Grebes are aquatic birds found across North America, Europe, Asia, Africa, Australia and South America. There are 22 recognized species of grebes that occupy a wide range of wetland habitats. Grebes have some unique adaptations that enable them to be excellent swimmers and divers. Their feet are lobed, their legs set far back on their bodies and their feathers are waterproof. These adaptations make grebes well-suited to their aquatic lifestyle. Understanding the feeding habits of grebes provides insight into their behavioral ecology and the aquatic ecosystems they inhabit.
Diet
Grebes are carnivorous and almost exclusively piscivorous, meaning fish make up nearly the entirety of their diet. They will also opportunistically feed on other small aquatic animals like crustaceans, mollusks, aquatic insects, tadpoles and even small amphibians like frogs. The proportions of each prey type in the diet vary by species, season and location. Here are some of the main prey items consumed by grebes:
Fish
Fish are the primary component in the grebe diet. Grebes are pursuit divers, so they chase down individual fish to capture them. Some common fish prey across grebe species include minnows, sticklebacks, perch, sunfish and carp. Larger species like the western and Clark’s grebe may also eat larger fish up to 30 centimeters long. Grebes usually swallow fish whole underwater.
Crustaceans
Grebes will opportunistically feed on small crustaceans like crayfish, shrimp and other freshwater macroinvertebrates. These may become more prevalent in the diet during the non-breeding season when grebes move to more coastal marine waters or during the time juveniles are developing.
Aquatic Insects
Grebes may consume aquatic insect larvae like those of mayflies, dragonflies, true flies, beetles and caddisflies. These invertebrates become more important prey items for grebes during the summer months when insect hatches occur.
Mollusks
Though not a primary food source, grebes may ingest mollusks like snails and small bivalves. Their crushing bills allow them to access the soft body parts inside hard shells.
Amphibians and Reptiles
On very rare occasions, grebes may feed on small vertebrates like frogs, toads, salamanders and even tiny turtles if the opportunity arises. These prey items are not actively hunted but may be opportunistically consumed.
Vegetation
Grebes ingest feathers, tufts of hair and plant material incidentally when feeding on animal prey. They do not actively consume or digest plant material. Feathers from consumed prey may help form pellets that are regurgitated.
Feeding Behavior and Strategies
Grebes have a variety of behavioral strategies and adaptations that facilitate feeding. Here are some of the ways grebes catch and consume their prey:
Diving
Grebes primarily locate and capture prey by diving and swimming underwater. Most dives are relatively short, ranging from just 5-30 seconds. However, grebes are capable of remaining submerged for up to one minute during pursuit of prey. Different species exhibit differing diving capacities based on bill shape and body size.
Fish Handling
Grebes utilize a variety of methods for handling different sized fish. Small fish may be swallowed head first underwater. Larger fish are typically carried to the surface where grebes manipulate them into position for swallowing head first. Occasionally grebes will carry larger fish to shore where they can be battered against the water or ground to immobilize them before swallowing.
Pursuit Diving
When actively seeking fish, grebes will swim underwater in pursuit of prey. Their streamlined bodies and lobed feet enable rapid propulsion to chase down fish. Their flexible necks allow for rapid strikes to capture prey.
Surfacing Strategy
Grebes exhibit a peculiar surfacing behavior where they partially emerge tilting their bills upwards to survey underwater prey activity. This allows them to conserve energy while scoping potential targets and helps grebes successfully capture prey.
Nocturnal Feeding
Some grebe species may feed nocturnally to take advantage of vertically migrating prey in dim light conditions where competition from visual predators is reduced.
Kleptoparasitism
Grebes sometimes steal food opportunistically from other waterbirds. They may rapidly swim at feeding ducks or loons in hopes of forcing the birds to drop prey that the grebe can scoop up.
Feeding Habitat
Grebes pursue prey across a diversity of wetland habitats:
Ponds
Small ponds, lakes and reservoirs provide excellent grebe feeding habitat due to an abundance of small fish, aquatic insects and vegetation that supports prey.
Rivers
Some grebe species feed in slow moving rivers, targeting small fish in eddies and calmer side channels. Fast currents limit extended diving and pursuit.
Marshes
Shallow vegetated marshes are used by some grebes like eared grebes and pied-billed grebes. The edges provide ambush opportunities.
Estuaries
Estuaries provide rich feeding opportunities during non-breeding seasons for species like western grebes and Clark’s grebes.
Coastal Waters
Coastal inshore waters out to several kilometers offshore are used for feeding by western, Clark’s and red-necked grebes outside of breeding season.
Open Ocean
Some larger grebe species may feed over 10 kilometers offshore, diving into deeper waters. Prey items shift to pelagic fish like anchovies.
Breeding and Wintering Habitats
The habitats grebes occupy while breeding are often different than wintering grounds where feeding occurs:
Breeding Habitats
– Marshes and shallow open water for colony nesting species.
– Secluded vegetated ponds for solitary nesting grebes.
Wintering Habitats
– Coastal bays, estuaries and open ocean.
– Large inland lakes and reservoirs.
– Sheltered rivers and channels.
Differences Between Species
While feeding habits are generally similar across grebes, some key differences occur between species:
Small vs Large Grebes
Larger grebes like the western and Clark’s grebe can consume larger prey items including larger fish and crustaceans. Their greater diving capacities allow them to access deeper water prey.
Bill Morphology
Western and Clark’s grebes have thicker bills adapted for crushing that provide access to heavily armored prey. Slender billed grebes have tools specialized for capturing fish.
Nocturnal Feeding
Smaller grebes with higher energy demands like the eared grebe exhibit nocturnal feeding behaviors more frequently.
Habitat Use
Marsh nesting species like the pied-billed grebe utilize vegetated wetlands while pelagic nesting grebes rely on open water.
Threats to Grebes
Declines in water quality, wetland habitat loss, and disturbance of breeding colonies are some threats that may impact grebe feeding:
Pollution
Agricultural runoff and industrial pollution can reduce water quality. This may decrease fish populations and aquatic insect hatches that grebes rely on for food.
Wetland Drainage
Drainage of wetland habitats for development eliminates key grebe feeding and breeding areas.
Disturbance
Recreational activities near breeding colonies can cause nest abandonment. This reduces grebe breeding success and capacity to acquire food for developing chicks.
Climate Change
Climate induced habitat shifts may force grebes to feed in suboptimal areas where prey is limited.
Overfishing
Overfishing of key prey species may restrict food availability and force grebes to travel farther to locate fish schools.
Conclusion
In summary, grebes are highly specialized aquatic birds that employ a variety of behavioral strategies to pursue fish and other prey across diverse wetland and coastal marine habitats. While feeding habits are generally similar across the 22 grebe species, differences in bill morphology, body size, habitat use and nocturnal behaviors lead to nuances between species. Conservation of wetland ecosystems and reduction in pollution and disturbance will help ensure healthy grebe populations into the future. Learning more about how grebes exploit resources can provide insight into aquatic ecosystem health and function.