Islands off southern Australia once harboured three subspecies of the mainland emu (Dromaius novaehollandiae), the smaller Tasmanian emu (D. n. diemenensis), and two dwarf emus, King Island emu (D. n. minor) and Kangaroo Island emu (D. n. baudinianus), which all became extinct rapidly after discovery by human settlers. Little was recorded about their life-histories and only a few historical museum specimens exist, including a number of complete eggs from Tasmania and a unique egg from Kangaroo Island. Here we present a detailed analysis of dwarf emu eggs, including a first record of an almost complete specimen from King Island. Our results show that despite the reduction in size of all island emus, especially the King Island emu that averaged 46% smaller than mainland birds, the egg remained virtually the same size and had slightly thinner eggshell, and we provide possible reasons why these phenomena occurred.

For intact eggs, all measurements were taken using dial calipers and rounded to the nearest 0.1 mm, with a micrometer rounded to the nearest 0.1 mm for eggshell thickness. Total length of complete eggs was taken along the long axis, and greatest width along the short axis at the widest medial point. Circulation of egg volume from museum eggs, including extinct taxa, used methodology described in [17]. Eggshell thickness was measured in the blowhole region of intact eggs, as described in [18], and recent museum and subfossil eggshell fragments were measured away from eroded or broken edges.

Table S1. Details of the museum and subfossil intact eggs used in this study, with location data and source information.

Table S2. Details of the museum and subfossil eggshell fragments used in this study, with location data, measurement, age, source and registration information.

Table S3. T-test for significance difference between mainland Dromaius n. novaehollandiae versus Tasmanian D. n. diemenensis for intact emu egg length, using two-tailed distribution and two-sampled unequal variance. P value = 0.05.

Table S4. T-test for significance difference between mainland Dromaius n. novaehollandiae versus Tasmanian D. n. diemenensis for intact emu egg width, using two-tailed distribution and two-sampled unequal variance. P value = 0.05.

Table S5. T-test for significance difference between mainland Dromaius n. novaehollandiae versus Tasmanian D. n. diemenensis for intact emu egg volume, using two-tailed distribution and two-sampled unequal variance. P value = 0.05.

Table S6. T-test for significance difference between mainland Dromaius n. novaehollandiae versus Kangaroo Island D. n. baudinianus and King Island D. n. minor eggshell thickness, using two-tailed distribution and two-sampled unequal variance. P-value = 0.05.