Canon gained a big advantage when it completely changed its mount design in 1987 when it introduced the EOS cameras. Nikon, on the other hand, has been dedicated to backwards compatability and has stayed with the same basic F mount since 1959. Most important for bird photography, Canon held a clear advantage in supertelephoto lens design for a long time. Their long lenses incorporated fluorite elements and also included image stabilization long before Nikon finally caught up with its own VR supertelephoto lenses (and now, fluorite elements, as well). Today, Nikon's long lenses are as good as Canon's (some nitpickers might dispute this, but any differences are inconsequential in actual use), and the Nikon mount design has proven adequate to incorporate all of the high-tech design features that the EOS mount made possible for Canon. Nikon is generally just a bit behind Canon, though not always (Nikon has released its excellent 200-500mm while Canon has not produced a lens that really competes with it).
As others have pointed out, Canon has the 400mm f5.6 teles, whereas Nikon does not. But I think that Canon's popularity and dominance over Nikon in the sports and wildlife photography areas are due to other factors, including Nikon's failure to keep up in lens design for many years and also Canon's superior support system, including its professional support service. Canon is a bigger, wealthier company. Nikon seems always to be playing catch-up. Nikon has not given new users any really compelling reasons to choose the Nikon system instead of the more popular Canon system.
That said, I think Nikon gear is terrific. I started with Nikon in 1968 and have stayed with Nikon since. I still can use the 50mm f1.4 lens that came with my first Nikon F camera. I think that's pretty cool.