2.1. DNA Sequence Data, Alignment and Gap Coding
The Early Bird [13] data matrix comprises ~25 kilobases (kb) of sequence data per species (before alignment) from 19 nuclear loci obtained from 169 bird species (supporting information, file 1). The 19 loci are located on 15 different chromosomes in the chicken genome [40], and they are likely to be unlinked in most or all avian lineages given the general conservation of avian karyotypes [41]. There was clear evidence that one locus (GH1) underwent a gene duplication within birds [42]; a single GH1 paralog was included for the taxa (Passeriformes) with two copies. Other details of the data matrix and alignment methods are provided in Hackett et al. [13] and Braun et al. [35].