Some of Bishop's other claims have also been challenged. While combat reports and claims of both sides are littered with well-intentioned errors and accidental duplicate claims, there are two phases of Bishop's life in which German records can provide no supporting evidence. In his book on Victoria Cross airmen of the First World War, author Alex Revell quotes aviation historian Philip Markham's view about German records of the events of 2 June 1917 (the day of Bishop's VC award): "Not a shred of evidence to support Bishop's claims." Referring to Bishop's claims in early to mid-1918, Revell says another aviation historian, Ed Ferko, carried out extensive research on Germans records in 1987. Revell says that Ferko failed "to match a single victory claim made by Bishop against a known German loss for the day, time or place in question." However, distinguished First World War aviation historian Peter Kilduff says in his biography (Billy Bishop VC: Lone Wolf Hunter) that Bishop may have had as many as 21 matches in piecemeal German records. Kilduff also makes a case for the unreliability of German records. He cites examples in which masses of data were destroyed by retreating German forces and instances of the German former air ministry having been guilty of "obfuscation" in denying losses when casualties had been incurred.