Predators
Here is a good list of clues that predators leave, it might tell you what happened to your birds:
Pests
Mites, lice and sometimes even ticks like to live on your birds. This will drain their energy and their health will decline.Here is a site that shows what the mites and lice look like and where it is likely to find them on your bird. http://ohioline.osu.edu/vme-fact/0018.html
Treatment of Pests
I have used several things with varying degrees of success and difficulty. Most of these treatments you want to do about 3 times a year.The first obvious choice is dusting powder. This one is a 10 on the work scale for me because I have a lot of birds but it would be good for small flocks. You have to catch each bird, dust it all over, (like you would dust a dog or cat with flea powder) especially under the wings and by the vent and work it in the feathers with your hands. You want to wear a dust/paint mask so that you do not breathe the dust. Then you have to dust each bird again 2 weeks later to kill the bugs that have hatched from the eggs. The dusting powder does not kill eggs. Remember that mites get off the birds during the day, so they are in your roosts and coops too. Dust all over the coop and roosts as well. Her is a link to the kind I have used; http://www.twincitypoultrysupplies.com/store/index.php?main_page=product_info&cPath=4&products_id=64
The next product I have used is call Frontline. It is a dog flea and tick product. I find this a 2 on the difficulty scale and a 7 or 8 success rate. I go in after dark and do it while they are on the roost. I buy the Frontline at Costco, buying the largest dog size you can get. (best savings) I squeezed the frontline out into a small bowl or container and then sucked it up with a 3 cc syringe (no needle). I used this to drop 2 drops on banties and 3 drops on large fowl. (on the skin on the back of the neck) Here is a link to the one I bought; http://www.frontline.com/prd_tools.asp. I got 3 ampules for around $38. I found that one ampule dosed 30-40 birds.
The next product I have used it called Ivomec Eprinex. It is a cattle product for outer and inner parasites. So it worms your birds while it gets rid of the bugs too! I like this product the best so far, because it does double duty! I also use a 3 cc syringe for this and use the following drops: Lf chickens get 5 drops my goose gets 10 my Muscovy girls get 6 and my drakes 8 my bantam chickens get 3 drops my standard chickens get 4 What i like about the Epernix is it is safe to continue to eat the eggs after using instead of having to toss then. I have been using these dosages for almost 3 years and have never had a problem.
