Specialist rodent-eaters are known for raising large families (up to eight or more young per brood in some species) in years when prey are plentiful, but few or none in years when prey are scarce, or they may crash from abundance to scarcity during a summer (Cramp 1985, Newton 2002). Juveniles formed 85% of 80 Northern Hawk Owls Surnia ulula obtained on irruption in northern Europe in 1950, 100% of 52 obtained in 1976, and 88% of 150 museum skins collected over several years (Cramp 1985). Not all owl invasions follow good breeding years, however, and after a known poor year, only four out of 126 Great Grey Owls Strix nebulosa trapped in Manitoba in 1995 were juveniles (Nero & Copland 1997), and a big invasion into Ontario in 2004 included virtually no juveniles (Jones 2005). In Snowy Owls Nyctea scandiaca, juveniles predominated in invasion years, but in other years when few owls appeared, the majority were adults and many were underweight (Smith 1997). Such observations reveal that, while irruptive owls move away from areas poor in food, they do not altogether avoid the adverse effects of fluctuating supply. The birds are likely also to suffer poorer survival in low food years, but I know of no relevant studies apart from accounts of unusual numbers found dead.
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