I have been told that if you give a Seagull an Alka-Seltzer it will
explode, because the birds have no way of releasing gases. Is this
true and why?

This is probably true for the reason that you have stated.

I've also heard that feeding them bread that has been soaked in a
solution of bicarbonate of saod has the same effect.

This is only hearsay ... I've got no first hand experience.

This is probably an urban myth. This has been discussed before on
Google Answers. See here
http://answers.google.com/answers/main?cmd=threadview&id=217385

Naw, the Alka-Seltzer bit is an uban legend. 

Much more fun is to loosely tie a bit of squid or fish to the ends of
a string and toss it in the air. One gull will grab it and head off to
eat when another will grab the other end and (usually) head in a
different direction.

Results-- sudden yanking of one or both birds from their respective flight paths.


I am ashamed of some of the stuff I used to do as a kid... but this
isn't one of 'em!