Weather is not the only thing which dictates what you wear when hunting Javelina. If you are coming to the southwest for your first hunt don't be fooled by the day time highs generally in the 68 - 72 degree range. T-shirt weather right?...wrong, I can't think of a bush, tree, or cactus that doesn't have some type of thorn, needle, or sticker on it. Long sleeve shirt (preferably cotton or cotton blend) is a must, not only will it keep you warm in the early morning, it will protect your arms from all the things that will poke, stick, and scratch you, as well as from sunburn.
Trick for keeping warm but traveling light
Morning lows in the southwest deserts generally will be in the low 30's (depending on elevation and location mid 20's are possible). Temperature variation from morning low to afternoon high often exceeds 45 degrees.
Be prepared. Yes, even snow is possible! The author outside of Tucson, Arizona, during the January archery hunt...try explaining to your friends how you became lost in a blowing snow storm, with no coat, wet tennis shoes, pitch dark, carrying a 50 pound Javelina, for over 6 hours--all in the supposedly hot and dry desert!
To keep warm but still travel light (no jacket or sweatshirt). Get your self a military field jacket liner from your local GI surplus store. A field jacket liner is best described as a very light weight (matter of ounces), polyester fiber filled, long sleeved undershirt. They are extremely warm. When dressing for the morning hunt wear a t-shirt, field jacket liner, and your long sleeved camo shirt (over the liner). You will be warm and comfortable. Once the day warms-up you can remove the field jacked liner and store it in your fanny pack.
Go for loose and comfortable. Don't bother with those "snake proof" pants. They make to much noise and snakes are not much of a problem during the January or February Arizona hunting season (the earliest I've seen rattlesnakes is mid March). The best choice is military style BDU pants in my opinion. They have extra large pockets for gear, reinforced seams, and are fairly quiet.
Yes, wear a hat. Sunburn is a concern even in January. Any baseball or "booney style" will do.
Foot wear selection is extremely important (maybe your most important decision). Lightweight hiking boots or even tennis shoes will do. The most important criteria is that they are well broken-in and very comfortable. If your hunting technique is walking the flats to cut Javelina "runs" and trails, you will be putting in several miles per day on your feet. A blister or severely sore feet will ruin your hunt.
When's the last time you walked 5 miles? If you have trouble remembering, you need to condition your feet and your foot wear prior to the hunt.
Key piece of equipment. Many excellent manufactures and options for under $60.
Miscellaneous things to carry in fanny pack
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