On 17 June 2013, an eastern bluebird (<i>Sialia sialis</i>) nest was observed containing one relatively large egg (bottom right) and three normal-sized eggs in State College, Pennsylvania.
Bailey R, Clark G
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<a rel="nofollow" class="external text" href="https://dx.doi.org/10.7717/peerj.273">Image file</a> from <cite style="font-style:normal">Bailey R, Clark G (2014). "<a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3961165">Occurrence of twin embryos in the eastern bluebird</a>". <i>PeerJ</i>. <a href="https://en.wikipedia.org/wiki/Digital_object_identifier" class="extiw" title="w:Digital object identifier">DOI</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.7717/peerj.273">10.7717/peerj.273</a>. <a href="https://en.wikipedia.org/wiki/PMID" class="extiw" title="w:PMID">PMID</a> <a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pubmed/24688852">24688852</a>. <a href="https://en.wikipedia.org/wiki/PubMed_Central" class="extiw" title="w:PubMed Central">PMC</a>: <a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3961165">3961165</a>.</cite>