Under the Devil’s Eye describes the thoughts of a British officer after the two failed attacks at Doiran:
So busy were we with considerations of the future and the present also, for a strong enemy counter-attack would have found our front very disorganised, that the remarkable news from the Serbs seemed to pass almost unnoticed.
The Bulgarian First Army had been astonished at the order to withdraw from its positions, now it was the turn of the BSF to be astonished. The British Official History describes how loud explosions at around 8am first gave an indication that something was happening, then reports from RAF reconnaissance flights reported large troop movements northwards – the Bulgarians were in full retreat. In the afternoon RAF D.H.9s from No. 47 Squadron returned to bomb the retreating columns:
The aircraft flew over the columns in relays, dropping bombs on them and then returning to their aerodromes for another load. In some cases they swooped down to within fifty feet of the ground, to rake troops and transport with machine-gun fire. The target was an extraordinary one. There is in existence a set of aeroplane photographs which show a solid stream of transport, for the most part double-banked, between Valandova and the Rabrovo cross-roads and also south and east of the latter point, with bombs bursting in its midst. At the cross-roads it can be seen that the columns are completely blocked by the bombing and are a congested mass, unable to make their northward towards Kosturino.
From Capt. Cyril Falls, (1935), Military Operations Macedonia – From the Spring of 1917 to the end of the War; HMSO, London.
There is a chilling photograph in the IWM collection (Q 80393) showing abandoned Bulgarian vehicles and equipment in the Kosturino Pass. You can view it here and I recommend that you spend time browsing the fantastic collection of photographs from the Macedonian campaign that are available online.