Hron et al. provide transcriptome evidence that three (1.1 %) of the 274 genes reported by Lovell et al. as missing in birds may actually be 'hidden' as a result of high GC content. Although this factor may explain some gene absences from genomic assemblies, we believe it is insufficient to account for the extensive syntenic losses described in Lovell et al. Please see related article: www.dx.doi.org/10.1186/s13059-015-0724-z.