What Is a Forked Tail? Bird Anatomy, Function, and the "Swallow Tail" Name

A forked tail (or bifid tail) is a tail feather arrangement where the central rectrices (tail feathers) are shorter than the outer ones, creating a distinct V- or U-shaped notch. The two elongated outer feathers form the "prongs."

Birds: The Classic Examples

• Swallows, swifts, martins (Hirundinidae/Apodidae): Deeply forked — "swallow tail" is the archetype.
• Fork-tailed Flycatcher (Tyrannus savana): Extreme fork — male's outer feathers 2–3× body length.
• Frigatebirds, tropicbirds, kites: Long, deeply forked — aerial agility.
• Terns, some gulls: Shallow fork — "notched" rather than deep.
• Hummingbirds (some): Forked or double-rounded.

Aerodynamic Function

• Reduced drag: Slotted tip vortices lower induced drag at high speed.
• Maneuverability: Independent control of outer feathers enables tight turns, sudden stops — critical for aerial insectivores.
• Display: Male fork-tailed flycatchers, barn swallows use exaggerated forks in courtship flights.

Beyond Birds

• Fish: Forked caudal fin (tuna, mackerel, fork-tailed catfish) — high-speed cruising.
• Insects: Some damselflies, mayflies — mating claspers or aerodynamic.
• Mammals: None have truly forked tails; some have tufted or bifurcated tips (manatee fluke notch).

Degrees of Forking

• Deep: Outer feathers ≥2× central (swallows, frigatebirds)
• Moderate: Outer 1.5–2× (kites, some terns)
• Shallow/Notched: Outer slightly longer (gulls, some passerines)
• Emarginate: Single notch on one feather (different morphology)

In ornithology, tail shape is a key field mark. A forked tail screams "aerial specialist"—whether it's a barn snatching mosquitoes over a pond or a frigatebird pirating fish 1,000 km from land.