Terns are seabirds in the family Sternidae known for their graceful flight and aerial acrobatics. There are over 40 species of terns that can be found worldwide, inhabiting coastal areas, wetlands, and inland waterways. Terns have a unique and specialized diet consisting primarily of small fish, crustaceans, and other marine life. Understanding what food terns eat provides insight into their behavior, adaptations, and role within marine ecosystems.
Small fish are the main food source for most tern species
The majority of tern species rely on small fish as their primary food source. Terns are well adapted for catching fish, with streamlined bodies, long pointed wings, and slender pointed bills. Their flight style involves hovering and diving to snatch fish from just below the water’s surface. Some species like the Artic tern even plunge dive from heights of up to 30 feet. Common fish consumed include herring, anchovies, smelts, sand lances, and juvenile stages of larger predatory fish like mackerel and tuna.
Terns may also opportunistically feed on schooling fish found near the surface like silversides. The size of fish preferred is generally less than 15 cm long, allowing for easy capture and swallowing. Fish are typically swallowed whole, head first. Certain species have bill morphologies adapted for feeding on specific fish types. For example, the large stout bill of the Caspian tern is suited for capturing larger and more robust fish compared to other smaller billed terns.
Crustaceans and marine invertebrates
While fish comprise the bulk of their diet, terns will also readily consume crustaceans like shrimp, marine worms, mollusks, and aquatic insects. Species that feed extensively on small crustaceans include the South American terns, the Inca tern, and some populations of Common terns. Terns employ a range of foraging strategies such as dipping, surface plunging, and shallow diving to capture fast moving invertebrate prey.
Crabs, sand fleas, marine worms, and tiny mollusks like sea snails may supplement the diet of coastal breeding terns. Rows of stiff rictal bristles around the mouth help filter and capture small invertebrate food items. Periodic mass emergence of flying ants, mayflies, grasshoppers, and beetles during the breeding season provides abundant aerial insect prey for inland nesting terns.
Other prey
While fish and invertebrates make up the majority of food consumed, terns exhibit opportunistic feeding habits and diet varies by habitat and seasonally available food resources. For example, some tropical terns will hawk flying termite alates during mating swarms. Species like the Black tern sometimes feed on tadpoles in freshwater marsh habitats.
Plankton, including fish eggs and tiny larval fishes, provides a supplemental food source, especially for terns foraging in open water habitats offshore. Vegetation and seeds from marsh grasses may also occasionally be consumed. Kleptoparasitism, where terns steal food from other seabirds, seabird chicks, and even other terns, plays a minor role overall but can be important to select tern populations.
Variation in diet by species
While there are overall similarities, different tern species utilize slightly different proportions of various prey types based on habitat preferences, morphology, and marine community composition.
Inshore vs offshore feeders
Terns can be categorized broadly into coastal inshore feeders and pelagic offshore feeders based on typical feeding habitats. Inshore species like Least terns, Little terns, and Common terns feed in shallow waters of estuaries, lagoons, shorelines, and reefs. Their diet consists mainly of small schooling fish and invertebrates like anchovies, herring, shrimp, and sandworms.
Pelagic species range widely over open oceans and feed on deeper swimming fish and plankton. Examples are the Sooty tern, Bridled tern, Crested tern, and Elegant tern. These species may also associate with predatory fish to scavenge leftovers and discarded bycatch.
Inshore terns | Offshore terns |
---|---|
Least tern | Sooty tern |
Little tern | Bridled tern |
Common tern | Crested tern |
Gull-billed tern | Elegant tern |
Geographic differences
Tern species that occupy extensive breeding ranges spanning temperate to tropical zones show adaptations in diet and foraging based on local prey availability.
For example, the Commic tern’s diet in the Southern hemisphere and tropics consists more heavily of small invertebrates like shrimp. But during the breeding season in the Northern hemisphere, small schooling fish comprise the bulk of food delivered to chicks. In some regions, the availability of certain pelagic fish like Pacific anchovies and sardines may also influence seasonal changes in tern diet composition.
Foraging ecology
Terns exhibit a wide array of foraging strategies and hunting techniques to effectively locate and capture prey.
Diving techniques
Plunge diving from flight is considered the signature foraging method of terns. Variations of this technique include straight plunges directly downwards, fast steep dives at sharp angles, and shallow inclined dives skimming just below the surface. Dive height ranges from 10 to 30 ft depending on species. Terns may fully submerge or dip only their head and neck below water when plunge diving.
Surface diving and dipping while swimming or hovering are used to capture prey near the surface. Partial hovering paired with dipping allows terns to scan for prey while remaining stationary. Deep plunge diving of up to several meters is occasionally employed, especially for species that pursue schooling fish pushed deeper by predators.
Aerial hunting
Terns utilize a range of aerial hunting techniques to capture insect and other aerial prey. These include hawking flying insects, hovering and dipping to pick prey off vegetation or the water surface, and rapid steep dives from flight. Terns that forage extensively on swarming aerial insects like the Common tern have evolved supreme aerial agility and speed.
Piracy
Stealing food from other seabirds is called kleptoparasitism or piracy. This surreptitious foraging strategy supplements tern diets at breeding colonies where dense concentrations of marine birds provide stealing opportunities. Terns may opportunistically harass other terns, gulls, petrels, and boobies to force them to surrender prey. Some species like the Great crested tern heavily engage in kleptoparasitism, chasing other seabirds over long distances.
Adaptive advantages
Terns possess a number of morphological and behavioral adaptations that aid feeding efficiency.
