It happens to us all eventually....can't tell you how many bird losses I have had due to my own negligence or ignorance at the time, just mistakes made that I wasn't aware of would result in dead birds/chicks or just not getting things done in a timely manner~I lost 20 chicks this spring due to not using preventative measures I've always used in the past but forgot to implement. I lost several young birds one fall due to hatching in the fall and then ranging them....didn't think about the lack of cover and the hawk migration causing a perfect storm for my free ranged birds, with the young suffering from the fall out. Up until then I had never lost a bird to aerial predation in almost 40 yrs or free ranging...ruined my perfect record and lost some good birds out of just not thinking, getting impatient for chicks, etc.
Lost another young juvenile to an owl this year from neglecting to put netting over the roosting area of the coop that I was tweaking and had just not got around to protecting that area once again.
It happens. It never feels good and feels doubly worse if it could have been prevented, but we'll never make those same mistakes twice, will we?
And that's how we learn, if we are wise enough to do so. It's incredible to me to read posts on BYC of folks who do the same thing over and over, expecting differing results each time but they still get dead birds. Over and over, every season.