perhaps it's the knock on effect the are concerned about.
Take hypothetical bird - "the yellow-winged ground duster"
The ground duster (lovely plumage by the way) eats two insects. From March to July the larva of the Red backed caterpillar and from August to November the Blue legged Beetle. The latter fattens it up so it can survive until March rolls round again.
Did you know the Red backed caterpillar is a farming pest? Nibbles the growing buds of Unobtainium vulgaris (a useful crop) destroying it. But their numbers are usually not excessive.
Now one year the Blue legged beetle population drops, precipitously. Yellow-winged dusters (you remember them? - #Connections ) can't find enough food in November and most are dead come the New Year. Consequently the Red-backed population explodes in Spring - crop gone. Finito. Not a sausage.
Biting the hand that feeds IT
