Mute swans are large, beautiful white waterfowl that can be found swimming gracefully across lakes and rivers in many parts of the world. With their long, curved necks and gleaming white plumage, mute swans are a distinctive sight. But when it comes to their diet, there is often some confusion over whether mute swans are herbivores or omnivores.
What is a herbivore?
A herbivore is an animal that gets its energy and nutrients almost exclusively from plant-based foods. Herbivores are specialized to digest fibers and other complex carbohydrates found in plants. True herbivores have digestive systems adapted to breaking down tough plant materials like grasses, tree bark, shrubs, and aquatic plants. Some examples of herbivores include cattle, deer, elephants, horses, and rabbits.
What is an omnivore?
An omnivore is an animal that eats both plant and animal matter. Omnivores are opportunistic feeders, meaning they can consume a wide variety of foods based on availability. They have more flexible digestive systems that can handle both plant and animal foods. Some omnivores like bears and raccoons will eat pretty much anything. Others like chickens have a more mixed diet of grains as well as worms and insects.
Mute swan diet
In the wild, mute swans spend much of their time in water, where they feed heavily on aquatic plants and algae. Their long necks allow them to reach food several feet below the water’s surface. Mute swans consume large quantities of emergent and submerged vegetation daily, including:
- Pondweeds
- Water lilies
- Duckweed
- Frogbit
- Chara
- Filamentous algae
Aquatic plants provide mute swans with carbohydrates and protein. But mute swans are also opportunistic feeders. At times they will also eat insects, crustaceans, mollusks, and even small fish to obtain additional protein and nutrients. The cygnets (young swans) in particular require extra protein from animal matter for healthy growth and development.
What do mute swans eat? A closer look
Let’s take a more detailed look at the plant and animal diet of mute swans:
Aquatic plants
Aquatic macrophytes comprise the bulk of the mute swan diet. Their long neck allows them to uproot plants in water up to several feet deep. Favored aquatic plants include:
- Pondweeds (Potamogeton species) – Abundant pondweed species are a staple food. The plants have high carbohydrate content in their stems and leaves.
- Eelgrass (Zostera species) – Eelgrasses grow in shallow coastal waters and estuaries, providing a good food source.
- Widgeongrass (Ruppia species) – Another protein-rich aquatic plant mute swans feed on.
- Wild celery (Vallisneria species) – Wild celery is a submerged plant rich in nutrients.
- Muskgrass (Chara species) – Muskgrass occurs worldwide and is part of the mute swan diet wherever it is found.
- Water milfoil (Myriophyllum species) – Water milfoils are submerged plants that mute swans will readily consume.
- Bladderwort (Utricularia species) – Small floating bladderwort plants provide supplementary food.
- Water lilies (Nymphaeaceae family) – Abundant water lilies are grazed on by mute swans.
- Duckweed (Lemnaceae family) – Tiny floating duckweed plants provide protein.
Algae
Mute swans will also feed on large quantities of algae, mainly:
- Chara – Also known as muskgrass, stoneworts are a type of macroalgae consumed by mute swans.
- Spirogyra – Spirogyra is a common filamentous green algae found in freshwater.
- Cladophora – This green algae has a thread-like structure. It occurs in nutrient-rich lakes and rivers.
Algae provide mute swans with essential fatty acids and protein. By grazing heavily on algae, mute swans help keep waterways clear of excessive algal overgrowth.
Insects and invertebrates
Mute swans will eat small aquatic invertebrates to supplement their diet. Reported animal prey includes:
- Insects – Dragonfly and mayfly larvae, caddisfly larvae, beetles.
- Crustaceans – Freshwater shrimp, daphnia, copepods.
- Mollusks – Small snails and mollusks.
- Worms – Aquatic worms.
The cygnets in particular selectively feed on protein-rich aquatic invertebrates to support growth. But mute swans of all ages will opportunistically take small insects, crustaceans, and mollusks year-round.
Small fish
On occasion, mute swans have been documented eating small fish like sticklebacks and minnows. However fish make up a very small portion of the mute swan diet. Their long neck does allow them to catch small fish in shallow water.
Terrestrial vegetation
While aquatic vegetation is their primary food source, mute swans will also supplement their diet with terrestrial plants from shorelines and wetlands, especially in drier conditions when aquatic food is more limited. Reported terrestrial foods include:
- Grasses
- Sedges
- Leaves
- Seeds
- Agricultural crops like wheat, rice, corn, soybeans, lettuce
Grassy shoreline vegetation provides an important supplementary food source for mute swans.
Bread and grains
In parks and captive settings, mute swans will readily consume bread, cracked corn, and other grains offered by people. However these foods provide little nutritional value compared to their natural aquatic plant diet.
Breakdown of diet composition
Researchers who have studied mute swan stomach contents and feeding habits estimate the breakdown of their natural diet composition in the wild is approximately:
- 90% aquatic vegetation (mainly submerged and floating pondweeds, eelgrass, algae)
- 10% animal matter (insects, mollusks, worms, crustaceans, fish)
So the majority of food is aquatic plants, with a small but significant proportion of animal prey, especially for young swans. Next let’s look at how their digestive system is adapted to this diet.
