One of the driving forces behind the neonicotinoids theory is a toxicologist called Henk Tennekes, who used to work as a consultant for Solvay, Roche, Monsanto en Sandoz (now known as Syngenta). Basically he claims that the toxicity was underestimated as it grows with time, the damage to the nerve system in insects is irreversible.

The pesticides also go into the surface water, where they found a link between higher surface-water concentrations of imidacloprid - a specific neonicotinoid - and a decline of birds that feed on insects.

Article in Dutch: https://www.ftm.nl/artikelen/toxicoloog-tennekes-had-al-die-...

On the toxicity of exposure time: https://www.sciencedirect.com/science/article/abs/pii/S03004...

On the decline of insectivorous birds: https://www.nature.com/articles/nature13531