Chromosome naming convention: Why are there chromosomes named "1" and "1A"?

Question

[beausoleilmo]

I've been stumbling on multiple genome of birds where there is a "1A" chromosome and a "1" chromosome. For example, the zebra finch has 1A and 4A. What does that mean?

Do you have any resource about naming conventions for chromosomes and if this naming convention is specific to birds?

[GenoMax]

While someone may answer you will likely have better luck posting this on a general biology forum like Biology stackexchange.

Here is a book chapter on chromosomes in birds. If you have institutional access check it to see if it throws any light on the naming.