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Supplemental Information

Cases of twinning in wild birds.

We found 14 documented cases of twinning for 13 species of free-living birds. Examples are known from either dissected unhatched eggs (n = 10) or from unassisted, hatched eggs where nestlings exceeded clutch size (n = 4).

DOI: 10.7287/peerj.preprints.186v1/supp-1

Image of bluebird clutch before hatching.

On 28 June 2013, an eastern bluebird (Sialia sialis) nest was observed containing one relatively large egg (bottom right) and three normal-sized eggs in State College, Pennsylvania.

DOI: 10.7287/peerj.preprints.186v1/supp-2

Photo taken on hatching day with one egg and four nestlings.

On 1 July 2013, likely the day of hatching, four nestlings were observed with one unhatched egg.

DOI: 10.7287/peerj.preprints.186v1/supp-3

Photo of five nestlings.

Five eastern bluebird (Sialia sialis) nestlings were present on 5 July 2013.

DOI: 10.7287/peerj.preprints.186v1/supp-4

Image of three live and two dead eastern bluebird nestlings.

At 8:15 am on 12 July 2013, the nest contained three live nestlings with two dead nestlings below them.

DOI: 10.7287/peerj.preprints.186v1/supp-5

Image of one eastern bluebird carcass.

On 12 July 2013, one carcass was found outside the nest box. It showed no obvious signs of structural problems or trauma.

DOI: 10.7287/peerj.preprints.186v1/supp-6

Additional Information

Competing Interests

The authors have no competing interests.

Author Contributions

Robyn L. Bailey conceived and designed the experiments, performed the experiments, analyzed the data, contributed reagents/materials/analysis tools, wrote the paper.

Gerald E. Clark conceived and designed the experiments, performed the experiments, contributed reagents/materials/analysis tools, collected data.

Animal Ethics

The following information was supplied relating to ethical approvals (i.e., approving body and any reference numbers):

Research was conducted under the Cornell University Institutional Animal Care and Use Protocol # 2008-0083.

Grant Disclosures

The following grant information was disclosed by the authors:

National Science Foundation grant No. 0540185

Funding

NestWatch is supported by the Cornell Lab of Ornithology and was founded under the National Science Foundation grant No. 0540185. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.


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