You're running on a time.sleep(60) timer. Sometimes that means we miss a minute because there's no time left to run raspistill and it takes more than a minute to capture an image, save it and run the timer. (In my case I'm storing the images on a NAS box using an NFS share so writing the image takes a noticable time).
I'm going to run a test where I log the timestamp for each image down to the microsecond level, that way we can see how long (on average) it takes from start to finish.
I suspect setting it to time.sleep(59.5) may be a good starting point. I'll also drop the 5 second delay on raspistill down to a 1s delay. Some tuning may be needed, YMMV.
It's only a minor issue because I'm still getting good pics of Mr & Mrs Blackbird and their best mate Robin on my bird table.