Well you said it already--they changed the perameters of what could be enrolled. And that is why you do not see the CRP in NW IA, or SW 1/2 of MN like you used to, it is still there-just do as much as before.
As for the plight of the ducks not being seen, remember CRP helps in the nesting, not neccessarily in keeping the ducks around in the fall.
Droughts can play a big factor in where they migrate. It the late 80s there wasa terrible drought in the PPR, and as a result the ducks headed for water--the lakes of MB and MN, so the flyway shifted east. Conversely when the water came back in the PPR in the mid 90s, they headed back to a more westerly flight. Transmigration can happen very easily(migrating east of west in aug/sept) . 2 years ago western MB and SK was very dry-the hunting for ducks was fairly poor as they had boogeyed the area. Only 2nd nest ducks were around. I know guys that did not shoot a mallard in 3 days of duck hunting up there. I found out a week later they headed NE 300 to 700 miles the the Interlakes region where there was more water. (reports of "fantastic" in that area). I have seen MN head to the Dakotas and conversly ND ducks to Mn when a drought there. Drought plays a big part in where the ducks are or arn't.
Also the type of farming has changed, and is an added factor. More rows crops and high protein crops for the ducks to feed on further North than before plus others farming practicves add to the mix.
Weather. The winters are warmer than 25 years ago. I know that once our season is over, there are Mallards hanging around in S MN until there is at least 6" of snow on the ground. They have wised up to this and are shortstopping. So in the mean time they have food and are not hunted-they will stick around. So their traditional migration to the deep south is not happening to the degree like it used to, or later. And even us Northern states are seeing the effects when our season closes and there still are a fair amount of ducks in CA because they are snowless.
Hunting pressure. That is also a big factor. Basically the ducks are doing more bypassing of less desireable places to the areas more suited for their needs. And example are Snow geese--they used to pour into ND, not any more, they go from CA to SD now, or even MO, overflying 1 to 2 states. All due to hunting pressure. Ducks are doing the same.
Habitat. As mentioned, 98% of the wetlands are gone in IA-duh and you wonder why they are absent? And the remaining ones are in degraded, if not in terrible ecologogical condition due to run off from fields, minnows/suckers invasions, etc... basically lack of good water.
All of the above are contributing to less ducks in NW IA IMO.
The Audacity of Bull Crap.
"Typical: Gun-loving, bitter bible-thumping white person" Barack Obama.
Hey I resemble that comment!!! Those are FIGHTING WORDS!!!