The biggest cause of these declining populations seems, to me, to be intensive agriculture which leads to both loss of habitat and various toxic chemicals (pesticides but also fertilisers and huge quantities of slurry). Another cause may be pollution but it seems to me that we’ve had pollution for longer. In the U.K. this intensive agriculture was started in the war as an effort to produce more food locally. It meant less land was left fallow and land which had been too poor quality to be viable was farmed (with low yields and lots of artificial inputs.) It also means that any agricultural runoff is worse for the environment it runs into (even fertilisers may cause algal blooms in rivers which can then remove the oxygen from the water.) Because farms have to stay competitive, there isn’t much choice but to continue these intensive practices. To some extent, switching to certified organic practices allows farms to stop such practices.

It’s not so obvious what the solution is. There are a lot of people to feed and reducing the productivity of the land in a rich country puts pressure on land in other countries. One suggested solution is people changing to more land-efficient diets with less meat, but I don’t really see this happening much.