Hi! Welcome to Whatbird!
eBird is an excellent website for learning about bird behavior. Here is the sightings map for migration in NYC of Yellow-shafted Northern Flickers. If it's any help, the red-shafted flickers I get here occasionally flock at my house over a period of a week or more, mostly during migration season (don't know why).
https://ebird.org/map/yesfli?neg=true&env.minX=155.69237000960777&env.minY=9.447971766050006&env.maxX=2.411120009607771&env.maxY=71.35671329491146&zh=true&gp=false&ev=Z&mr=8-11&bmo=8&emo=11&yr=all&byr=1900&eyr=2018