About 63% of known bird species have a diploid chromosome number (2n) of 74-86. A further 24% have 2n = 66-74.
Extremes at the lower end of the spectrum are the trumpeter hornbill Ceratogyman bucinator and stone curlew Burhinus oedicemus (with 2n = 40 and 42 respectively). The highest recorded chromosome number in birds is in the common kingfisher Alcedo athis (2n = 132 or 138) or the Grey (or Southern) Go-away bird Corythaixoides concolor (2n = 136-142). Understandably, accurate chromosome counts become difficult with these numbers. [Source: Christidis, 1990, Animal Cytogenetics 4, Chordata 3 B: Aves]
In general, chromosome number in birds is both higher than in most other vertebrates, and less variable. The typical avian karyotype is composed of macro- and microchromosomes, with a continuous size distribution. Exceptions include the falcons and parrots, where chromosomal fusions and other rearrangements are common, and have reduced chromosome number and size variation in these groups as a whole.
For a recent review of this topic, see: The evolution of the avian genome as revealed by comparative molecular cytogenetics, Cytogenet Genome Res. 2007;117(1-4):64-77
