The 2004 invasion of northern bullfinches into parts of western Europe was the largest on record. It also involved some birds that gave the normal piping call of the species, and others that gave a different ‘trumpet’ call, previously unknown to most observers in the invasion areas. This suggests that many birds in the 2004 invasion were drawn from areas outside the usual breeding range of immigrants to western Europe, and a summer sound-recording of a similar trumpet call had previously been obtained from a bullfinch in the Komi Republic of northern Russia. Measurements of δD values in feather samples suggested that bullfinches in the 2004 invasion could have come from a wide area of northern Europe eastward into Russia. No difference in the range of δD values was apparent between the 2004 birds obtained in Scotland compared with Denmark, nor between birds obtained in the 2004 invasion and others from the 1910 invasion to Scotland. Northern birds obtained in autumn-winter in Denmark and Scotland showed a wider spread in δD values than resident bullfinches from either Denmark or Scotland. In almost all the samples, females had generally lower δD levels than males. The reason for this sex difference is unknown, but could not be attributed to differences in moult timing, location or diet between the sexes.