999a).

Paruk (2000) found loons nesting on large lakes with multiple loon pairs had significantly shorter incubation bouts than a single pair of loons occupying an entire lake. Eggs are turned at time of nest exchange and during long incubation bouts. Nest exchange usually occurs on or within 50 m of nest, but pair members may meet and exchange duty in mid-territory; alternatively, relieving loon may crawl on land while mate is still on nest. If anxious, incubator will wail, and if mate does not respond, continues wailing even after leaving nest to seek mate (JWM, JDB).

Nest exchanges are not random, peak times occur within 1 h of sunrise and 11:00-13:00 (n= 363 nest exchanges, Paruk 1999a). Some patterns of nest exchange are very predictable (e.g., at 40 min intervals on successive days) and others are unpredictable (e.g., by 3-4 h differences) (Paruk 1999a). Frequency of nest exchanges happening at night vs. day is 23% (Paruk 1999a). Goodale et al. (2005) documented female-only incubation at night for one nest in Maine, based on continual monitoring by a camera.

Nests are unattended sometimes between dawn and 08:00 (Barr 1996, Paruk 1999a). Incubating parent is more attentive to nest after first chick pips and calls hatched chick into water.

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

Not studied, but eggs known to hatch after being chilled and covered with dew. Overheating likely affects embryo development as in other avian species.

Hatching

Preliminary Events And Vocalizations

Chick peeps from inside egg before pipping.

Shell-Breaking And Emergence

Usually asynchronous. Interval longer at higher latitudes: 0-52 h (average 8 h) in e. Ontario (n = 50; JFB); 17.3 h in Saskatchewan (n = 39; Yonge 1981). Extreme asynchrony usually accompanied by long interval from pipping to hatching, often produces weak chicks that die within hours of hatching (JFB).

Parental Assistance And Disposal Of Eggshells

Eggshell of first-hatched chick almost always removed from nest by attending parent; deliberately sunk in water several meters from nest. Shell from second egg sometimes left on nest.

Young Birds

Condition At Hatching

Semi-precocial, but dependent on parents for food. Chicks are dry and often active within hour of hatching; usually leave nest with parents within day of hatching, weather permitting. Hatched chicks weigh on average 93.9 g (range 71.4–108.5, n = 18), about 60% of mass of newly-laid egg (JFB). Chicks covered with dense down; sooty black except for white belly. Tarsus, toes, web, and bill grayish black (colors 82–83 according to Smithe 1974); egg tooth lighter. Iris walnut brown (see Appearance). Egg tooth retained into week 2. Feeding movements jerky and uncertain on first day; coordination and ability to focus on food item improve rapidly.

Growth And Development

Most information from hand-reared birds (n = 5; JFB). Chick at hatching is about 2% of adult female body mass. Rapid growth occurs first in structures required for swimming and food capture (hindlimb, head); these structures grow more slowly after week 8, while rapid development of pectoral muscle and flight-feathers continues through week 11, when bill is 85% adult length, primaries are 80% grown (average 145 mm, n = 7), and chick is capable of flight. Chick mass is predictable for the first 8 weeks using a linear regression model, irrespective of chick sex and geographic origin (r2=0.95; Table 1). Weights of chicks from field (Barr 1996) and lab (Fornier 2002b) captive research projects tend to differ from average field weights (BRI unpubl. data), particularly for the field captive study.

Based on Barr (1996), natal down replaced by second (brown-gray) down by week 3. 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. Down still present on nape and dorsal tract by week 7. Chicks fully feathered by week 8. Juvenal primaries erupt by week 3; growth accelerates in week 4, remains rapid to week 11, then declines gradually. Primaries fully grown (average 185 mm, n = 7) by week 15. Eyes still walnut brown.

Control of body temperature unknown, although unable to regulate well at least for 1 wk; often observed brooding on parents’ back or under folded wing of adults to 3 wks. Chick on water is normally alert; pecks indiscriminately at moving objects when hungry—e.g., insects, sibling. Begs by peeps or yelps, pecking at side of parent’s bill, cheek, or neck. Peers into water, dives, and chases minnows by end of day 2.

Gives low-amplitude and low-frequency peep, yelp, hoot, and mew call when content, but amplitude and frequency rise when agitated, hungry, or stressed. Gives prolonged whine when distressed, paralleling progression in wail of juveniles and adults.

Inter-sibling conflict common, with first-born (dominant) chick indiscriminately pecking at second-born (subdominant) chick (Dulin 1988). Chicks may aggressively confront each other in upright stance, stabbing at or grasping each other’s head with bill; frequently both chicks tumble while holding onto each other until one chick submits. Subsequent conflicts may involve dominant chick, holding head high, approaching submissive chick that responds by cowering. Peck order is usually stable at 4 d, but is observed frequently during feeding bouts. Direct siblicide, though rare, has been observed (Strong and Hunsicker 1987, JDP, DCE).

