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

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.

Benefits and limitations of three-dimensional printing technology for ecological research

“…Using photogrammetry, photos can be digitized and 2D x,y coordinates from the photo converted into a 3D image [27, 42]. Photogrammetry may be the easiest and most cost effective method, especially if a scanner is not available.…”

“…In addition, photogrammetry can be used to augment an image produced by 3D scanning: in the creation of 3D printed northern map turtle decoys, the carapace and legs of a dried specimen were scanned and the head was digitally rendered using photographs [32]. Alternatively, mathematical formulae may be used to generate different shapes, such as the surface of a bird egg [27] or the curvature of a flower corolla [30]. Finally, it is possible to draft the object completely from scratch (e.g., [38]), although a higher proficiency on the appropriate drafting software is necessary (Additional file 2).…”

“…For testing hypotheses in both lab and field conditions, 3D printing may be incredibly useful for making precise, repeatable models. Three-dimensional printing has already been used to create precise models of bird eggs to test egg rejection behavior in the context of brood parasitism [27], zebrafish shoals to test the effect of body size on zebrafish shoaling preferences [28], artificial flower corollas to test the effect of floral traits on pollinator visitation [29–31], and female turtle decoys to test the effect of body size on mate choice [32] (Table 1). In these studies, 3D printing was chosen for its ability to create identical experimental stimuli because alternative methods, such as constructing models by hand, could introduce unintentional variation that makes it difficult to determine whether study subjects are responding to intentional or unintentional variation in experimental stimuli.…”

“…In addition, 3D printing is often a faster method for creating models than making them by alternative methods [13]. There may be scenarios where 3D printing will not produce more biologically accurate models than other methods, but in many cases, 3D printing will increase the types of behavioral questions that can be asked [27]. For example, northern map turtles ( Graptemys geographica ) are sensitive to captivity, and using 3D printed decoys of females permitted field studies of male mating behavior whereas using live females for the same study would have been detrimental to their survival [32].…”

“…The use of 3D technology facilitates these broad-scale studies through the sharing of identical tools, models, and/or equipment that can be used in multiple systems. For example, 3D printed models of brown-headed cowbird ( Molothrus ater ) eggs [27] and Texas horned lizards ( Phrynosoma cornutum ) [13] can be used to test patterns of brood parasitism and thermal tolerances, respectively, across their geographic ranges. Similarly, for widespread invasive species like the emerald ash borer, sharing effective trap methodology [12] among scientists and agencies can potentially accelerate the rate at which the impact of the species is mitigated.…”