Using 3D printed eggs to examine the egg-rejection behaviour of wild birds

PeerJ. 2015 May 26;3:e965. doi: 10.7717/peerj.965. eCollection 2015.

Abstract

The coevolutionary relationships between brood parasites and their hosts are often studied by examining the egg rejection behaviour of host species using artificial eggs. However, the traditional methods for producing artificial eggs out of plasticine, plastic, wood, or plaster-of-Paris are laborious, imprecise, and prone to human error. As an alternative, 3D printing may reduce human error, enable more precise manipulation of egg size and shape, and provide a more accurate and replicable protocol for generating artificial stimuli than traditional methods. However, the usefulness of 3D printing technology for egg rejection research remains to be tested. Here, we applied 3D printing technology to the extensively studied egg rejection behaviour of American robins, Turdus migratorius. Eggs of the robin's brood parasites, brown-headed cowbirds, Molothrus ater, vary greatly in size and shape, but it is unknown whether host egg rejection decisions differ across this gradient of natural variation. We printed artificial eggs that encompass the natural range of shapes and sizes of cowbird eggs, painted them to resemble either robin or cowbird egg colour, and used them to artificially parasitize nests of breeding wild robins. In line with previous studies, we show that robins accept mimetically coloured and reject non-mimetically coloured artificial eggs. Although we found no evidence that subtle differences in parasitic egg size or shape affect robins' rejection decisions, 3D printing will provide an opportunity for more extensive experimentation on the potential biological or evolutionary significance of size and shape variation of foreign eggs in rejection decisions. We provide a detailed protocol for generating 3D printed eggs using either personal 3D printers or commercial printing services, and highlight additional potential future applications for this technology in the study of egg rejection.

Keywords: 3D printing; American robin; Artificial egg; Brood parasitism; Cowbird; Egg rejection; Experimental techniques; Molothrus ater; Turdus migratorius.

Grant support

Funding for this research was provided by the Human Frontier Science Program (RGY83/2012) to MDS and MEH, the offices of the Provost and the Dean of Arts and Sciences at Hunter College to MEH, the Air Force Office of Scientific Research (FA9550-13-1-0222) to MDS, the National Science Foundation to HUV (#0956306) and MH (#1247550), and by the Camille and Henry Dreyfus Foundation to MH. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.