Genomic data reveal unexpected relatedness between a brown female Eastern bluebird and her brood

Because plumage coloration is frequently involved in sexual selection, for both male and female mate choice, birds with aberrant plumage should have fewer mating opportunities and thus lower reproductive output. Here we report an Eastern Bluebird (Sialia sialis) female with a brown phenotype that raised a brood of four chicks to fledging. The brown female and her mate were only related to their social offspring to the second degree and one of the offspring was a half-sibling. We propose four family tree scenarios and discuss their implications (e.g., extra-pair paternity, conspecific brood parasitism), but regardless of the tree, the brown female was able to find a mate, which may have been facilitated by the bottleneck created by the severe snowstorms in February 2021.

Methods
These data are a result of low-coverage whole genome sequencing from individual Eastern bluebirds (Sialia sialis). We isolated genetic material from blood samples of bluebirds collected in northeast Arkansas, USA. We filtered the raw reads and mapped them to an existing reference (GenBank accession GCA_009812075.1) to call single nucleotide polymorphisms (SNPs). We then used the SNPs in a relatedness analysis to assess the familial relationship among samples. We also assembled the mitochondrial gene nad2 from the filtered genomic data. We used the nad2 sequences to assess maternal lineages among our samples.