*What are the practical limitations of a glider made of bronze age materials and pulled by well trained birds?*

Due to psychics able to control animals, and a cultural fixation with domestication, nearly every animal of any potential use (even just as a status symbol for psychics) was domesticated hundreds of millennia ago in this timeline.

*You can draw from any animals living in the past ~400k years in your answer*, notably including: Haast's eagle and various massive Teratornithidae birds (but excluding the heaviest ever flying bird Argentavis because it lived millions of years ago).  

Given both the limited fossil record for the largest birds and selective breeding you have some flexibility in terms of exact wingspan and wing shape.

The aircraft can be directed by ~3ft, ~40lb hominids and/or *extremely* well trained crows bred for incredible endurance, memory and diligence (with the latter being much cheaper).

*Given only natural materials (which includes cheap spider silk) seem viable here, how much could such an aircraft possibly carry?  

Will air travel be limited to the tiny halflings for a very long time, or is carrying a human totally feasible?*

Remember the birds can hold on to the aircraft with their wings extended even when not flapping so one must factor this into surface area.

If your answer is contingent on the use of long lines/tethers connected to or held by birds: Please consider how tangles are avoided when dealing with turbulence, as whether the aircraft crashes whenever it encounters a strong wind is an important consideration.

Part of series with:
Just How Omnipresent Would Cheap Cross Laminated Spider Silk Be?  

How much would this alien parasite increase crop yields?
What Bronze Age Jobs Couldn't be Replaced by Extremely Well Trained Animals?