Food Habits

Main Foods Taken

Aquatic invertebrates (insects, mollusks, crustaceans) and occasionally small fish or spawn. Vegetation (seeds, tubers) typically comprises a small percentage (<25%) of diet (Munro 1939, Bellrose 1980).

Microhabitat For Foraging

Shallow water along shorelines (usually < 4 m deep). May feed occasionally in deeper water (6–7 m, Phillips 1925; rarely up to 9 m, Johnsgard 1978). Prefers open water without emergent or dense submerged vegetation. May feed alongside or in sparse stands of bulrush. Broods feed in similar habitat, although females may lead broods to more productive areas (Eriksson 1976, 1978, Wayland and McNicol 1994). Class-I (approximately 1–21 d) and Class-II (22–42 d) ducklings feed in water 0.5–2 m deep (69% of 166 observations < 1.0 m depth in n. Ontario; JME). Older ducklings (Class III, 45–63 d) feed farther from shore in deeper water (66% of 400 observations in water 1.0–2.0 m depth; JME).

During winter, foraging habitat on salt water includes rocky shorelines, mussel beds, mudflats, and estuaries (Vermeer 1982).

Food Capture And Consumption

A diurnal feeder, although some reports of nocturnal feeding on clear nights (Palmer 1976b). Adults and young feed by diving. Duration of dives and dive frequency may vary considerably with sex, location, and breeding status. Dive duration of adults varies between 10 and 55 s, averaging 25 s (Palmer 1976b, Bellrose 1980). Interdive intervals range from 5 to 15 s (Phillips 1925, Bellrose 1980). In n. Ontario, paired females averaged 19.9 s/dive and 19.1 s between dives; males averaged 22.4 s/dive and 33.3 s between dives (HGL). In n.-central Minnesota, paired females averaged 16.8 s/dive and 10.8 s between dives; males averaged 15.1 s/dive and 9.3 s between dives; females with broods averaged 19.0 s/dive and only 2.7 s between dives (M. C. Zicus pers. comm.).

Downy young dabble at surface during first few days after leaving nest but thereafter feed primarily by diving (94.5% of 311 feeding attempts by ducklings in n. Ontario; JME). Dive duration of downy young averages 16 s (SE 0.26, n = 294; JME), with interdive intervals of 8 s (SE 0.18, n = 294; JME). Dive duration and interdive interval increase significantly with duckling age (JME).

Birds capture most prey on bottom or on subsurface vegetation; can consume multiple prey in a single dive (Palmer 1976b). They generally consume food underwater, although sometimes they bring larger items to surface. They may overturn small rocks and stones while seeking prey.

Individuals dive with wings closed and tail spread (Palmer 1976a). Distances between submerging and surfacing range from 0.5 to 5 m, depending on depth and current. During nonbreeding periods, flocks often dive synchronously (JME).

Young in broods also demonstrate a tendency toward synchronous dives (JME). Class-II ducklings in n. Ontario traveled 0.9 m (SE 0.007, 95 observations) underwater; Class-III ducklings averaged 1.9 m (SE 0.09, 199 observations; JME).

Major Food Items

Diet varies with season and habitat (fresh water vs. salt water; see Appendix 1). Of 395 adult Common Goldeneyes collected throughout the year in a variety of habitats (Cottam 1939), crustaceans (32% by volume), insects (28%) and mollusks (10%) were prominent (based on gizzard contents). Seeds and other plant material comprised 26% of diet, whereas fish and spawn contributed 4% by volume. Important crustaceans: crabs (Hemigrapsus, Cancer, Pagurus, Cambarus, Astacus), amphipods (Gammarellus, Hyalella, Ischyrocerus, Pseudalibrotus), shrimp, isopods, and barnacles (Palmer 1976b). Insects include caddisfly larvae (Trichoptera), water boatmen (Corixidae), dragonfly and damselfly nymphs (Odonata), mayfly nymphs (Ephemeroptera), and beetles (Coleoptera). A variety of mollusk species are consumed (Mytilus, Lymnea, Macoma, Littorina, Nucula, Goniobasis, Nassarius, Lacuna, Bittium, Mitrella). Fish in diet are predominantly Gasterosteidae, Cottidae, Cyprinidae, Poeciliidae; also parr and eggs of Salmonidae. Eggs of whitefish and lake herring (Coregoninae) are believed to be important food items in some Minnesota lakes (M. C. Zicus pers. comm.). Vegetation includes seeds of pondweeds (Potomogeton, Zostera, Ruppia, Najas, Zanichellia), spatterdock (Nymphaea), and bulrush.

