A group of jackdaws is called a train, a term that captures the long, orderly lines jackdaws sometimes form when flying to and from communal roosts at dusk and dawn. Jackdaws are among the smartest members of the crow family, known for strong pair bonds and impressively organized social structures within their flocks.
Jackdaw roosts can number in the thousands, and the daily commute to and from these shared sleeping sites often produces exactly the strung-out, linear formation “train” describes, birds streaming across the sky in a continuous procession rather than a tight, unified flock.
Bonus fact: jackdaws are one of the few birds known to use eye contact meaningfully. Researchers have found they can follow human gaze direction, an ability once thought unique to primates.
Jackdaws are also known by other collective nouns depending on context, including a band and a pair for their tightly bonded mated couples.
Related: Collective Nouns for Birds · What Is a Group of Jackdaws Called? · A Band of Jackdaws