Species identification; clusters of big plump red bugs in Taipei
I'm fairly certain these true bugs belong to the species Leptocoris vicinus, and carry the nickname of "soapberry bugs", which is specific to the subfamily Serinethinae. They're quite common in urban areas of Southeast Asia, which coincides nicely with where you encountered them. 

Also, you had mentioned,


  There are plenty of birds around (this is quite a green area) but I didn't notice any interest by birds in eating them.


Soapberry bugs, as well as many other types of insects, are able to freely congregate in large numbers, and in such exposed places, due to their bright coloration. Having such a bright color may indicate to some predators that the prey in consideration is toxic, a phenomenon referred to as aposematism.

 source

 source

And then, here's a map of their distribution, with Taipei holding marker #37. (source)





An interactive version of this map can be found here.