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Why can tree swifts perch while true swifts usually cannot?

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Illustration of a chestnut-collared swift (Cypseloides rutilus) in flight, showing its dark brown body and wings with a distinctive reddish-brown collar around its neck. — Apodiform Chestnut-collared swift (*Cypseloides rutilus*)

The primary reason tree swifts (family Hemiprocnidae) can perch while true swifts (family Apodidae) generally cannot lies in the anatomical development of their feet.12

In tree swifts, the hallux (hind toe) is directed backward and is not reversible, which makes their feet capable of effective perching. This adaptation allows them to use high branches as vantage points from which they launch to forage for insects.123

Conversely, true swifts have tiny, weak feet with a hallux that is lateral or nearly so, and in some species, it is reversible. These feet are specialized for clinging to vertical surfaces rather than grasping twigs or wires. Because their feet are so weak in proportion to their body size, true swifts are unable to perch in the traditional sense and may be unable to take flight if they land on flat ground. While tree swifts are considered less specialized in this regard, both groups share a general lack of ability to walk or engage in non-flying locomotion.143