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Access through your institution Buy or subscribe Renée Duckworth and her colleagues at the University of Arizona in Tucson discovered that female western bluebirds (_Sialia mexicana_;
pictured) that live in areas with many neighbours and few nesting sites laid eggs containing more androgen — a hormone that boosts aggression in the offspring — than females facing less
competitive pressure. Those first eggs also tended to produce more males, which can compete for and colonize new territory. When the researchers increased the number of nesting sites in
study areas in western Montana, however, the females produced eggs with less androgen, and fewer male offspring in the early eggs. This eventually allowed the western bluebird to boost its
numbers and displace its competitor, the mountain bluebird (_S. currucoides_). This is a preview of subscription content, access via your institution ACCESS OPTIONS Access through your
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FAQs * Contact customer support RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Competing bluebirds make tougher sons. _Nature_ 518, 458 (2015).
https://doi.org/10.1038/518458a Download citation * Published: 25 February 2015 * Issue Date: 26 February 2015 * DOI: https://doi.org/10.1038/518458a SHARE THIS ARTICLE Anyone you share the
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