Best tripod head for BIF... Dead arm syndrome in full effect

After four days of hand holding my 400mm from sun up to sundown at Bosque del Apache I realize it is time to invest in a good tripod head for panning. On a day outing hand holding is not a problem but the cumulative effect of the weight and swinging the camera around took a toll on me. You guys definitely have an advantage in the strength department!  I would really appreciate any suggestions you might have.

I will be renting and eventually hope to invest in a 500mm lens so I will need something sturdy. If y'all ever get the chance to visit Bosque, it is truly amazing. The morning light at the ponds is awesome and the late afternoon glow at the reserve when the Cranes fly in will take your breath away. A photographer set up near us with a 600mm. If I didn't think my husband would bury me in the desert for buying something that expensive I would dearly love to have one. It is a thing of beauty.