I have seen plenty of people asking about keeping their birds safe from avian predators. I have been working with birds of prey for over 12 years, and have learned a few things. I will attempt here to create a general guide to avoid these predators, and keep your birds safe.
I first want to state that IT IS A FEDERAL CRIME TO KILL ANY BIRD OF PREY. This is punishable by up to 5 years in prison and a $200,000 fine. If you have a predatory bird that you cannot protect against, your local Wildlife Division may be able to assist you in removing the bird. Occasionally you can get a depredation permit to shoot a bird of prey if they have caused significant loss of property. I urge and encourage you to use this as an absolute last option.
I am not going to go into too much detail about identifying the raptors, because there are many different species, and some of them are geographically isolated.
Here are some Common US Hawks: http://www.birds-of-north-america.net/hawks.html
How to tell if a raptor has killed a bird:
All of the hawks and falcons once catching a prey bird, will start to pluck. If you find a large quantity of feathers surrounding the kill site, it is a telltale sign that the culprit is a hawk or falcon. This is especially true if the prey animal is too big to carry off. If you find the bird missing all together, and you are raising smaller birds, then a hawk may also be to blame. If you check below nearby trees, and telephone poles you may find a pile of feathers, and other “leftovers”. Hawks and falcons, usually eat the head/breast first, and then work over the rest of the bird. Being sight predators, if the hawk cannot see the poultry, or does not have a "clear shot" at it, it will not attack. Usually the birds that get caught are the ones hanging around in the open. Raptors will trigger off of certain movements. Bathing, flapping, and other motion intense activities can trigger an attack. They can make the chicken look injured or in trouble, which is too good for a raptor to pass up.
I have seen trained hunting hawks refuse game that is sitting in the open and not moving at all. It is not until that the game moved, that the hawk engaged.
A note here, I talk about falcons, but hawks are far more likely to attack poultry than a falcon. Falcons prefer to take birds on the wing, and will rarely attack a bird that is on the ground.
Here is an example of what a hawk kill looks like:
Photo Credit: PandoraChick
As far as owls go, your biggest threat is your local Eagle Owl, for those in the Europe it is the Eurasian Eagle Owl, in North America it is the Great Horned Owl. It is possible that other large owls will target poultry, but those instances are rare. Eagle owls normal diet is anything unlucky enough to look edible. I have handled owls that have eaten skunks (based on smell and stomach contents). Unlike other owls that have narrow mouths and swallow their prey whole, Eagle owls have big mouths and will tear up prey. Owls tend to prefer carrying off their prey before eating, so often the only telltale signs are a missing bird in the morning. Eagle Owls are crepuscular, meaning they usually hunt mornings and evenings, during low light periods. They will carry their hunts into full dark/daylight if they need. I have not seen evidence of owls just eating the head and leaving the rest, but owls do not have a crop, so they cannot eat as much in a sitting as a hawk, so it is possible that they will only eat a smaller portion and either come back, or abandon the kill.
Eagles will also target poultry, they are large enough to carry them off, or you will find very little left. Eagles can eat a large amount in a single sitting.
In the fall and winter months, raptors migrate southward. Attacks during these times are largely done by these migrating birds. The raptor will likely be in an area for a week or two, and them move on. This can make attacks very unpredictable, as birds move through the area. In the Spring and Summer months birds are mating and rearing young. These birds will know the best hunting spots and will quickly clean out a whole coop if they find easy pickings.
What to do about them:
The only 100% safe way is to have fully enclosed pens for your poultry. A determined hawk can be very aggressive. I have seen hawks bodily slam into cages trying to get at the contents. Here are some other ideas:
Wire/Rope Netting: This is a visual deterrent. It will deter a casual passerby, but a truly determined bird can bypass it with little effort: adding some cloth streamers or flashers to the netting/wire will also help to make it look harder to navigate, and help so scare the bird off. Ropes should be spaced 24" or less, less being better.
Roosters: Roosters make an ok early warning system, but otherwise should not be relied on to protect against a raptor. Altercations usually end in favor of the raptor.
Dogs: Dogs are a great deterrent, unless trained otherwise wild raptors are afraid of them and will give them a wide berth.
Plastic Owls: These can be effective, or they can totally backfire. Owls are everyone’s mortal enemy and will often get attacked on sight by everything from finches on up. This can actually serve to attract other raptors to your yard. If you do go this route, make sure to move the owl from time to time. I have seen pigeons perching on them because they had grown accustomed to it being there, and learned that is was fake.
Bushes/Cover/Undergrowth: (thanks for BigECarter for asking about this)
Providing cover will help. It can give the poultry something to hide in, and juke around if trying to evade. Most of your hawks will not target something already in cover, and having it close by will certainly help if your poultry have warning, and know to take cover. I have seen many a Hawk chase something into bushes and catching it however. Usually because whatever it is stops inside thinking its safe, and the hawk comes crashing in right behind. Most domestic poultry have an advantage in a ground race, but if incapable of strong flight, they will loose to a raptor on the wing.
Trees: Trees can be a good or a bad thing. With the exception of the Accipiters, most hawks and all of the falcons do not like hunting under the canopy. They prefer wide open fields. Trees can therefore provide protection for your birds. The other side of this, is that trees are the preferred roosing, and hunting grounds for owls. Owls are ambush predators, that like to hide in the foliage and then drop on to unsuspecting critters.
These can be a good deterrent, and can be a good cheap way to help scare away raptors. CDs, mirrors, pieces of metal, or other reflective objects hung over the coop or around the yard to flutter in the wind will help. Here is a great idea that Grizzly Mills used https://www.backyardchickens.com/t/...redneck-backyard-hawk-deterrent-disco-cone/10
Image Credit: Grizzly Mills
Roofs/Fencing/Netting: This is the only sure fire way to keep your poultry safe. Any hole smaller than 6” square will keep chickens safe, and 2” square or smaller for quail. This will not prevent a hawk or owl from trying to grab a bird straying too close to the edge, but it will keep them out.
Scarecrows (scarehawks?): I do not have any experience with hawks and scarecrows. I do know that a hungry hawk will not immediately attack prey if humans have been recently associated with it. (This is a great aggravation when trying to trap them for falconry). The idea has merit, and is certainly worth a try if you are fortunate enough to have a raptor take up permanent residence in your area. I would create the scarecrow on a portable base and move it around every few days. A hawk will grow accustomed to it if it is in the same place for too long.
I hope this helps answer a few questions and helps keep your birds safe!