Male zebra finch mysteriously dead

Question
I have a pair of zebra finches, and usually they are quite cheerful despite the extreme weather in my city (about 34+ degrees C in summers, and about 15 degrees Celsius in winter).
As per the past behavior of the finches goes, they were quite friendly with each other, but the male one was slightly dominating over the female. They bath very often, and I clean their cage every 2 weeks or so. Also, the finch pair had also laid about 6-8 eggs over the past 1.5 years, but unfortunately, none of them ever hatched.
They are usually very active, but yesterday, I noticed that both of them seemed much weaker than usual. When that happens, I just take them out of their cage (about 3x2x2 feet), give them a solution of warm water and just a pinch of glucose powder, and let them fly around freely in a large room for about half an hour. So I did just that yesterday. the male one is extremely shy and fast, and I am usually unable to grab him, but the female one is much easier to grab. the whole giving-them-water-and-letting-them-fly-around routine helped, and I just let them sleep over for the night. The next morning, I went to school (my brother usually puts them out in the open for the day), and when I came back, I didn't check on them. but when i was putting them back inside the house about 15-20 minutes after dark, I noticed that the male one was missing, so I looked in the small earthen pots in their cage, which they use as a nest by stuffing them with grass etc, but I couldn't find him, though the female one was relaxing in the 'nest'. I check the floor of the cage, and i found the male one's dead carcass. its body was all stiff and cold, and he was missing his head and his wings, and his most of his feathers were scattered around as if he was killed by something, and it seemed as if it was partially eaten by something larger, but that doesn't seem possible as the cage remains locked. however, i have noticed for the past few days that a hawk circles above the cages and sits on a power line nearby, and I know that it scares my birds, but I don't really worry about any harm because the cages are locked securely, and are made of strong steel wire.
I have no idea how or why this happened, and the mystery of his missing head and wings beats me. On the other hand, the female one is doing absolutely fine.
Also, last year, 5 of my budgerigars died in a period of 3 days, with no apparent reason. I am extremely distraught on this matter, and if this is some kind of disease, please advise me on what to do about it, because I also have two healthy cockatiels and one budgerigar in a separate cage, and I have them since I was a little kid, and I don't  want anything to happen to them. Also, please tell me what to do with the remaining female zebra finch, because it saddens me to watch her all alone in the cage. I want to get another male one for her, but there are hardly any good breeders here in India, and it pains me to watch them die one by one by unknown causes.
Please help me out. thanks !

Answer
I am assuming that the birds live outside. I NEVER recommend having birds outside cage or no unsupervised. It does sound like a predator got them. Most likely the hawk. I suggest keeping them inside the house and only in the outside when you can watch them. If you can't watch them, they go inside the house.