I was soloing The Nest (normal zone version)and wanted to see how hard these "a fallen nestling" could be. I went up the bridge, got in line of sight (they did not see invis).. and cast mire thorns. He resisted and did come single, but he was running faster than my SOE. the damage was 709, 533, 1104, 1060, 314 and then I used exodus to get away. I didn't notice if he summoned because he was on me the whole 5 seconds. Good luck!
No they do not summon, in the non-instanced at least. I used ensnare and it landed, just root/rotted it to 40%, epic clicked and nuked it down, at under 20% it went into flee mode making it easy to finish.