I saw a Van de Graaff generator spark when a demonstrator touched it. should read I saw a Van de Graaff generator spark $\color{red}{\text{just before the}}$ demonstrator touched it.
What happened was that the electric field strength between the VdG and the demonstrator was high enough to ionise (breakdown) the air which produced a conducting path between the VdG and the demonstrator and this resulted in the spark.
In general when birds are sitting on high voltage power lines the electric field between them and adjacent power lines is not sufficient to produces sparking but there may be a corona discharge from pointed parts of the bird eg the pointed beak oe end of wing feathers which produces an insufficient conducting path to cause the bird permanent harm.
Of course if the bird is sufficiently large the gap between the bird and the adjacent power line may be sufficient small for the air to ionise which a result as in the photograph below.
Engineers who design high voltage power lines are well aware of the problems that birds may cause both to the bird itself and to the power line which would temporarily trip if the air around the cables becomes conducting to a significant degree.
Here is a diagram of the sort of size parameters which engineers use when designing power lines.
The paper Bird protection on medium voltage power lines gives an indication
care taken to avoid harm to birds during the design process.
