Complex sentence: Goshawks are less successful when attacking larger flocks of wood pigeons than they are when attacking smaller flocks, because the larger the flock size, a bird is more likely to notice the hawk sooner and fly away.
Split into simpler sentences: Goshawks are less successful when attacking larger flocks of wood pigeons than they are when attacking smaller flocks. This is because the larger the flock size, the more likely it is that one bird will notice the hawk sooner and fly away.