The following has been disproven, based on this study:
James J. Krupa. A CLASSROOM EXERCISE FOR TESTING URBAN MYTH: Does Wedding Rice Cause Birds To Explode or Were Ann Landers, Martha Stewart & Bart Simpson Wrong? The American Biology Teacher 67(4):223-230. 2005
In 2005, James J. Krupa, an Associate Professor of Biology, asked 600 students from his biology classes at the University of Kentucky if throwing rice at weddings would be harmful. 45% percent of the students alleged it was unsafe mentioning exploding birds as the reason.
Krupa realizing this a teaching opportunity had his students test the claim as a lesson on the scientific method. The students carried out a series of experiments on:
- the expansion of contrastive types of grains
- the strength of avian digestive system
- the snack preferences of several common bird species.
In results, issued in the journal cited at the beginning of this answer, the students found that the only grain that expanded enough to possibly pose a danger was instant rice not the kind usually thrown at weddings.
Nevertheless, the students prevailed on their teacher Krupa to feed instant rice to his own birds and monitor the outcome.
Confident that the birds won't explode, Krupa agreed and fed 60 doves and pigeons a diet of nothing but instant rice and water for a day, monitoring them for signs of distress or discomfort.
The results were out. There was no sign of burst gizzards, deaths, or even regurgitation.
They just loved it, and now they're kind of addicted to it.