Morphological adaptations
- Streamlined body shape to enhance agile flight and diving
- Long, narrow, pointed wings provide speed and maneuverability
- forked tail for stability and steering
- Pointed bills to snatch and grasp slippery prey
- modified bill shape in some species for holding certain prey
- partial webbed feet for swimming and surface dipping
Behavioral adaptations
- Rapid nimble flight and sudden turns enhance prey capture
- Hovering provides ability to scan for prey from vantage point
- Diving from heights of 10-30 ft gives speed to surprise and catch fast fish
- Plunge diving at steep angles aids capturing evasive prey
- Social foraging in groups disorients and concentrates fish prey
Breeding season strategies
Tern foraging and diet changes substantially during the breeding season when adults must provision chicks. Terns prioritize hunting the most energy rich and easily captured prey.
Energy rich fish
Schooling oily fish like anchovies, herring, and sardines become target prey as they efficiently deliver calories and fat to growing chicks. Adults may travel farther and switch locations to track abundant schooling fish.
Coordinated flock feeding
Synchronized flock foraging helps concentrate dispersed fish into bait balls, forcing fish towards the surface where terns can more easily plunge dive and capture them.
Closer foraging ranges
Terns reduce foraging distance from the colony when provisioning small chicks, capturing more easily accessible fish and invertebrates close to shore.
Kleptoparasitism
Some terns will engage in increased kleptoparasitism, stealing food from other birds to deliver back to chicks with minimal energy expenditure.
Diet studies
Determining tern diet composition relies heavily on direct observation of foraging, analysis of food remains and regurgitations, evaluating chick prey deliveries, and collecting data from tagging studies.
Field observations
Recording terns in the wild provides details on key prey types, foraging habitat, diving techniques, and flock feeding behaviors. Observations help clarify differences in diet and foraging ecology between species, populations, seasons, and life stages.
Stomach content analysis
Analysis of food remains and regurgitations of chicks or collected adults gives quantitative data on specific prey proportions. Otoliths, fish bones, shells, and exoskeletons can often identify prey to species.
Prey deliveries
Recording prey deliveries to mates and chicks establishes diet composition during the breeding period. This helps reveal preferential prey targeted when meeting chick nutritional needs.
Tracking technology
Tags, satellite transmitters, and data loggers provide insight into at-sea ranging, habitat use, diving depths, and association with prey. This technology clarifies offshore feeding behaviors difficult to directly observe.
Key research findings
Detailed dietary analyses have revealed some notable findings about tern feeding ecology and diet preferences.
Opportunistic feeding
While terns exhibit preferential prey targeting, they largely demonstrate opportunistic feeding, exploiting seasonal abundance of energy-rich food available within habitat range.
Diet affects breeding success
Access to high-quality prey fish during breeding strongly influences nesting success and chick growth rates. This underscores the importance of prime foraging areas for terns.
Flexible foraging
Terns show adaptability in switching between solitary and coordinated flock feeding strategies in response to prey availability and distribution.
Diet partitioning
Where multiple tern species breed sympatrically, partitioning of prey resources through differential habitat use and foraging behavior reduces interspecific competition.
Role in the ecosystem
As avian predators of plankton, fish, and invertebrates, terns play an integral role in marine and freshwater ecosystems.
Influence on prey populations
Terns can locally suppress populations of small fish and invertebrate prey. Large tern colonies may substantially reduce prey abundance during the breeding season. However, their overall impact on total prey biomass is relatively limited.
Nutrient transport
By feeding on marine prey and nesting inland, terns transport significant nutrients from ocean to land. Their guano accumulates around breeding colonies, providing nutrient subsidies to terrestrial food webs.
Keystone predation
Terns help regulate ecosystem balance through preferential feeding on dominant prey species. This prevents overpopulation of fast reproducing fish and invertebrates.
Cascading indirect effects
As major fish predators, terns influence community structure across multiple trophic levels. Declines in tern numbers can release prey populations, increasing competition for food and potentially affecting reproductive success of other marine birds.
Anthropogenic impacts on diet
Several human-related activities have altered historic tern diets and raised conservation concerns.
Prey population declines
Overfishing of small pelagic fish like sardines, herring, and anchovies depletes key food resources, especially during breeding when chick provisioning is most demanding.
Competition with fisheries
Commercial fishing forage fish stocks directly competes with terns seeking energy-rich foods. Declining prey fish accessibility threatens nesting success.
Pollution and bioaccumulation
Pollutants become concentrated up marine food chains. Toxic chemicals accumulated in prey fish can impair tern reproduction and have caused deformities.
Invasive species
Introduced fish species may displace or compete with native prey fish depended on by breeding terns.
Habitat degradation
Coastal development, dredging, and water diversions can destroy or reduce small fish habitat. This limits food supply vital for successful tern nesting.
Conservation implications
Supporting abundant food resources is critical for maintaining tern populations. Several conservation initiatives can help secure prey availability.
- Protection of key breeding and foraging habitats
- Limiting commercial fishery impacts during nesting
- Reducing pollution in marine ecosystems
- Restoring coastal and wetland nursery habitats
- Invasive species control
- Public education about terns
Given their reliance on fish and invertebrates, terns serve as excellent indicators of marine ecosystem health. Ongoing monitoring of diet provides insight into changing conditions in the food web on which terns depend.
Conclusion
Terns demonstrate a remarkable degree of adaptation in their feeding ecology and diet. Their specialized morphology and hunting strategies allow them to successfully exploit seasonal coastal and pelagic prey resources. Small fish form the nutritional foundation, supplemented by invertebrates and other marine organisms. Closer study continues to reveal the complex dietary behaviors and strategies terns employ in response to prey availability, competition, and reproductive pressures. As sensitive bioindicators, the food requirements of terns highlight the need for responsible management of marine habitats and fisheries on which they rely.