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Main Foods Taken
Primarily live fish, opportunistically taken (see McIntyre and Barr 1997 for list of known species), although preferences are known. Also some crustaceans during the breeding and winter periods.
Microhabitat For Foraging
Prefers clear waters of littoral zones; tends to avoid foraging in deeper parts of lakes. Breeding adults and their young generally forage in relatively shallow areas: < 5 m in depth and within 50 -150 m of shoreline (Strong and Bissonette 1989, Ruggles 1994). Preferred prey species and size classes, such as 10 - 15 cm yellow perch (Perca flavescens), are found in this zone (Barr 1996).
Food Capture And Consumption
From Barr 1973, 1996; also JFB. Finds prey by peering into water while swimming, eyes beneath surface; also by searching and probing around vegetation and objects in water column and on bottom while swimming underwater. While searching for prey, holds body at an angle, anterior end lowest, with neck forming low S-curve. Jerks head and neck forward synchronously with propulsion stroke of legs, then retracts them between strokes. Eyes are briefly fixed relative to surroundings, an advantage in detecting motion. As pursuit starts, jerking stops; eyes are fixed on prey.
Underwater pursuit is foot-propelled by simultaneous thrust of feet, except in turns, when inside foot and tail are used as rudders. Grasps prey between mandibles and manipulates it to swallow it headfirst. Rearward-pointing, sharp denticles on roof of mouth and tongue assist retention of prey by interdigitating with scales or carapace. Using tongue, presses prey against roof of mouth while repeatedly retracting tongue in conjunction with extensions of head and neck. No taste buds found, but tactile corpuscles are present at base of tongue denticles. Proventriculus is completely lined with large digestive glands; large muscular gizzard effectively grinds bones and chitin.
Does not regurgitate indigestible materials. Gizzard maintains 10–20 pea-sized stones (smaller grit in chicks), which are routinely replaced as needed (average 11.6 stones, range 0–34, n = 74 gizzards; JFB). Stones aid in grinding hard material before it passes through pyloric sphincter into the intestine. Sexual difference in size of digestive organs and prey taken suggests that pair members may partition resources (Barr 1996). Chicks that lack grit in stomach are stimulated to eat gravel when gizzards are congested with fish bones (Barr 1973).
Adults ingest most prey underwater where taken. Larger items are brought to surface and repeatedly manipulated before swallowing or discarding. Will shake large crustaceans and flail them against surface to dismember them before swallowing. Adult transfers food to chick bill-to-bill at surface; acceptance of food and manipulation for headfirst ingestion are instinctive (Barr 1996).
Breeding adults do not regularly carry prey from satellite lakes to their natal lake although exceptions occur (Parker 1985) and may be related to acid-rain-induced lowering of prey concentrations (Alvo et al. 1988).
Major Food Items
Breeding season diet. Loons are obligate fish-eaters. They are opportunistic predators, and favor fish that have an erratic swimming behavior and fusiform shape (Barr 1996). Yellow perch (Perca flavescens) and centrarchid species such as pumpkinseed (Lepomis gibbosus) and bluegill (Lepomis macrochirus) are favored prey items for these reasons. In New England, in situ mercury (Hg) studies by Evers et al. (2004) substantiate the preference of perch by loons; adult blood Hg levels strongly correlate with yellow perch Hg levels (r2=0. 72 for males and r2=0. 75 for females) on lakes with cold-water and warm-water fisheries.
Loons will feed on salmonids; however, the straight-lined escape method salmonids use helps them evade capture. Loons foraging on lakes inhabited by salmonids apparently prefer other prey items such as perch (Evers et al. 2004) and chubs (Seiler et al. 2004). Burgess and Hobson (2006) further document close associations of yellow perch as prey for loons. These findings indicate low preference for sucker (Carpiodes spp.) and salmonid (Oncorhynchus spp.) species when perch or centrarchids are present. Fish larger than 30 cm are regularly taken as well, including northern pike (Esox lucius), chain pickerel (Esox niger) and lake trout (Salvelinus namaycush) (BRI, unpubl. data and LPC, unpubl. data). At Seney National Wildlife Refuge, Michigan, adult loons regularly forage on bullheads (Ictalurus spp.) (DCE).
Common Loons forage on many other fish species and frequently rely on the temporary abundance of a prey item to meet their dietary needs. Because loons capture and swallow small prey items underwater it is difficult to document all possible types prey items selected. Chicks are regularly fed macroinvertebrates including odonate larvae and leeches, although fish are generally considered an essential dietary component for successful fledging (Gingras and Paszkowski 2006).
Daily fish intake by an adult measured at approximately 960 g; a family with two chicks can consume upwards of 423 kg in one breeding cycle (Barr 1996). The relatively consistent sexual dimorphism measured in loons (see Appendix I) may be partly explained by enhanced foraging efficiency that provides a broader prey base through reduced competition within pairs which in turn provides a larger available prey base.
