The biggest problem I have capturing images of wildlife is not getting close enough to the subject. I never have enough time to sit and wait for them to come to me or they are easily spooked, and fly away at the first sign of movement. Using an simple, widely-available PIR motion sensor, I can leave my DSLR in the yard to capture images while I run errands, cut the grass, or even leave it out overnight.
The Simple PIR Sensor DSLR Camera Trap uses an industry standard PIR sensor to send a signal to a DSLR camera to fire the trigger when it detects motion. I have access to Canon cameras so this Instructable is specific to Canon DSLRs with an N3 connector or stereo connector, but this will work with all DSLR cameras that have a shutter input port. A PIR sensor is what is commonly used in home security motion detectors. Instead of tripping an alarm, we are going to use the PIR sensor to fire a camera.
This Eastern Bluebird was fairly predictable on where he would land, only if the yard was free of people. By placing the Simple PIR Sensor DSLR Camera Trap aimed at his perch, I could set up the sensor, set focus, exposure, shutter speed, and ISO on the camera, and walk away. This image was captured at distance of 10' allowing for greater detail and a pleasing background blur.