This is a trap we built copied from one provided to us by Falcon Environmental Services at JFK Airport. It is highly effective at catching crows that use a communal foraging area, such as a public compost pile. Other methods are required for catching crows on their territories (such as cannon-nets).
The four lower panels of the trap are the same dimension: 8 by 4 feet. One side has a door that is 2 by 4 feet. The two parts of the pitched roof measure 2 feet high by 3 feet wide.
The ladder, which is about 23 inches wide, sits between the two parts of the roof and it is here that the most important measurements lie as this is the working part of the trap. Rungs define the area that crows will drop into the trap. They are 8 inches apart (that is, the HOLE is 8 inches - I realize the picture looks as if the measurement is from one leading edge to the next, but 8 inches refers to the size of the hole). The rungs begin and end 18 inches from each end of the ladder. This part is covered in chicken wire and it is important not to have rungs close to the end of the ladder as crows will pull themselves out of the trap at this point. Running back and forth through drilled holes in the rungs is a length of wire. The distance between each course of wire is about 3.75 inches. This creates a hole that is 8 by 3.75 inches for the crows to drop through.
Each panel, the two pitched roofs, and the ladder are constructed separately. The trap is then held together by plastic zip ties. Extra perches can be secured with zip ties inside the trap and the door can be secured with either zip ties or a lock (but given that the trap has no floor, a door lock provides little security). A dish of water should be provided as well as a sign indicating permit number contact and a little warning about west nile virus (to keep the riffraff away).
Below is what a loaded Wattman Number 1 filter blot quarter looks like:
Note that the band number is written on the blot itself. Blots are cut into quarters from 2-inch circles of filter paper. They are allowed to air-dry and then placed in a small plastic bag with accompanying information. Blots can be stored at room temperature or, for the long term, in a standard refrigerator.
See also http://www-rci.rutgers.edu/~lreed/crowtrap.htm