Date Published: January 30, 2018
Publisher: Public Library of Science
Author(s): Ricardo Camargos de Meireles, João Paulo Gusmão Teixeira, Ricardo Solar, Bruno Nery F. Vasconcelos, Raphael B. A. Fernandes, Leonardo Esteves Lopes, Petr Heneberg.
Substrate type is a key-factor in nest-site selection and nest architecture of burrowing birds. However, little is known about which factors drive nest-site selection for these species, especially in the tropics. We studied the influence of soil attributes on nest-site selection by the campo miner Geositta poeciloptera, an open grassland bird that builds its nests within soil cavities. For all nests found, we measured the depth of the nest cavity and the resistance of the soil to penetration, and identified the soil horizon in which the nest was located. In soil banks with nests, we collected soil samples for granulometric analysis around each nest cavity, while in soil banks without nests we collected these samples at random points. From 43 nests found, 86% were located in the deeper soil horizons (C-horizon), and only 14% in the shallower horizons (B-horizon). Granulometric analysis showed that the C-horizons possessed a high similar granulometric composition, with high silt and low clay contents. These characteristics are associated with a low degree of structural development of the soil, which makes it easier to excavate. Contrarily, soil resistance to penetration does not seem to be an important criterion for nest site selection, although nests in more resistant the soils tend to have shallower nest cavities. Among the soil banks analyzed, 40% of those without cavities possessed a larger proportion of B-horizon relative to the C-horizon, and their texture was more clayey. On the other hand, almost all soil banks containing nest cavities had a larger C-horizon and a silty texture, indicating that soil attributes drive nest-site selection by G. poeciloptera. Thus, we conclude that the patchy distribution of G. poeciloptera can attributed to the infrequent natural exposure of the C-horizon in the tropical region, where well developed, deep and permeable soils are more common.
Birds select nest-sites by assessing many biotic and abiotic environmental factors (e.g., [1–5]). Among the abiotic factors, the type of substrate chosen for nest construction is particularly important for cavity-nesting birds [6, 7, 8]. However, the attributes of the substrate that drive nest-site selection are still poorly studied for excavating birds, even though the first observations on the subject were published decades ago [9–11]. Although some authors have addressed this topic in recent years, these studies were mostly conducted in temperate regions (e.g., [6, 7, 12, 13, 14, 15, 16]), and seldom in the tropics (e.g., ).
We found 45 active nests (25 nests in 2015 and 20 nests in 2016), but only 43 nests were used in this study since two of them were built within cavities supposedly excavated by the campo flicker Colaptes campestris (Picidae). Most of the nests were found in the C-horizon (72%, n = 31), followed by the B-horizon (14%, n = 6), the BC-horizon (11.6%, n = 5), and the Cr-horizon (3%, n = 1). The granulometric analyses showed that horizons BC and Cr, which are transitional horizons, had great similarities with the C-horizon, all possessing high silt and low clay contents. In contrast, the B-horizon had high clay and low silt contents (Fig 4).
Soil resistance to penetration significantly affected nest cavity architecture of G. poeciloptera, which builds shallower nests at sites with higher soil resistance to penetration. However, this soil attribute does not seem to interfere in the selection of nesting sites, since soil resistance to penetration was found to be similar between steep soil banks with and without nest cavities. Contrarily, soil resistance to penetration is a major factor in nest-site selection for the passerine bank swallow Riparia riparia and non passerines blue-tailed bee-eater Merops philippinus, the european bee-eater M. apiaster and the eurasian kingfisher Alcedo atthis, all of which select sites with greater or lesser soil resistance, depending on the species requirements and the conflicting requirements of tunnel stability and ease to excavation [13, 16].
Soils of the study area have low permeability and show narrow superficial horizons. These soils are subjected to naturally occurring erosive process that produces a landscape full of deep ravines and gullies where the subsurface C-horizon is frequently exposed. Once exposed, the C-horizon becomes the preferred site for nesting by G. poeciloptera due to the absence of structure and the low performance of cohesive forces between solid particles. This finding is important for understanding the distribution and abundance of G. poeciloptera, which, although globally rare, is surprisingly common in some isolated sites. We propose the hypothesis that this patchy distribution of G. poeciloptera can be explained by the fact that the natural exposure of the C-horizon is infrequent in the tropical region, where well developed, deep and permeable soils are common [35, 36]. Therefore, soil attributes must be considered when investigating the driving forces that shape the distribution and abundance of tropical birds that nest in soil cavities.