INTRODUCTION
The interactions between birds and insects have been the subject of extensive research (Morse, 1971; Willis & Oniki, 1978). Bird-insect interactions in the form of predation, parasitism, and mutualism have received the most attention because of their ubiquitous occurrence and importance as study systems for understanding coevolution (Holmes et al., 1979; Maziarz et al., 2021; Price, 1997; Quinn & Ueta, 2008). Other less conventional or more cryptic interaction types also occur along the antagonism-commensalism gradient, such as competition (e.g. for food resources; Smith & Balda, 1979) or ‘anting’ (i.e. the use of ants as a treatment against parasites; Camacho & Potti, 2018), but they often go undetected or underreported and their true nature and biological significance remain to be elucidated in most cases.
Of all insects, ants (Formicidae) are the most abundant in most terrestrial ecosystems (Del Toro et al., 2012). There is ample literature documenting antagonistic ant-bird interactions, primarily in the form of bird predation on ants (reviewed in Avilés, 2023). There are also studies reporting negative effects of ants on aspects such as bird abundance, behaviour, and reproduction, primarily through predation on eggs and chicks (Alvarez-Blanco et al., 2020; Davis et al., 2008; Haemig, 1999). However, assessing the impact of ants on more mobile segments of a population (e.g. juveniles and adults) is challenging and our understanding of the nature and magnitude of these effects is therefore limited.
This study takes advantage of a long-term capture-recapture program to examine the nature and frequency of injuries (toe amputations) of a migratory insectivorous bird –the red-necked nightjar (Caprimulgus ruficollis )– in their breeding area in S Spain. The study was motivated by observations of nightjars missing toes and particularly of an adult bird recorded in 2015 that had what appeared to be the mandible of an ant embedded in the medial toe of one of their feet. Six years later, in 2021, we found another adult that had the full head of an ant embedded in a central toe. Since then, we engaged in a systematic revision of the feet of all captured nightjars to evaluate the frequency and severity of toe injuries. Observations of insect remains attached to injured toes in three nightjars provided an opportunity to identify the agents of damage based on a taxonomic analysis. We close by discussing the incidence of such injuries on nightjar populations and identify further research needs.