Tall trees drive the nest-site selection of wild Crested Ibis Nipponia nippon

Understanding how birds select breeding sites plays an important role in habitat protection, especially for the conservation of endangered species. With the increase in population size of the endangered Crested Ibis Nipponia nippon, its distribution range has expanded from mountain areas to plains located outside protected areas, representing a new challenge for conservation of the species. Identifying the current nesting habitat requirements is thus needed and can provide valuable information for the planning of new nature reserves. In this research, we surveyed a total of 117 nests across the whole distribution range from 2015 to 2019. We used generalized linear mixed-effects models to assess nesting preferences of the Crested Ibis in the wild. Results showed that in mountain areas, Masson pines Pinus massoniana were preferred (64.6%), whereas elms Ulmus pumila (44.9%) and aspens Populus davidiana (40.6%) were used more frequently lower down, probably because of their higher availability. In both mountain areas and plains the ibises selected tall nesting trees with larger diameter at breast height and preferred nesting rather high above ground, especially in plains where taller trees provided higher suitable nesting positions. The ibises also preferred nesting close to tree trunks, especially in mountain areas, probably for more safety from collapsing. Furthermore, in mountain areas, slope and distance to path had positive effects on nesting occurrence, and understorey coverage was avoided by nesting ibises, while these variables had little impact in plains. Our results indicate that, despite their range expansion, Crested Ibises rely on very specific habitat characteristics for nesting. We suggest relatively tall trees like elms and aspens should be preserved in plains. In addition, we highlight how selection patterns of Crested Ibises may vary, and that such variation should be addressed in conservation planning, especially in future reintroduction.