Most bird strikes on aerodromes involve a small range of species: primarily gulls (especially black-headed and common gulls), waders (mostly lap wing, but also golden plover and oystercatcher), pigeons, corvids, starling, skylark, swift, swallow and martins. There is a clear relation between bird mass and risk that the aircraft will be damaged: small birds (less than 100g) cause damage on less than 3% of occasions when they are struck; medium-sized birds (101g-1000g) -12%; and large birds (over 1000g) - nearly 23%. There is also a strong relation between bird numbers and the chances that the aircraft will be damaged: single birds cause damage on 8% of occasions; small flocks (2-10) - 14%; and larger flocks (11-100 birds) - 40%. Thus, species which are larger than 100g or occur on aerodromes in flocks are most likely to cause damage to aircraft. Gulls, waders, pigeons, corvids and starling all occur commonly on aerodromes, are responsible for the majority of bird strikes and are most likely to cause damage. However, they also respond well to habitat management and active dispersal techniques and are, therefore, controllable. These birds can be classed as the Priority Group for control. Many other species, most of which are less susceptible to available control measures, are involved in aerodrome bird strikes. Total numbers of bird strikes are not very useful in assessing an aerodrome＇s bird hazard or performance in controlling it. However, a more detailed breakdown can provide useful insights, as follows: ? Priority group species. If a high proportion of an aerodrome＇s bird strikes involve priority group species, then the hazard level is probably high and control standard low. ? Multiple strikes. If there are multiple strikes with priority group species, it is an indication that flocks are permitted to build up and remain on or close to runways. Bird strikes, especially multiple strikes, with these species commonly cause damage and, even if the reporting standard is low, these incidents cannot be suppressed. Thus, the hazard is obviously high and there are problems with controlling it. ? Small birds. All aerodromes have many small birds which are inevitably involved in strikes: Skylarks all year but especially in autumn; finches in winter; and swifts, swallows and martins in summer. If the common small birds are absent from an aerodrome＇s strike statistics, it can be assumed that the true situation is not reflected: the reporting standard must be suspect. ? Less common medium-sized and large species. The significance of the occurrence of these species in an aerodrome＇s strike records can only be properly interpreted with a detailed knowledge of local conditions. By adopting priorities of minimizing strikes with: (1) the greatest potential for serious accidents (multiple strikes with large birds); (2) priority group species; and (3) controllable species, and by using the prevalence of strikes with small birds as a check of reporting standard, the data can provide indicators of hazard level and performance which are independent of comparison with other aerodromes.1. Medium-sized birds (101g-1000g) cause damage on less than of occasions when they are struck