These guys are hungry. Bone thin. By policy if the bison leave the park into Montana they can be killed. This is the only way to keep brucellosis out of the Montana beef population. The winter of 2007/2008 was a particularly bad year with over 1,700 killed and an additional 400 to 700 lost to winter kill. That means that about 1/2 of the Yellowstone bison population is gone.

From Yellowstone Insider:

Winter kill adds to the record loss of bison
[Friday April 18, 2008] The other hoof dropped (on a Friday no less) - it was news that on top of the roughly 1700 bison killed by hunters or sent to slaughter this winter, the annual winter kill (bison who don't survive the rigors of winter) appears to be between 400 and 700, bringing the total loss of the Yellowstone bison to about half the herd. Something like this should have been expected, as a tough winter is the reason so many bison were heading for lower, warmer terrain near the park border.

The 2007 herd was estimated at 4700 head; now 1700 are dead by the hand of man, and roughly 500 dead by this point from winter conditions; that equals approximately 2500 head (+/- 200). The figure is still above the cutoff point of 2100 set by the Interagency Bison Management Plan; the point at which non-lethal measures are required to manage the bison. The target bison population is 3000. However, winter isn't over in Yellowstone, as spring storms, hungry grizzlies, and wolf packs typically take a significant toll at least until June. Whatever the herd population come the end of spring, this winter of 2007-2008 has set a record for the number of bison killed and could be heading for historic lows in the bison population.

Bison kill for 2007/2008