Birds are definitely limited by aerodynamics, biology just says well screw this it ain&#x27;t worth it because life forms are limited by the amount of energy available to them.
If we take the heaviest flying bird today it has a wing span of about 3M and a total wing surface of slightly under 1 m&#x2F;sq the Quetzalcoatlus you&#x27;ve mentioned has a wing span of about 15m and a total wing surface of about 35 m&#x2F;sq, back of the envelope math means that about a mass increase of about 12 times over the Kori (heaviest bird capable of flight) results in about 30-35 times the increase in lift surface (and we are still unclear if the damn thing was even capable of powered flight or not).<p>And again you are missing the point Solar Impulse can lift about 1000 lbs of w&#x2F;e if you want to scale it up either increase the thrust by 50 or increase your lift surface by 50 to get to that 50,000 lbs cargo you want.
If we ignore load baring and drag than 3 Solar Impulse aircraft combined could lift 3000 lbs that&#x27;s it.<p>We can argue about this till the sun doesn&#x27;t shine it won&#x27;t change the fact that solar flight will never be a viable option for cargo transport.
But it can replace other things, things that for example are well in range of the 1000 or lbs of payload that any reasonable designed solar aircraft could carry - surveillance, communications, weather and atmospheric monitoring equipment etc.
They could potentially replace satellites, and increase the endurance of both military and civilian drones considerably but no one is looking at them to carry cargo because it&#x27;s simply silly.<p>Also if you want to move 50,000 lbs today by air it usually won&#x27;t take less than 6 days, the only thing you can really ship overnight are small things that end up going on commercial flights not cargo flights.
And while it&#x27;s also true that a container ship can take 21-30 days from say China to the US about 14-15 of those days are spent in local transit and loading, the sailing it self  a cargo ship can do within 10 days of good weather with the average being probably closer to 12 or so days due to weather and fuel economy.