Materials, coating, and shape all contributes to what is commonly referred to as stealth. You can't just put radar absorbing paint on a 747 and it suddenly becomes stealth, otherwise all of the USAF would do that
Birds do not get picked up by radar set to war-time sensitivity levels. A a former UAV pilot who flew the Shadow 200 TUAV for 3rd Brigade 2nd Infantry Division in Iraq from 2003-2004, we did training with our attached air defense patriot battery. The Shadow is designed to have an exceptionally low radar and thermal cross signature. We wanted to prove it when doing our combat certifications before going to war. They told us they had to set their gear sensitive enough to pick up birds of prey (eagles, falcons, etc) to see anything meaningful and that they aren't able to do that in combat due to the numerous false positives. After initial takeoff and climbing up to mission altitude, those planes are effectively (meaning in practice) "stealth" although I'm saying that very loosely