Red-throated Loon Gavia stellata
Version: 2.0 — Published April 16, 2020
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Diet and Foraging
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Main Foods Taken
Primarily live, marine fish. Published reports from both North America and Eurasia include 27 species of marine fish and 9 species of freshwater fish, and, less frequently, invertebrates (186, 45, 34, 41, 187, 166, 13, 37).
Microhabitat for Foraging
Feeds away from breeding sites, primarily along the coast at tidal estuaries and over mudflats in streams and rivers, or larger lakes (188, 189, 34, 190, 187, 32, 15). On Haida Gwaii, British Columbia, most foraging (70%) is in water ≤ 1 m deep (191).
Water clarity, vegetation density, and prey availability not documented for foraging sites. On the Yukon-Kuskokwim Delta, Alaska, Red-throated Loons forage in water that is highly turbid with sediment (DJR). In United Kingdom, marine feeding close to shore during windy, rough conditions, farther offshore when calm (166).
Food Capture and Consumption
Opportunistic forager; see Diet. Searches for prey by peering from surface or hunting underwater, where it propels itself by simultaneous thrusts of legs (as Common Loon does; see 78). Captures fish by grasping in bill, orienting head first to swallow. Except for largest, ingests most prey underwater. Does not regurgitate or cast pellets. Digestive tract is like that of Common Loon (192), including sharp, chitinous denticles on tongue and roof of mouth to assist in holding and ingesting fish; a large glandular proventriculus to break down protein is followed by a muscular gizzard containing pea-sized stones for grinding bones and chitinous material (JFB).
Major Food Items
Diet has been poorly studied as most prey are ingested during dives, therefore there are few data. Diet is composed of mainly fish, but also leeches (Hirudinea), crustaceans (Crustacea), polychaetes (Annelida: Polychaeta), mollusks (Mollusca), and aquatic insects, some probably from stomach contents of ingested fish (186, 45).
Prey include: in Labrador: capelin (Mallotus sp.) from the sea; also brook trout (Salvelinus fontinalis) and stickleback (Gasterosteus sp.); in Oregon: sculpin (Leptocottus sp.); in Firth River estuary, Yukon Territory: stickleback; Igloolik Island, North West Territories: tomcod (Microgadus tomcod), sculpin, and squid, as well as moss (Hypnoceae) in juveniles (45, JFB).
Outside North America, Atlantic herring (Clupea harengus), sprat (Sprattus sprattus), and sand eel (Ammodytes marinus) in United Kingdom (50); arctic char (Salvelinus alpinus), sculpin, cod (Gadus sp.), sand lance (Ammodytes sp.), marine worms, copepods (Copepoda), and crustaceans along Greenland coast (45). In Greenland during breeding season, mainly arctic char inland; later, cod in coastal waters (45).
Stomach contents of Red-throated Loons drowned in fishing nets in the southwestern Baltic Sea indicated fish species were taken in proportion to their availability with zander (Sander luciopera) the dominant winter prey (70% of total prey biomass) and Atlantic herring dominant in the spring (77.2% of total prey biomass); the average calculated fish biomass per individual was 251 g ± 160 SD (n = 82 birds, 688 fishes; 193).
In North Sea, wintering Red-throated Loons eat mostly fish; cod (3–25 cm) constituted > 50% of total food intake, found in 70% of 203 individuals; other prey included gobies (Gobius spp.), stickleback, and herring. Stomach contents of ≥ 80% of birds contained only one fish species (186). Molecular analysis of feces from Red-throated Loons wintering in the German Bight of the North Sea indicated a diverse diet including 19 taxa dominated by five groups: clupeids, mackerel, gadoids, flatfish, and sand lances (194).
For chick diet, see Breeding: Parental Care.
In Alaska, along the Chukchi Sea coast, average adult diet composition during late-incubation was estimated indirectly based on fatty acid composition of adipose tissue from loons and lipid from potential prey fishes (37). This approach provides an estimate of proportional diet composition integrated over several weeks and can only separate prey species that have distinct fatty acid compositions. The average diet was estimated as: 40% (95% confidence interval [CI]: 31–49%) capelin and/or Pacific sand lance and/or rainbow smelt (Osmerus mordax; all had similar fatty acid compositions); 22% (95% CI: 16–28%) slender eelblenny (Lumpenus fabricii); 17% (95% CI: 8–26%) saffron cod (Eleginus gracilis) and/or fourhorn sculpin (Myoxocephalus quadriconis; which had similar fatty acid compositions); 9% (95% CI: 3–15%) Arctic flounder (Liopsetta gracialis); 6% (95% CI: 2–10%) threespine stickleback (Gasterosteus aculeatus); and 3% (95% CI: 1–5%) least cisco (Coregonus sardinella; n = 32 loons).
In coastal breeding areas, parents feed chicks marine fishes almost exclusively and no one species predominates the diet. Across studies, 19 marine species and 2 freshwater species of fish observed in chick diet (45, 34, 191, 23, 37). For quantitative analysis of chick diet, see Breeding: Parental Care.
Food Selection and Storage
Does not store food. Few data on selection, but see Breeding: Parental Care.
Nutrition and Energetics
Body condition during late-incubation, as indicated by total body water measurements using deuterium dilution, was not related to distance from the breeding lake to the nearest marine foraging habitat (all lakes were within 7 km of coast; 37). Deuterium dilution indicated an average body fat content of 10% ± 5% SE (n = 35).
Daily energy expenditure (DEE) estimated using doubly labeled water administered to 4 adult loons during late-incubation and early-chick rearing in Alaska averaged 3,508 kJ/d ± 1,799 SD (range 1,916–5,934 kJ/d; DJR).
In Arctic Alaska, chick DEE during the postnatal period increased in proportion to body mass and maximum energy requirement occurred on day 25 post-hatching (peak daily metabolizable energy = 1,214 kJ/day; 25). Over the average 49-day postnatal period (hatching to fledging), chicks metabolized a total of 49.0 MJ (mega Joule) of energy; to meet this demand, parents provisioned 58.6 MJ of food. These empirical values matched those predicted based on chick body mass at fledging. However, chick growth rate was higher than predicted based on body mass at fledging and chicks fledged before reaching adult size; chick body mass at fledging was only 60%, and outermost primary feather length only 66%, that of adults. Taken together, these results suggest a growth strategy that reduces parental provisioning effort when chick daily energy demands are high by fledging well before adult size is reached; the young birds complete their growth during the post-fledging period in the ocean where prey intake rate is likely higher than what parents can provide while chicks are in the natal lake (25).
Metabolism and Temperature Regulation
Drinking, Pellet-Casting, and Defecation
Water is likely ingested with food consumed underwater, as active drinking was not observed in adults raising young on freshwater and brackish lakes (DJR). Supraorbital salt glands for removing sodium chloride from the bloodstream are prominent, as in other loons (e.g., see 195); other details unstudied.
Both chicks and adults may defecate on shore and in water, although defecation on shore of breeding lakes is not as common as in other loon species (e.g., Pacific Loon and Yellow-billed Loon; DJR).