I have had this flash for 7 years and have never used it. Brand new. Until today.
The Nikon Speedlight SB-900 Flash was introduced in 2008. Robin and I purchased one in 2010 right after Nikon introduced the D70 Camera. I had the Nikon SB-800 Flash and was plenty happy with it. So, I never had the need to use it. Robin hadn't gotten around to use it, so it sat. With today being a dark, overcast day I decided to unpack it and give it a play.
I spent some time playing w/ the menus and did some searching to determine best settings for using it for bird photography. In the menu for the Nikon D500 Camera I went to Flash/Bracketing Menu and set it for syncing at 1/250 (Auto FP). In this mode the camera will operate using TTL up to 1/250 sec, and use High-Speed Sync up to 1/8000 sec. The flash was set at [TTL][FP]. Note: there has a lot of discussion about when to use [TTL][FP] vs. [TTL][BL][FP], which is also a menu option. The latter tends to correct for ambient light, but tends to produce darker backgrounds.
When I switched to Manual Mode and selected a Shutter Speed of 1/1000 sec. at f/5.6 the SB-900 exposed this American Goldfinch nicely (a bit overexposed, but nothing that Lightroom couldn't correct easily). Compare it to the same exposure w/ the flash off. 1/1000 sec., f/5.6, ISO 2800.
The flash did a nice job exposing feather detail. It will produce some steel-eye, but that can also be corrected w/ Photoshop. Tomorrow, I'll have to try it out some more and see what kind of adjustments have to be made to get consistently-good exposures.
BTW, the TC17EII Teleconverter worked just fine on the 300/2.8 VRII lens. Vibration reduction worked quietly, and autofocus was smooth and silent. That makes me happy.