What do common goldeneyes eat in the wild?

The common goldeneye (Bucephala clangula) is a medium-sized sea duck that breeds across the Northern Hemisphere in boreal forests. Their diet consists primarily of aquatic invertebrates and occasionally small fish. Common goldeneyes have a diverse diet that varies based on habitat, time of year, and availability of food sources. In this article, we will explore the typical diet of wild common goldeneyes throughout the year.

Spring Diet

In spring, common goldeneyes feed on a variety of aquatic invertebrates to rebuild energy reserves after winter and prepare for breeding season. Some of their main food sources include:

• Aquatic insects – Common goldeneyes consume aquatic insects like mayfly and caddisfly larvae, dragonfly nymphs, and water beetles.
• Crustaceans – Crustaceans like freshwater shrimp, amphipods, isopods, and cladocerans are eaten by common goldeneyes.
• Mollusks – Snails, mussels, and clams supplement the spring diet when available.
• Annelids – Aquatic worms like leeches and polychaetes provide protein.

Insects tend to dominate the diet in spring, providing an abundant source of protein for female goldeneyes before nesting. Crustaceans and mollusks are also key sources of calcium for eggshell formation.

Summer Diet

During the summer breeding season, common goldeneye diets shift to support the energy demands of egg-laying and caring for young. Some key foods include:

• Aquatic insects – Adults continue foraging on aquatic insect larvae and nymphs while breeding.
• Crustaceans – Small crustaceans are an important staple for females and growing ducklings.
• Mollusks – Mussels and snails remain essential sources of calcium.
• Fish eggs – Eggs from other fish species supplement high protein requirements.
• Plants – Adults may graze on aquatic vegetation and in some cases pass onto ducklings.

Females need substantial amounts of protein while forming eggs. Ducklings rely heavily on small crustaceans and insect larvae during their first few weeks. As ducklings grow, their diet expands to include plants, mollusks, and fish eggs.

Fall Diet

In fall, common goldeneyes prepare for migration by building fat and muscle reserves. Their diet includes:

• Insects – Larvae, nymphs, and adults are consumed in large quantities before migration.
• Crustaceans – Amphipods, isopods, and shrimp supplement insect intake.
• Fish – Small forage fish like minnows become a larger part of the diet in fall.
• Mollusks – Mussels and snails are eaten less as fat-rich foods are favored.
• Seeds/nuts – Some seeds and aquatic nuts provide carbohydrates.

The emphasis shifts to foods with higher fat and protein content. Insects, crustaceans, and small fish help goldeneyes build nutrient stores for long migrations and winter. Foods like mussels and snails decline in dietary importance during fall.

Winter Diet

Common goldeneyes winter in coastal marine habitats and forage primarily on crustaceans, mollusks, and fish:

• Crustaceans – Marine amphipods, isopods, shrimp, and copepods become the main food source.
• Mollusks – Mussels, clams, snails, squid, and other marine mollusks also feature prominently.
• Fish – Small forage fish like smelt are consumed when available.
• Aquatic insects – Some marine insect larvae are still eaten.
• Aquatic plants – Common goldeneyes may graze on eelgrass and algae.

The winter diet is higher in fat-rich marine invertebrates that help maintain energy reserves during harsh conditions. Fish and algae supplement the lipid-heavy diet. When inland waters freeze, marine foods sustain goldeneyes until spring.

Key Factors Influencing Diet

Several factors contribute to variation in common goldeneye diets across seasons and regions:

• Breeding status – Requirements differ greatly between breeding and non-breeding birds.
• Age – Duckling diets are more limited than adults.
• Sex – Females require more calcium and protein during egg production.
• Habitat – Food availability depends on wetland type, latitude, and local ecology.
• Season – Prey abundance, fat needs, and migration influence seasonal shifts.
• Population density – Competition may restrict niche foods at high densities.

These intrinsic and environmental factors interact to determine the composition of goldeneye diets across their range. They exhibit a flexible foraging strategy, allowing them to exploit diverse food resources.

Foraging Behavior

Common goldeneyes employ a range of foraging techniques to capture prey:

• Diving – They dive underwater to depths of up to 20 meters to pursue prey.
• Dabbling – On the water’s surface, they feed on insects and plants.
• Probing – They probe in sediment and vegetation for concealed invertebrates.
• Scavenging – They will scavenge dead fish and invertebrates.

Diving and dabbling are their primary feeding modes. Diving allows them to access a diversity of benthic and pelagic prey. Dabbling helps them conserve energy while foraging on surface foods. They are visual predators and use their keen eyesight to detect prey while swimming and diving.

Diet Adaptations

Common goldeneyes have several key adaptations related to their diversified diet:

• Serrated bill edges that help grip slippery prey.
• Strong jaw muscles for crushing shelled mollusks.
• Nictitating membrane that protects eyes while diving.
• Dense plumage that resists water penetration while swimming.
• Salt glands that remove excess salt from marine foods.
• Rapid digestion to assimilate high protein crustaceans and insects.

Morphological and physiological adaptations allow them to pursue mobile aquatic prey, access a diversity of food resources, and switch between freshwater and marine habitats seasonally.

Conclusion

In summary, common goldeneyes are dietary generalists that consume a wide range of aquatic invertebrates, fish, and plants. Their diet changes seasonally based on breeding requirements, prey availability, and fat storage needs related to migration and winter survival. Key foods include insects, crustaceans, mollusks, and fish supplemented with some aquatic vegetation. They employ flexible foraging strategies to take advantage of diverse prey across freshwater wetlands and marine environments throughout their Holarctic range.