I made several similar mistakes when I was setting up my D7100.
The manual gets to it, in its usual ponderous way, but the simple answer is: Bulb is available only in MANUAL mode, which is controlled by the button on the left side of the camera, having among other interesting markings...M manual A aperture S shutter and P program. I can't for the life of me see why they don't just label them S P A M !

If I read my manuals correctly, BULB is only available in MANUAL mode; then you use the main command dial (rear) to select the shutter speed, which will then eventually read BULB on the control panel, which is the little TV-like screen on the right rear of your camera.

If, like me, you have/had difficulty understanding your camera, SKIP THE FACTORY MANUAL! A book, "The Nikon D7100 for Dummies" can be ordered from Amazon. It has helped me greatly. I wore out my first one, and had the second spiral bound.
Anyhoo; Good luck, and keep at it! One day, Knowledge will gently descend, and smack you in the head with her wand!

Rivers usually have a plethora (I always wanted to use that word) of birds in the area. The Snake River is no exception. On my walk the other day I saw ducks, geese and more Cormorants. The Cormorants were exceptionally neat to photograph and to just watch. I photographed many Cormorants in the Santa Barbara, California area but they were not as close then as they are here. These are more into mimicking America's Bald Eagle with wings outstretched for freedom.

I hope you enjoy these bird photographs. Please let me know when you are inundated with too many photos.

Dennis

In a desert, or at least very dry environment, the most dependable attraction for birds is water. This tiny pool in the coastal range of mountains in Central California, attracted a steady stream of birds coming in the very early morning (Golden Light). Some people like the reflections; others do not. I fall in the category of liking the reflections. Shot with a Canon R5, 600 mm f/4.0 lens off a tripod from a blind.
California Towhee Shot with ISO of 2000, SS 1/400, F/4.0.
Northern Flicker Shot with ISO of 2500, SS 1/800, F/4.0
Spotted Towhee Shot with ISO of 12,800, SS 1/800, F4.0

I ended up working all-day Friday – heading home, the news talked about this historic geomagnetic storm that will make for a beautiful display in night skies across the country. When I got home, I told my amazing (and understanding) wife, load up, because we’re headed to Crater Lake. Around 4-hours later (as the sun was setting), we’re setting up cameras and waited for the light show of the decade (maybe the century). Around 2:00 am, we’re loading up and heading back to home.

Was it worth it, I would have to say “Most Definitely”.