The black tern is a small migratory seabird found across North America. Their diet consists primarily of small fish and insects which they catch by diving into water. Understanding what black terns eat is important for conservation efforts as their populations have declined in recent decades.
Fish
Fish make up the majority of the black tern’s diet. They feed on small fish that swim near the surface of lakes, rivers, and coastal waters. Some of the common fish they prey on include:
- Minnows
- Sticklebacks
- Killifish
- Silversides
- Smelt
Black terns are aerial hunters and will hover 10-30 feet above the water searching for fish. When they spot prey, they will plunge headfirst into the water to grab the fish in their bill. They surface and return to land or a perch to consume the fish.
Insects
Although fish are the primary component, insects also make up a notable part of the black tern’s diet especially during the breeding season. They feed on insects like:
- Dragonflies
- Damselflies
- Grasshoppers
- Crickets
- Beetles
- Moths
- Mayflies
The abundance of aquatic insects near wetland nesting areas provides an important food source for nesting and fledgling terns. The adults will capture insects on the wing or pluck them from vegetation near the water’s edge.
Hunting and Foraging Behavior
Black terns have unique hunting and foraging behaviors to catch fish and insects.
- They patrol open water in search of fish, flying back and forth over the surface and hovering in place.
- They plunge dive at steep angles, submerging briefly to grab prey.
- They snatch insects from the air during flight.
- They scoop up insects while swimming and floating on the water’s surface.
- They perch on emergent wetland vegetation and pick off insects.
- They glean insects disturbed by grazing animals in marshes and fields.
Their agile flying skills and ability to catch prey both in flight and by diving in the water allow them to exploit a wide range of food sources.
Food Preferences by Season
The types of prey black terns eat changes throughout the year as they migrate and breed:
Spring Migration
During spring migration in April and May, black terns feed predominantly on small fish species in coastal estuaries, lakes, and rivers. They build up fat reserves fishing to sustain their continued migration to breeding areas.
Breeding Season
At breeding grounds starting in May and June, the diet shifts to include more insects to meet higher energy demands. Aquatic insects become important for adults and fledglings. Fish are still eaten regularly by breeding adults travelling to foraging sites further from the nest.
Fall Migration
In August and September during fall migration, black terns again feed mainly on fish as they travel to staging areas along migration routes. Good fish availability is crucial prior to their transoceanic flights to South America.
Wintering Grounds
On South American wintering grounds from October to March, small schooling fish are the primary prey. Terns also mix in some insects when fish are scarce.
Amount of Food Consumed
The daily amount of food black terns eat depends on their energy needs at different stages of life and times of year:
- Non-breeding adults eat approximately 12-15 g of food per day.
- Breeding adults consume 25-30 g per day at peak demand while raising young.
- Nestlings can eat up to their body weight or 10-15 g per day.
- During fall migration terns may eat 30 g or more per day to fuel long distance flights.
Their high metabolism and energy expenditure from flying requires them to eat large quantities of prey. In prime foraging habitat, they may eat 50-100 items per day during the breeding season.
Foraging Habitat
Productive wetland and coastal habitats provide the best areas for black terns to find sufficient food. Ideal foraging sites include:
- Shallow lakes and ponds with plenty of small fish.
- Slow rivers and streams.
- Marshes and wetlands teeming with insects.
- Coastal inlets and estuaries.
- Flooded agricultural fields.
Loss of these productive foraging grounds due to wetland drainage and degradation is a major threat to black terns. Protecting important feeding areas is vital for conservation.
Adaptations for Feeding
Black terns have several key adaptations that help them catch prey:
- Aerobatic flight: They are agile fliers with quick, darting movements to chase down insects.
- Plunge diving: They can dive from heights of 10-30 feet to pursue fish underwater.
- Skewed bill: Their bill is angled to help grab and hold slippery fish.
- Hind toe: The hind toe allows them to grasp perches while eating and feeding young.
- Salt glands: Glands above the eyes remove excess salt from food in marine habitats.
Feeding Behaviors
Black terns exhibit fascinating feeding behaviors and strategies:
- They often forage in groups which may maximize finding prey patches.
- They perch and consume prey or carry it in the bill back to nests or young.
- They are nomadic and will congregate in areas of high food abundance.
- Parents will provide fish for mates and feed young by carrying prey back in the bill.
- They cache excess food items in aquatic vegetation near nests.
- They may rob prey from other tern species or pirate from other diving birds.
Threats to Food Supply
Although they are adaptable feeders, black tern populations still face threats to their food supply including:
- Wetland drainage and destruction – reduces major feeding habitat and prey availability.
- Invasive species – disrupt native fish and insect populations.
- Pollution – affects water quality and survival of prey.
- Climate change – alters prey abundances and distributions.
- Overfishing – depletes food supplies, especially on wintering grounds.
Maintaining productive and pristine foraging habitat will be key to preserving black tern populations into the future.
Conclusion
Black terns are predators adept at catching small fish and insects. Fish comprise the majority of their diet, especially small schooling fish like minnows, smelt, and silversides. Insects become important during the breeding season when terrestrial and aquatic insects nourish adults and young near wetland nesting colonies. Black terns frequent productive lakes, rivers, wetlands, and coastal habitats where prey is abundant. They exhibit specialized hunting behavior and adaptations to catch food on the wing, by plunging from the air, and gleaning from the water’s surface. Conservation of high quality feeding areas is essential for preserving black tern populations which face threats from habitat loss, pollution, invasive species, climate change, and overfishing. Understanding details of their diet provides insights into protecting this unique seabird.