The X-Men grieve the death of Thunderbird in "Mourning!"
Reprint of and retroactive backup story to The X-Men #95 - Warhunt, retroactively the earliest appearance of Thunderbird II, aka Warpath.
The story opens with Prof. Xavier sitting in the Arizona desert contemplating the recent death of his student John Proudstar, the mutant known as Thunderbird, whose remains he has brought to his family. With John's death Xavier realizes how lucky he's been to not have lost any students yet*, given the nature of their mission.
In addition to losing a student, Xavier had been in telepathic contact with John during the moments of his death, a feeling that he fears will haunt him for the rest of his life. He is soon joined by Jean Grey, who does her best to console him, as he had consoled her after witnessing the death of her close friend as a child.
The two mourners are interrupted by what they perceive as a cry for help off in the distance. A telepathic scan reveals the source of the cry to be Colossus. Upon investigation they find the source of Colossus' outrage: the body of John Proudstar has been stolen.
The other X-men who have accompanied Xavier to pay their respects seem equally outraged, save for Storm, who muses that no further harm can be done to John, to which Banshee replies, "in your beliefs, perhaps.. ..ours are different."
A priest says he will call the police, but Cyclops says it will take them too long to arrive and volunteers the help of himself and his colleagues instead. John's parents, however, ask that the X-men not get involved, saying that, while they appreciate the offer, they have done enough, and ask to be left in peace.
The X-men are baffled by this reaction, but Wolverine decides to search for the thief anyway, stating that, "me, I was taught a team takes care of it's own." Storm and Cyclops agree and the X-men set out to follow Wolverine into the desert, while Jean and Xavier share their impressions of John's parents, who seemed to suspect the thief was actually John's brother, James.
As the X-men attempt to catch up to Wolverine, Banshee, Nightcrawler, Colossus, and Storm each recount some of their last interactions with John before he died, struggling to make sense of the haunted and mysterious friend they have lost to what some members of the team are beginning to suspect was suicide.
Finally Wolverine catches up to the thief, who is indeed John's younger brother, James. Seeing that James is attempting to honor his late brother's memory by giving him a ceremony in the traditions of his people, Wolverine respectfully stays out of sight, watching but not interfering. From where he sits he finds some of John's remains that James had dropped, including some photos and his complete and impressive military awards. He realizes that John had lied about his age to get into the military, and from that deduces that whatever happened to him during that time had continued to haunt him ever since. He heads back and tells the team to call off their search, for whatever's happened to John, he's now at peace.
At the site of James' funeral for John we see him holding up his brother's X-men uniform and swearing revenge on the X-men for the death of his brother.