The study and collecting of postcards is termed "deltiology".

It is possible to form a large collection of postcards around the theme of parrots and parakeets which were very popular subjects, especially on the early comic or saucy postcards. At the peak of their popularity, in the 1930s, sales of saucy postcards alone reached a massive 16 million a year.

To search eBay UK for parrot postcards, click here, and for eBay USA, click here.

A quick comparison - '30s to '60s!

These two postcards came up for sale in October 2011. You can't get a much quicker way of comparing two decades than these two!