The word "toop" is an onomatopoeic rendition used in Nethersole-Thompson's book on Scottish crossbills.
Description: Many types of crossbills (an exception is Type 1) incorporate harmonic frequencies in toops. The calls may be given in relatively rapid (3-4 per second) series or intermittently.
Important note: Species differences among toops are extensive. However, types 2 and 4 are almost indistinguishable. This is generally not a problem in field identification because the flight calls of these two types are radically different (see the diagnosis page).