Colony-site tenacity commonly reflects the stability of nesting habitats. The match between nesting substrate and coloration pattern of the eggs in two reproductive groups of the Gull-billed Tern (Gelochelidon nilotica) in the south of Iberian Peninsula was investigated to tests the adaptiveness of colony-site tenacity. The terns showing high colony-site tenacity had 1) a closer match in color between the eggs and the nesting substrate and 2) a lower richness and diversity of egg colors and 3) a lower rate of egg loss from predation than the low colony-site tenacity ones. These results appear to be adaptive, resulting from a long-standing renesting in the same colony-site. However, such mimicry would be difficult to achieve for terns with low site-tenacity, where breeding occur in unstable habitats. These birds were frequently obliged to move from one location to another due to weather irregularity and water level in the reservoirs they bred. In addition, the occasional high predation pressures on both adults and chicks, manifested by the low-tenacity tern group studied, was also involved. The attainment of such egg-crypsis could be an important adaptive advantage favoring colony-site tenacity. In unstable habitats, however, there would be a trade-off between the benefits linked to colony-site tenacity and the negative effects of suboptimal reproduction. Additional experimental design and studies are, however, necessary to confirm these results and predictions.