Terns are seabirds in the family Sternidae known for their aerial acrobatics and long migrations. There are over 40 species of terns found worldwide, ranging in size from the 8-inch Least Tern to the 24-inch Caspian Tern. All terns are fish eaters, catching their prey by plunge diving from the air into water. Terns build very minimal nests or none at all, choosing instead to lay their eggs on bare ground.
Do Terns Make Nests?
Most tern species do not build substantial nests. They lay their eggs directly onto the bare ground, sand, or gravel substrate of their breeding colonies. Some species may construct a tiny scrape in the substrate as a nest, but most do not add any materials or structure around their eggs. The eggs and chicks are essentially exposed. This lack of nest building sets terns apart from most other seabird families like gulls, auks, and cormorants that construct nests out of vegetation, mud, or other materials.
There are a few exceptions among tern species that do build more defined nest structures:
- The Sooty Tern collects debris like sticks and seaweed into a small mound or platform nest.
- The Sandwich Tern may assemble a scrape nest lined with shells and vegetation.
- The Royal Tern and Elegant Tern build shallow scoops in the sand, lined with bits of shell, seaweed, or fish bones.
- The Roseate Tern constructs a scrape nest, occasionally lined with seaweed or grass.
However, even in these cases, the nests are very simple, shallow, and exposed compared to the elaborate enclosed nests built by many other bird families. Overall, minimal nest building is characteristic of terns.
Why Don’t Terns Build Nests?
There are several hypotheses for why most tern species do not construct substantial nests:
- Ground nesting adaptation – Most terns nest in colonies on open, flat ground on islands and beaches with little access to nesting materials. Constructing nests may not be an option in their breeding habitat.
- Predator avoidance – Built nests can draw the attention of predators. Eggs and chicks scattered directly on the ground are more camouflaged.
- Heat management – Incubating eggs directly on the warm sand or gravel may help regulate egg temperature for embryos.
- Frequent movement – Terns frequently relocate nest sites. Minimal nests allow eggs to be moved easily.
Researchers hypothesize that the lack of nest building is an evolutionary adaptation in terns that enhances egg crypsis and temperature regulation in their open, ground-nesting breeding habitat, while also facilitating frequent relocation of eggs as needed.
Nesting Behaviors of Terns
While they may not build nests, terns do exhibit particular nesting and parental care behaviors:
- Both male and female assist in choosing nest sites.
- Nest colonies can be very dense, with territories as little as 1 square foot.
- Eggs are laid in clutches, usually of 1-3 eggs.
- Parents may move eggs frequently within the colony in response to disturbance or changing habitat conditions.
- Terns feign injury to divert predators from eggs/chicks.
- Chicks hatch precocial and mobile. They leave the nest site within 1-3 days.
- For the first few weeks after hatching, chicks are brooded and fed by parents away from the nest site.
While minimal nest building is a tern characteristic, the birds demonstrate strong parental investment through egg incubation, chick rearing, and defensive behaviors against predators.
Comparison of Tern Nests to Other Seabirds
The table below compares tern nesting habits to other seabird groups:
Bird Family | Nest Structure | Nest Materials |
---|---|---|
Terns | Scrape or none | None or bits of vegetation, shells |
Gulls & Skimmers | Scrape or mound nest | Vegetation, debris |
Auks | Enclosed nest in crevice or burrow | Vegetation, feathers |
Cormorants | Stick nest in tree/cliff or ground scrape | Sticks, seaweed, debris |
Petrels & shearwaters | Burrow nest | None |
This comparison shows that most other seabird families construct more elaborate nests than terns. The minimal nesting of terns is unusual among seabirds.
Examples of Tern Nesting Habitats
Terns nest in a variety of coastal and inland habitats, but most prefer flat, open substrates near water:
- Beaches & sandbars – Common and Least Terns nest directly on sandy beaches along coasts, rivers, or lakes.
- Islands – Many terns nest communally on rocky or sandy islands off shore or in lakes/rivers.
- Gravel bars – Forster’s and Caspian Terns nest on gravel bars and shoals in river deltas.
- Salt marshes – Some terns nest in coastal salt marshes, like Roseate and Sandwich Terns.
- Dunes & flats – Barrier island dunes and tidal flats are used by terns like the Gull-billed Tern.
- Roof tops – Urban nesting on gravel rooftops occurs in terns like the Black Tern.
Access to fish and protection from mammals often determines ideal tern nesting habitat. Nests may be in the open or around scattered vegetation.
Beach Habitat
Beaches and sandbars along coasts are a prime nesting habitat for terns like Common Terns, offering protection from predators and access to fishing areas.
Island Habitat
Islands, like this one used by Roseate Terns and Common Terns, offer isolation from mainland predators for tern colonies.
Gravel Habitat
Gravel rooftops mimic the rocky island habitats terns evolved with. Urban sites like this are used by Black Terns and Caspian Terns.
Threats to Tern Nesting Habitats
Terns face challenges to finding undisturbed nesting habitats:
- Coastal development encroaches on beach habitats.
- Human recreation disturbs nesting colonies.
- Invasive plants overgrow nesting islands.
- Flooding and erosion alter island and beach habitats.
- Predators like gulls, foxes, and raccoons prey on eggs and chicks.
Conservation measures like habitat protection, public awareness, and control of problematic predators help maintain tern nesting habitats.
Conclusion
Most terns build little to no nest structure, laying their eggs directly on bare ground or gravel substrates. This is unusual among seabirds, and likely an adaptation to ground nesting on open beaches and islands with access to fishing areas. Though they lack substantial nests, terns demonstrate strong breeding site fidelity, egg brooding, and chick rearing behaviors. Protecting tern nesting habitats like coastal beaches and off-shore islands from human disturbance and predators is an important conservation priority.