Dogs may not have a lot in common with birds, but they do build nests. Traditional nesting behavior is common in pregnant dogs, who need a safe, warm and comfortable place for delivering and raising pups. Other dogs build beds in the form of a den, which is used a private, secure space whether they live in the wild or enjoy a life of luxury as your pet.
Pregnant dogs build nests out of instinct, because in the wild, they would need a safe and private area where they can give birth and raise the puppies while they're small, vulnerable and dependent. Generally, this isn't a particularly complex area -- just someplace relatively dark, enclosed and warm. They need the privacy because they and their pups are vulnerable to predators, and because the puppies need shelter from the elements.
A dog doesn't have to be pregnant to seek out a nest-like area. Dogs enjoy the comfort of a den, which is an enclosed, personal space just big enough for a single dog. He has room to stand up and stretch or lie down, and when he feels frightened or stressed, he can go there to feel safe. In domestic dogs, this usually takes the form of a crate, which you can line with a blanket for additional comfort.
Inside and outside the crate, some dogs like to turn in a few tight little circles before lying down. This is part of nesting behavior that is hard-wired in your dog's instincts -- in the wild, circling a few times before getting cozy made the grass softer and more comfortable, and drew out any undesirable bedfellows, like snakes.
Dogs like a clean room, so they keep their nests/dens as tidy as possible. Most notable is the fact that a dog won't go to the bathroom in his den or nest if he can avoid it, which is why indoor crates are widely used as housebreaking tools. In the birthing nest, a mother will consume her pups' waste, as leaving it in the nest is a health risk, and the odor attracts predators.
A dog's den is his sanctuary, and you have to treat it with the same respect that he does. Using his den should be his choice -- one of the biggest training mistakes that a pet owner can make is to use the crate as punishment. This violates your dog's instincts and confuses him by telling him that going into the den is a bad thing, which causes him anxiety and disrupts the training process. When you're training him to stay in his crate, always treat it like a reward, and give him a treat when he goes in -- this reinforces his natural instincts, and he'll be able to go in there whenever he wants without feeling like he has done something wrong.
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