During a four-year study of the breeding biology and ecology of an atypical population of Barn Swallows nesting in 13 abandoned post-war bomb shelters, the unexpected presence of foreign juveniles in active nests with nestlings was recorded on eight occasions. In five cases, single birds were noted, and in the other three, two foreign individuals were observed. The average age of the nestlings joined by foreign juveniles was 11.6 days (SE = 1.08, range 8–16), while the average age of the latter birds recorded with the nestlings was 23.2 ± 1.02 days (range 20–25). The mean distance between the hatching and parasitised nests was 0.9 ± 0.11 m (range 0.5–1.2). This unusual behaviour in swallows seems to be deliberate and is aimed at choosing a nest with nestlings in order to obtain extra food from experienced adult birds rather than an error caused by the darkness in the shelter.