Munro (1939) described winter diet in British Columbia: mostly salmon eggs and flesh on coastal rivers and inland lakes (46–66% by volume), along with sculpins, caddisfly larvae, mollusks, and small amounts of vegetation. On estuaries and salt water, crustaceans and mollusks made up entire diet of 9 specimens. On Chesapeake Bay during winter, primarily crustaceans (crabs, amphipods, crayfish), mollusks (mussels, snails, little surf clams), small fish, and plant seeds (Bellrose 1980). In Nova Scotia during winter, plant seeds (58.1%), crabs (19.4%), and snails (14.5%) (Martell 1969).

On breeding grounds, adults eat mostly animal matter, especially dragonfly and damselfly nymphs and caddisfly larvae (JME; see Appendix 1).

Downy young eat primarily animal matter: aquatic beetles, caddisfly larvae, dragonfly and damselfly nymphs, water boatmen, and mayfly nymphs (Cottam 1939, Eriksson 1976, JME; Appendix 2). Plant seeds are less important (2–10% of diet). Prey of ducklings are 1–50 mm long, most 11–20 mm (JME). Mean prey size increases slightly with duckling age: age 1–20 d: 14.2 mm (SE 1.1, n = 11); age 21–40 d: 17.1 mm (SE 1.6, n = 12); age 41–60 d: 15.7 (SE 0.9, n = 6) (JME).

Quantitative Analysis

See Appendixes 1 and 2 . Note that many of these data were obtained from analyses of gizzards, so animals with hard parts (e.g., mollusks, some crustaceans) and plant seeds may be over-represented (Swanson and Bartonek 1970).

In Ontario, ducklings appear to be highly selective foragers; e.g., they consume dragonflies (Anisoptera) and damselflies (Zygoptera) nymphs, caddisfly larvae, and water boatmen adults at greater proportions than available (availability assessed using samples of invertebrates from benthos, water column, and submergent vegetation; JME). They choose mayfly nymphs less frequently than expected. In areas affected by acid deposition, Common Goldeneyes may shift diet to feed on less diverse, more abundant, acid-tolerant invertebrates (McNicol et al. 1987b). Little information available on prey choice relative to prey abundance for adults or other study areas.

Adults are estimated to expend approximately 70 Kcal/d at basal metabolic rate (BMR) and body mass of approximately 1 kg; females probably expend less, breeding adults more. If caloric content of invertebrate prey is 1.18 kcal/g wet weight (Ricklefs 1973), approximately 59 g wet weight of invertebrates would be required for self-maintenance. Estimates for mayfly, caddisfly, and dragonfly and damselfly average 0.035 g wet weight/prey item, so (on its breeding grounds, where such prey are staples) an adult Common Goldeneye would require almost 1,700 prey items/day.

Koskimies and Lahti (1964) estimated BMR of a 1-d-old duckling (32.4 g) at 8.6 cal/g/h, and thus daily requirement of 206 cal/g/d. For a newly hatched duckling (35 g), this equates to a daily main-tenance requirement of 7.2 kcal and an estimate of 174 prey items needed/day. For a half-grown duckling of 400 g, assuming that BMR decreases to approximately 3.8 cal/g/h (estimated using curves in Ricklefs 1973: 245–246), the daily maintenance requirement would be 36.5 Kcal and > 883 prey items/day. Energy required for growth could almost double these energy and nutritional demands.

Little information. Koskimies and Lahti (1964) found that 1-d-old Common Goldeneye ducklings were able to maintain body temperatures during prolonged exposure to cold temperatures and suggested this was an adaptive response of the species to its typical breeding chronology and environment.

Scoops up water with bill when on water. Defecates in water and occasionally while taking flight. Early in incubation, females frequently defecate on nest if disturbed or flushed.