The Left Fork of North Creek (aka "the Great West Canyon") is one of the most beautiful and most popular canyons in the backcountry of Zion. The most famous section of this canyon is known as "The Subway" -- an amazing half-mile stretch that resembles a tubular tunnel complete with tracks going down the middle (but requires a permit). The Left Fork of North Creek has much diverse terrain, and as such, there are several different possible routes. The habitat includes most dry desert, some riparian and canyon habitats, but at the trailhead is mostly mixed oak and juniper woodlands.
I can't speak to actual birding on the trail, just at the trailhead and along the road through the awesome mixed woodlands. The habitat is great for specialty species like JUNIPER TITMOUSE and SPOTTED TOWHEE year round. During the breeding season both BLACK-THROATED GRAY and VIRGINIA'S WARBLER can be found here, while CHIPPING SPARROW, WARBLING VIREO, and BLACK-HEADED GROSBEAK are usually seen as well. SCOTT'S ORIOLE has been reported from here in the past, and is likely overlooked in the juniper stretches here. Birding from the road with a few quick stops you can usually pick up all the expected species without much work. On the way up to Kolob Terrace this is one of the usual spots I stop to look for the oak and juniper species for a few minutes.
All Zion National Park hot spots directions in Zion Canyon, or to the east will orignate from the visitor center parking lot for consistency. Anything in the north and west sides of the park will come from the nearest major instersection. From the U.S. Post Office in Virgin, go east on SR-9 for .2 miles. Turn left onto Kolob Terrace Road and continue north for 8.2 miles. The trailhead will be on your right.