Original sentence: Evolutionary constraints may operate so parasites avoid killing their hosts, or the natural variability in host defensive strategies may be suffice to keep host populations viable.For example, tapeworms can make some fish behave in such a way that a predatory bird can catch it.However not all parasites are concerned with keeping their hosts alive.Specifically, the tapeworm Schistocephalus solidus turns infected threespine stickleback white, and then makes them more buoyant so that they splash along at the surface of the water, becoming easy picking for a passing bird.The predatory bird becomes the next host for the parasite in the next stage of its life cycle.here are  multistage life cycle parasites who go to some lengths to make sure their host dies
Edited sentence: Evolutionary constraints may operate so parasites avoid killing their hosts, or the natural variability in host defensive strategies may suffice to keep host populations viable.For example, some tapeworms can make some fish behave in such a way that a predatory bird can catch it.However not all parasites want to keep their hosts alive, and there are parasites with multistage life cycles who go to some trouble to kill their host.Specifically, the tapeworm Schistocephalus solidus turns infected threespine stickleback white, and then makes them more buoyant so that they splash along at the surface of the water, becoming easy to see and easy to catch for a passing bird.The predatory bird is the next host for the parasite in the next stage of its life cycle