Diet of Baillon's CrakesZapornia pusilla: Assessing Differences in Prey Availability and Consumption during the Breeding Season in the Senegal River Delta, West Africa

Abstract

The Baillon's Crake Zapornia (Porzana) pusilla is considered as one of the least known Rallidae species of the Palaearctic. Very little information exists about its ecological requirements and knowledge on diet refers to very few observations. Based on the analysis of faecal samples (N = 59) from two study sites in Djoudj National Park (NW Senegal), we describe the major diet components and examine how seasonal and environmental factors influence its dietary composition. All faeces contained remains of invertebrates. Coleopterans were the most frequent prey items with an occurrence in 95% of the samples. Other important food items were: Odonata (82%), Aranaea (78%), Nematocera (59%), and Brachycera (44%). Remains of gecko skin were the only evidence for vertebrate prey. 75% of the faeces contained plant matter, especially seeds of Eleocharis mutata which constituted in some individuals > 90% of the sample content. Generalized linear models (GLMs) were used to assess whether occurrence of prey items was an effect of selection or environmental variation, considering both consumed items as well as prey availability. Sweep netting was used to provide an estimate of relative abundance of potential invertebrate prey.Despite pronounced seasonal changes in temperature and humidity, models revealed a lower influence of meteorological variables on prey composition and availabilty. Rather we found water level, date and site having the highest impact in the models. In contrast to decreasing diversity of available food items in the course of the season, diversity in the faeces remained constant indicating Baillon's Crakes prey less selectively when resources diminish. Furthermore, diversity of Baillon's Crakes' diet was lower at higher water levels, suggesting stronger selectivity when prey abundances are high as implied by positive relationships of several invertebrate groups with water level. Despite the rapidly declining water levels and decreasing abundances of e.g. Nematocera, Odonata and Mollusca in the course of the season, we found no clear shift from aquatic to a more terrestrial dominated composition of taxa in the birds' diet. This might be due to the selection of profitable prey such as araneans or molluscs but could also be explained by better accessibility due to physical changes in the habitats e.g in the case of consumed Odonata and Saltatoria. High disintegration of invertebrates in the faeces rendered quantification of prey items impossible. Biomass estimates could support the assessment of specialization of the Baillon's Crakes as well as the detection of seasonal succession of the wetlands' biotic communities.