IPoD actually has really high throughput.

According to wikipedia Carrier pigeons typically travel ~1000km at ~100km/h and can carry 75g comfortably. a microsd card weighs about 0.5g and we have 1Tb ones now so our pigeon could carry about 150Tb per trip (sidenote that'll cost ~20K so packet loss would really suck) . that's an impressive 33Gb/s at the 1000km range. the 30million ms ping might be annoying though.

relevant XKCD

This really needs an update for each carrier to transport multiple packets at once.
Based on RFC2549 it seems each carrier can transport up to 10g. That's roughly 40 MicroSD cards. The current largest MicroSD cards are 2TB, so that's 80TB/carrier. It seems the smallest response time is 3,000s.
That means the theoretical top transfer rate could be roughly 213Gbps.

Edit: Although it seems the carriers could do as much as 75g. That's 300 MicroSD cards or 600TB. At 5km that makes 1.6Tbps!