In 2009 they tested this with a 700MB file. It took 70 minutes for the pigeon to do it. The ISP failed completely. A car took twice as long (but could carry a lot of tapes.) The pigeon, basically, has 50% the latency overhead of the car but can't carry a fraction as much information. The ISP is effectively useless.
If in 2009 the pigeon could do 700MB in 70 minutes, today it could do 512GB in the same time! That's like an 800x increase in speed.
Current pigeon speeds would be 7GB per minute or 122MB/s which is 975Mb/s. Pigeons in Australia are basically matching Gigabit Fiber speeds!!