Flocks of birds can fool weather radar, but I don't think an individual bird is going to give a strong return on a radar designed to detect larger metal objects. But I'm not sure.
Well, it's only a few pixels across, and would likely be mostly gliding at that altitude. But the general "blob" shape does seem to resemble a balloon more than a bird. Very hard to say though. At a distance planes and birds also look like blobs.
Remember those are military grade radars. They are the best sensors in the world. Their mission is to track stealth planes at extreme range.
The radar cross section of a stealth fighter isn't different from a bird according to some sources (it's all classified of course). An F-22 is supposedly "comparable to the radar cross sections of birds and
bees" (source:
https://www.globalsecurity.org/military/systems/aircraft/f-22-stealth.htm)
So of course those radars can distinguish birds but they probably filter them out normally (remember F-18 radars are so sophisticated they can recognise the exact aircraft type based on the radar return of their turbine blades which are characteristic for each model
https://www.princeton.edu/~ota/disk1/1993/9351/935106.PDF so it's safe to assume they can distinguish the radar return of a bird). But if you ask them to calculate the range to a large bird you are tracking with ATFLIR they won't have any problem at all.
Also, 4 miles is not a lot in terms of the capabilities of those sensors. You are around 10% of the total range of ATFLIR (40 nm). A big bird should be pretty clear and distinguishable.
And just how big is the "bird"?
Not sure if someone did this already but to calculate approximate size:
Pixels of object / total pixels of the video on my screen: 13/644=0.020
ATFLIR NFOV Zoom level 1 field of view 0.70 deg
https://www.explorescu.org/post/nimitz_strike_group_2004
So apparent size is 0.020*0.70=0.014126° which at a distance of 3.4 nautical miles=6296 meters means the
object is around 1.55 meters wide.
No problem tracking anything that big with an F-18. And its shape should be distinguishable as this is a very normal range for those sensors.
