Mutational spectrum of the mitochondrial genome

Birds are a vertebrate class with one of the highest basal metabolic rates across all animals. So the question arises: does their mtDNA have nucleotide content and mutational bias shift due to Ah->Gh and Ch -> Th (h – heavy chain) mutations? 

Our hypotheses:

1. Birds should have more G and T on the mtDNA in four-fold positions
2. Amount of G and T should grow with increase with each gene specific time being single stranded
3. Birds with high basal metabolism rate – antarctic ecozone and herbivore aquatic trophic niche should have higher amount of G and T due to Ah->Gh and Ch->Th
4. Mutagenesis is the main reason for birds mtDNA nucleotide content shift