Examinations were carried out in a breeding flock of 5-year-old rheas (Rhea americana). A total of 62 eggs, coming from around peak laying period, were subject to incubation. During hatching, there were instances of giving assistance to chicks. Based on the performed analysis of post-hatching refuse, three groups were separated: I – egg shells of chicks hatched unassisted, II – egg shells of chicks hatched with personnel’s assistance, III – embryos died during embryogenesis. The results were subject to one-way analysis of variance using applying a Duncan’s test. It was found that rhea chicks frequently required assistance during hatching, and their relative weight (nearly 60%) on the day of hatch reached a low level. The analysis of egg shell ultrastructure showed that largest mammillae occurred in the egg shells after chicks hatched without assistance. On the other hand, significantly smaller mammillae were observed in the egg shell groups where embryos died during embryogenesis or chicks needed assistance during hatch. Moreover, it was found that the largest number of mammillae per area unit is characteristic of the egg shells after chicks hatched without assistance. Therefore, one may think that the structure of this layer, in particular the density of mammillae, is related to hatching performance results.

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