I'm just going from memory, but not only ducks were affected. The hardest hit species were apparently predatory birds. The numbers of raptors are recovering at a very comforting pace - unless you're an envirnazi, because then no matter how prolific a species is, it's 'endangered'.
Most people just figure that the lack of insects were starving the animals that prey on the insects, which then caused starvation in animals that preyed on them.
NOW...because the study results are worthless, I must make the point that we'll never know whether raptors were really affected by the DDT, or some other, entirely unrelated factor. The Greenies and their useful idiots are proof: science and politics NEVER mix. Also, as I pointed out, we have no effective substitute, because ALL pesticides are more and more and more and more closely regulated and then later banned - not because of SCIENCE - but because of pseudoscience, the bastard child of politics and science, which borrows heavily from late '70's, early '80's 'wisdom'.
Egg shell thinning was an enormous hoax. It is caused by an astonishing number of environmental factors, ranging from oil spills (as was the case on Annacappa), dehydration, low calcium diet, and stress.
The science has been done, but the ecofreaks keep repeating the old crap simply because it suits their purpose. You'll hear all about the persistence of breakdown products of DDT such as DDD or DDE. The test methods used are a joke, sometimes as crude as measuring total chlorine content in seawater. These idiots have been caught claiming existence of DDT breakdown products in soil samples taken before DDT was ever produced! Even after they are confronted with the facts, they keep right on going.
Try 100 things you should know about DDT. It's a pretty good compilation of the case against this crap. It's useful to have the facts when people repeat these bogus claims. I have more if you need it.