Blue Tit Chick Deaths

Hi all, This is my first time on here so bear with me. We spend part of the year in Portugal so I decided to put up a nesting box, because we had seen Blue Tits about. To our great delight we saw a Blue Tit going in and out. We have two camera boxes at home but no camera in this one. After a while we heard the chicks making a lot of noise and the adult birds were in and out with food. Then all of a sudden no sound, also the adult bird would only go to the entrance and not inside the box. We thought the young had fledged, but then the adult bird would not still be coming with food. We decided to open the box up and found the four chicks dead. They were fully feathered and had no marks on them that would indicate and attack of any sort. The only thing we think might of happened is, where we live the villages use strong chemicals on their crops and we wonder if the adults birds have fed them contaminated food. Could this be possible? We now have a camera in the box, but as of today there is no signs of any birds taking up residence.