Sorry for not wading in sooner, my day job interfered. Many of the other posts have raised the fundamental question - How do you know that there hasn't been any damage to the engine? The answer is you don't and there is lots of data to show that relying on cockpit displayed engine paramaters to form a judgment on the condition of the engine is not wise. Strike data and engine manufacture information shows that in many cases the engine will suffer a "delayed damage" failure. The only way to know for sure is to have the engine inspected by qualified personnel using boroscopes and/or black lights.
I am not proposing that every bird strike requires a return to land. Obviously some thought must be given to the size and number of birds struck. However, if the strike occurs in IMC (yes birds do fly in IMC) or at night, observations are problematic. To assist with making the decision you might want to frame it based on what engines are certified for with respect to bird strikes. With the excception of the very large engines like the GE90 RR Trent on the B777, the majority of jet aircraft engines are certified to ingest, at the aircraft liftoff speed, one 4-pound bird OR 8 1.5-pound birds OR 16 3-oz birds may not cause the engine to-- (1) Catch fire; (2) Burst (release hazardous fragments through the engine case); (3) Generate loads greater than those ultimate loads specified in FAR 33.23(a); or (4) Lose the capability of being shut down. (b) Ingestion of 3-ounce birds or 1 1/2 -pound birds, under the conditions prescribed above, may not-- (1) Cause more than a sustained 25 percent power or thrust loss; (2) Require the engine to be shut down within 5 minutes from the time of ingestion; or (3) Result in a potentially hazardous condition.
Another guideline to apply is that as altitude increases although the strike rate decreases BUT the the average weight/size increases and you are most likely going to hit a flocking bird.
Taking into account all the above you can see that if you ingest seagulls or waterfowl, particularly of you ingest more than one you will most likely have damaged the engine.
And we haven't even talked about airframe or collateral system damage that can occur from hitting a flock of birds - something not catered too in the certification standards.
Food for thought (sorry I couldn't resist)
Richard - A320 Pilot and Bird Strike researcher