Chicks can enter water, swim, and follow parents within hours of hatching. They swim like adults, except that, being buoyant, they use extended wings in conjunction with feet in attempts to dive and remain underwater. Use of wings to assist propulsion declines quickly; seldom apparent after first week. Able to dive a few centimeters below surface on day 1; to swim to bottom, probe, and search around objects and chase fish by end of wk 1; to swim underwater erratically 15 m by wk 2; and to chase fish 30 m by week 3 (capture rate 3%) (JFB). By wk 8, almost fully feathered, streamlined. Ability to compress feathers coincides with ability to forage independently.

Parental Care

Brooding

Occurs on nest immediately after hatching, with chick nestled under adult’s wing; afterward, chick(s) either on top of the parent’s back, or under the wing between flank and sides. Both chicks may ride on the back of one adult at age 1 wk, and then less frequently at wk 2. Chicks observed more often on back of males than females (Mager 1995, Paruk 1999a), but this varies from pair to pair. Chicks are occasionally fed while on the back of one of the adults.

Feeding

Parents offer food on day of hatching, but regular feeding may not be established until day 2 or 3 (Barr 1973). Initially both parents stay with chicks and feed them; one parent remains on surface while other dives for food. Compared to female, male more commonly observed in vigilant behaviors next to chicks (Mager 1995, Paruk 1999a). During period that chicks are 1-2 wk of age, one parent remains with young while the other is frequently away between feedings. Feeding bouts are brief, but frequent. Beginning wk 2, both parents simultaneously dive for food while chicks remain on surface. As chicks mature, feeding bouts are less frequent, but longer. Feeding most frequent when chicks are 4 wk old (Paruk 1999a). Chicks ≥ 6 wk old are left alone for longer intervals; however, one or both parents remain aware of chicks’ location, returning quickly at signs of danger.

Chicks are completely dependent on parents for food initially but capture about 50% of their daily food by week 8, 90–100% by week 11, although still fed occasionally by parent in early morning or evening. Considerable variation in this pattern, but no apparent difference between parents in overall time spent feeding young (Mager 1995, Paruk 1999a). No significant difference in mean length of prey females and males fed to their young; more invertebrates fed to young during the first two weeks after hatch (Paruk 1999a). Adult may offer food item repeatedly if chick does not take it. Frequently dips prey in water before offering it and occasionally drops it in front of chick by week 3 (Barr 1973). Chicks grasp presented items crosswise in bill, shifting it with a series of head jerks so the fish rest parallel to the bill and then are swallowed headfirst. Chicks swallow prey by shaking their heads sideways or by extending neck when fish is large. Chicks manipulate large fish repeatedly, frequently attempting to swallow tail-first.

Diet of foraging chicks includes invertebrates, fish, and vegetation (Barr 1973, McIntyre 1983, Parker 1988, Alvo et al. 1988, Paruk 1999a). Chicks initially bring all captured food to surface; begin ingesting some prey underwater as they approach fledging. Prey size gradually increases commensurately with energy demands of growing chicks. Fish (1–2 g) offered from 1 d after hatching, when available. Chicks at week 1 eat fish up to 3 g; at week 2, up to 10 g. Fledged juveniles and adults eat fish of similar size.

Nest Sanitation

Adults do not defecate on or near nest unless fleeing. Chicks defecate after entering water, usually after feeding bouts, but also on shore (see Food habits). Early departure from nest after chicks hatch and simplicity of nest structure are not conducive to harboring invertebrate nest parasites; mosquitoes (Culicidae) and black flies (Simulium euryadminiculum) feed on both adults and chicks at nest.

Cooperative Breeding

Very rare. Three cases in Wisconsin (1) and Michigan (2) where three adults (2 males and 1 female) cared for chicks (< 6 weeks). In each case, care of chicks by the males appeared to be divided by night vs. day (DCE).

Brood Parasitism

None known. No exhaustive observational evidence to suggest otherwise.

Fledging Stage

Details from Barr 1973, 1996. Chicks begin exercising wings at 8 wk by orienting into wind and attempting takeoff; exercise infrequently during calm weather, but make frequent, repeated attempts with breezy, cooler weather. Exercise most in early morning, but bouts alternate with rests throughout day. Orientation into wind appears instinctive. During same period, chicks rear upright and, while treading water, flap wings and engage in mock battle by repeatedly grasping wrist of own semi-extended wing in bill. During prolonged calm periods, low stimulation results in lack of exercise, so first flight may be delayed until week 12 or 13. First takeoffs are usually brief, with juvenile staggering, even falling, but soon can make short flights (~800 m) with slight headwind (12–14 km/h);

Immature Stage

Juveniles become independent mid-Sep through mid-Nov as they begin to fly and catch their own food. One parent may depart territory when chicks are <10 wk old, but usually one parent remains until chicks are >12 wk of age. Chicks remain on natal lake 1–3 wk after parents leave. Migratory pattern of juveniles not well understood; pre-migratory and migratory staging occurs (McIntyre and Barr 1983). After first winter, first-year and second year individuals molt remiges too late (May to Jul; earlier tendency for second-years) to join adults on spring migration (DCE). See Migration, also Appearance.