Hurricane's egg also looks like every egg that was affected by DTT or DDE - it's the way eggs are damaged if the shell is too thin.
I wouldn't take the blog post as scientific evidence, it isn't. It's not even a theory, just a supposition, which isn't a bad thing, just need to refrain from following the zebras (more on zebras later). Feral Pigeons are what we have here though there are some species there that are huge. And because of the density of humans in the UK, they are pretty much everywhere and eating pretty much anything. I know the Clyde area very well and I don't recall them having any more or less than anywhere else. And are pigeons all their birds eat, heck no. Are they even most of what they eat, unlikely given peregrine prey preferences, all the prey species in the UK, the number that don't need to migrate (like the peregrines) and the lack of other predator species to eat them. And while pigeons are seed eaters, much of what peregrines eat are not.
So could pesticides be contributing in this particular case? Possibly, if there is a localized over-abundance that is affecting all of the local prey species. If there was a wider trend of fewer/smaller eggs amongst the Scottish peregrine population, then yes, you could say that perhaps it's time to look at pesticides again (and there are folks doing that) - but keep in mind, not all pesticides will affect egg-shell thinning, different chemicals do different things to the birds. Flame/fire-retardant is world-wide in the environment now and doesn't appear to be affecting eggshells, though there is research looking at whether it can cause behavioural changes. Only way they are going to be able to say that the Clyde pair were affected by pesticides will be to check their chemical loading - unlikely while they are still alive as the UK tends to be very hands-off their wild birds. The alternative is to do necropsies when they are dead, but you have to find their bodies to do that and while these birds may not migrate as far away as our birds do, they probably won't drop dead in the nest so finding them might not be an easy task. Add to that, given the birds' "fame", there may be a backlash against doing a necropsy.
I know the urge is to look for zebras when one hears hoofbeats, but with a 16+ year old male and a 13+ year old female, age could be the case. Madame was an old bird by the time she laid her last eggs and she was laying fewer despite having a much younger male (Trey) as her third mate. Most birds don't live as long as Madame or the Clyde birds so comparing numbers of successful eggs to younger pairs is skewing the results I'm afraid. One of the most famous peregrines was the Sun-Life Female in Montreal, she lived a very long time pre-DDT and didn't raise nearly as many young as she should have been able to. There was another female who fostered more than she hatched successfully but I'm afraid I don't remember who/where she was at the moment.
As for malformed egg - hmmm, can't say that from the picture they attached - Mistral came from an egg that small and she was fine whereas her sister from a perfectly normal egg died of a brain aneurysm. And the colour isn't an indicator, that is laid down as the egg passing through the channel on its way to being laid and I'm not sure that odd colours can be definitively linked to egg problems. Would I expect an egg that small to hatch, no. Would I expect an oddly coloured but otherwise normal sized egg to hatch, I can't say that I would expect it not to, but I would be interested if it did.
The UK/Europe has lots of very good bird researchers and Europe seems to be much more critical of chemicals on crops/in the environment than we are here (or that is my perception) so I'm sure someone is looking at birds-of-prey and the current "crop" of pesticides/chemicals. And given how widely forum members read on the subject of peregrines, I'm sure when there are papers written, folks here will find them.
Just my thoughts ...
