On 2 January, Nivelle instructed the to co-operate with the British to investigate German defensive systems that spies and repatriated civilians had reported. Not until 26 January, did a British intelligence summary report a new line of defence between Arras and Laon. In February, attempts to send more aircraft to reconnoitre the line were hampered by mist, snow, rain, low cloud and an extremely determined German air defence. British air reconnaissance discovered diggings between Drocourt and Vitry en Artois at the end of January and on 15 February, found a line between Quéant and Etaing. The British were able to trace the new line (named the Drocourt–Quéant Switch) south to Bellicourt on 15 February and St Quentin on 25 February, the day after the first German withdrawal on the Ancre. British aircraft losses on these flights were severe due to the presence of "Jagdstaffel" 11 (the Richthofen Circus) near Douai; six British reconnaissance aircraft were shot down on 15 April, along with two escorts.