One of the bulkiest items on your gear list for a
bivy hunt is probably your sleeping bag. If it has not been replaced in the last couple of years, chances are you can not only shave off some weight, but also significantly reduce the bulk. Almost all of the major sleeping bag manufacturers are offering sub 2 pound 3 season down bags now. Some have reduced the weight down to as little as a single pound and can be stuffed into a sack half the size of a nerf football. For my early season hunts in August and the first week of September I like a bag rated at about 30-35 degrees. Any later in the high country and you are increasingly flirting with the chances of sub-freezing temperatures. For those hunts I drop down to a 20 degree rating. Some of my favorite bags are from Western Mountaineering, Marmot, Mountain Hardware and Feathered Friends.
Down is graded by measuring how many cubic inches it takes to weigh one ounce. A 600 fill is 600 cubic inches per ounce. Several things to keep in mind as you shop for a featherweight bag is the higher quality the down, the lighter the weight and the better the insulation it provides. You should be looking for a minimum of 700-fill-power, with 800-900 being preferred. The difference between a 600 and a 800-fill-power bag of the same temperature rating can be over half a pound. The higher the number, the greater the loft and compatibility for the same fill weight.