After a life in dark barns with little human contact, transport is emotionally stressful. Birds get dislocations and broken bones during catching, loading, and unloading. Many chickens overheat or freeze to death on the truck.
Both meat birds and laying hens have their food withdrawn up to 8 hours prior to transport, and lights are lowered. Meat birds are herded into a fenced area before catching. Laying hens are grabbed from inside their cages. The weak birds are vulnerable to injuries.
A crate is filled every 7 seconds! Humane handling is impossible. Birds suffer dislocations and broken bones due to rough handling.
Mechanical catchers can be more humane but are also prone to abuse. The emphasis is speed, not animal welfare. Images from Mercy For Animals videos
Amsterdam-based Eyes on Animals shows current and improved ways to catch and load chickens in a short video called Calming The Catch (the better way is shown towards the end):
Crates are stacked onto trailers. Fragile, featherless hens are protected with only tarps even in -31°C wind-chill. Birds die from excessive cold or suffocate on different parts of the same trailer. Transport may legally be 28 hours with no food or water. For most birds, this is their first and last experience of the outside world.