What is this Renolds Transport Theorem(RTT) which appears frequently in fluid mechanics problems actually doing?
RTT basically converts some property variation from material volume(Lagrangian) which is formed by the same fluid element set in motion to spacial(Eulerian) volume which is fixed in space which includes different set of fluid particles at different time
Visualisation is what is most important
I would love creating geometry of what you are gonna ask
Is it easier for trailing birds to fly when they are in a bunch?
Yes. The disturbance(wake) in the air created by leading birds reduces the drag felt by the trailing ones which are modeled as the reduction in induced angle of attack caused by line vortices created by the leading birds.