Crustaceans—e.g., crayfish (Decapoda) constitute major part of diet when fish are scarce or water is murky (1.0 m visibility), up to about a third of diet for males and more for females (Barr 1973). On some Wisconsin lakes, observed adults and chicks regularly observed foraging on snails (W. Piper, pers. com.). Leeches (Hirudinea) are occasionally an important food, and individuals that are stressed or ill sometimes eat vegetation (Barr 1973).
Migration and winter season diet. Migratory staging areas are important refueling and resting sites. Loons actively feed at fall staging areas, such as at Lakes Winnibigoshish and Mille Lacs in Minnesota (McIntyre and Barr 1979, Hertzel et al. 2000), Walker Lake in Nevada (L. Neel, pers. com.), Twin Lakes, ID (JDP), and Lake Erie (Roblee 2002). Fall migration timing may also overlap with shifting marine prey resources. For example, mid-November influxes of fall loon migrants into Chesapeake Bay coincide with Atlantic menhaden movements (Spitzer 1993).
In winter, loons use two general foraging strategies: solitary and group foraging. Solitary foraging likely results in reliance on evenly-spaced fish prey, such as Atlantic croaker (Micropogonoas undulatus) and spot (Liostomus xanthurus), while group foraging is more effective for patchy prey abundance such as schools of gulf silversides (Mendidia peninsulae) (Vlietstra 2000). Large prey items that are difficult to swallow underwater, such as crabs, flounder and lobster (Creaser et al. 1993), are also observed as a winter food source.
No data available for wild birds.
Food Selection And Storage
See Diet, above.
Nutrition And Energetics
Average daily energy expenditures (DEE) from free-living chicks at 10, 21, and 35 days old were 686 kJ/day, 768 kJ/day and 1, 935 kJ/day, respectively (Fournier et al. 2002). The difference between DEE in chicks 10 and 21 days was not significant, suggesting 10 day-old chicks had relatively high DEE. This nonlinearity may be due to the delay of homeothermic independence in younger chicks. At 12 wk, DEE peaks at around ~3,300kJ/d. Fournier et al. (2002) estimated loon chicks consume between 161-383 g wet mass of fish per day (depending on age).
Hand-reared loons exposed to same environmental conditions as wild counterparts in adjacent territories found daily food intake is 40% of chick’s body weight at week 1 (90 kcal) and gradually declining to 22% (1,086 kcal) at fledging (11 wk after hatching). Remains approximately at same level (average 900 g/d, 1,214 kcal) into second year; varies with environmental conditions and activity. When stressed, food intake of chicks 1–2 wk old doubles (>80% body mass/d). Sedentary conditions in confined, sheltered environment result in reduced food intake, as low as 10% body mass/d (J. Pichner pers. comm., JFB).
Metabolism And Temperature Regulation
Foot-waggles (one foot repeatedly held in air and shaken), interspersed with dips in water, may serve for cooling, or for heat conservation, when foot is placed under wing after shaking. Paruk (2009) studied the frequency of foot-waggling with four environmental variables: wind speed, incident light, and both air and water temperature. Wind speed had a negative effect and incident light had a positive effect on adult loon foot waggle frequency. The number of foot waggles observed on sunny days compared to overcast skies increased by at least 10-12%. Ambient air temperature and water temperature had no effect on the frequency of foot waggling. Adult loons with young foot waggled four to five times more per day during Jul-Aug than they did in May-Jun, when they were without young. In hot weather (25°C), incubating individuals frequently pant (Paruk 2000). Chicks foot waggled 3x more than adults. It is likely that younger chicks are faced with greater thermoregulatory challenges than adults. Fournier et al (2002) suggest loon chicks may not gain homeothermic independence from adults until 8-10 days old.
Drinking, Pellet-Casting, And Defecation
Routinely drink by scooping water into beak, raising head, and swallowing. Probably ingest water with food captured underwater. Mucous glands of mouth, tongue, and esophagus are well developed and extensive. Not known to cast pellets (JFB).
Both adults and chicks defecate in water and on shore. Defecation common during panic departure from nest. In a second, stylized defecation, loon climbs onto shore, occasionally moves 2–4 m from water, turns to face water, defecates, and returns to water. Observed both as isolated incidents, and executed sequentially by both pair members (JFB, DCE, J. Fair, pers. com.). Chicks routinely follow parent’s lead. Function unknown; result is readily visible from the air, and may act as territory marker. Also reported in Pacific Loon (JFB) and found regularly on shorelines of lakes occupied by Yellow-billed Loon territories (DCE, J. Fair pers. com.).
Evers, David C., James D. Paruk, Judith W. Mcintyre and Jack F. Barr. 2010. Common Loon (Gavia immer), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/313