Mute swan digestive system
Like other waterfowl, mute swans have a digestive system well adapted to their diet consisting mainly of aquatic vegetation. Key adaptations include:
- A muscular gizzard – The gizzard grinds up and mechanically digests tough plant fibers.
- Long intestines – Their intestinal tract is very long, allowing more time to chemically break down and absorb nutrients from fibrous plants.
- Powerful enzymes – Mute swans produce cellulase and other enzymes that digest cellulose and complex carbohydrates in plants.
- Abrasive gastroliths – Mute swans swallow small stones that sit in their gizzard and help grind up food.
At the same time, they are able to digest animal proteins and fats when animal matter is available. So mute swans have a flexible digestive system adapted for their mix of plant and animal foods.
Benefits of mixed plant and animal diet
The combination of aquatic vegetation along with supplemental animal prey in the mute swan diet provides a number of benefits:
- Aquatic plants provide abundant carbohydrates for energy.
- Invertebrates and fish supply high-quality protein for growth and reproduction.
- Algae and aquatic plants deliver essential fatty acids like omega-3s.
- Chewing on fibrous stems and leaves keeps their gizzard in good shape.
- Animal foods deliver more concentrated nutrients and minerals like iron, zinc, and phosphorus.
By consuming both plant and animal matter, mute swans obtain a nutritious balance of carbohydrates, protein, fats, and micronutrients. The small fraction of animal prey in their diet provides benefits beyond the abundant vegetation.
How do mute swans catch animal prey?
Mute swans employ a few different techniques to capture animal prey:
- Dabbling – They will duck their head underwater and dabble to grab slow moving invertebrates.
- Surface skimming – Swans skim floating algae and emerging insects from the surface while swimming.
- Trenching – They plunge their head and neck underwater while pivoting to search muddy trenches for prey.
- Excavating – Some swans will dig into submerged mud or leaf litter to uncover hiding prey.
Group feeding behaviors also help mute swans round up small fish into shallow water where they can be more easily captured. Their varied feeding techniques allow them to take advantage of seasonal abundance of different prey.
Do mute swans eat fish eggs?
Mute swans are not often considered major fish egg predators. However some studies have found evidence they may eat significant numbers of fish eggs in certain situations:
- Beds of stickleback eggs are consumed during breeding season when abundant.
- Salmonid fish eggs may be taken opportunistically at spawning sites.
- Invasive mute swans may deplete eggs of threatened native fish species.
Still, fish eggs make up a very small proportion of the mute swan diet compared to aquatic plants. Any impacts on fish egg abundance more likely depend on high densities of mute swans rather than egg predation being a major feeding strategy.
Are mute swans mostly herbivores or omnivores?
Based on the evidence, mute swans are best described as omnivores. Here’s a quick summary of why:
- 90% of their diet consists of submerged and floating aquatic plants.
- They heavily graze on algae which provides protein and fatty acids.
- About 10% of their diet is made up of aquatic animal prey.
- Their digestive system is adapted to digest both plant and animal matter.
- They employ various feeding techniques to catch animal prey.
So while mute swans focus mainly on aquatic vegetation, they do supplement their diet with enough insects, fish, and other prey to be considered omnivores. They cannot digest cellulose as efficiently as obligate herbivores like cattle. But they are not predators either. Their ability to consume both plant and animal food sources fits the definition of an omnivore.
Do mute swans change their diet seasonally?
Mute swans do exhibit some seasonal shifts in their diet composition:
- Spring – Focus is on high protein aquatic animals to fuel breeding. More insects, fish, and crustaceans are consumed to build up reserves.
- Summer – Abundant aquatic vegetation dominates the diet, along with algae which peaks in summer.
- Fall – Priority is plant foods to store fat before winter. Seeds and tubers are favoured.
- Winter – Less vegetation is available, so more reliance on seaweeds, algae, and animal proteins during the lean season.
While aquatic plants remain the dietary staple, mute swans appear to opportunistically shift their consumption of animal prey depending on seasonal availability and nutritional needs.
Mute swan diet changes by life stage
The balance of plant vs. animal foods also shifts for mute swans depending on life stage:
- Cygnets – Require abundant protein for rapid growth, so animal prey makes up a larger percentage of the diet for young swans.
- Yearlings – Still consume more animal prey than adults but begin relying more on vegetation as they approach adult size.
- Adults – Focus heavily on aquatic vegetation. But females continue to take animal prey before breeding season and during egg laying to help meet protein demands.
As mute swans mature, the proportion of animal foods decreases but remains an important part of their nutritional intake.
Conclusion
In conclusion, mute swans follow an omnivorous diet consisting predominantly of aquatic vegetation along with supplemental animal prey. Their ability to digest both plant and animal matter allows them to thrive across different aquatic habitats. A diverse diet with some animal protein provides mute swans with balanced nutrition throughout the year as conditions and needs change. So the answer to whether mute swans are herbivores or omnivores is clear – they are definitively omnivores due to their adaptations and flexible feeding behaviors.