Fig. 1

Experimental design. In the conspecific cross-fostering experiments with A zebra finches (ZF) and C Bengalese finches (BF), the second and third eggs were swapped between conspecific nests. C In the heterospecific cross-fostering experiment, the second and third eggs from ZF nests were transferred into BF nests after removing the second and third BF eggs. We assessed the microbial similarity between (i) age groups, (ii) different sample types and (iii) genetic vs. foster relatives. D Sampling scheme over bird ontogeny: 5, 10, 35 and 100 days post-hatch (dph). This figure was created by Omar Castillo-Gutiérrez using bird illustrations designed by Sonja Engel, the copyright holder