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Science of the Total Environment 744 (2020) 140895 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv Revisiting an old question: Which predators eat eggs of ground-nesting birds in farmland landscapes? Carolina Bravo a,b,⁎, Olivier Pays b,c, Mathieu Sarasa d,e, Vincent Bretagnolle a,f a Centre d'Etudes Biologiques de Chizé, UMR 7372, CNRS and La Rochelle Université, F-79360 Beauvoir-sur- Niort, France b LETG-Angers, UMR 6554, CNRS, Université d'Angers, 49045 Angers, France c REHABS International Research Laboratory, CNRS-Université Lyon 1-Nelson Mandela University, George Campus, Madiba drive, 6531 George, South Africa d BEOPS, 1 Esplanade Compans Caffarelli, 31000 Toulouse, France e Fédération Nationale des Chasseurs, 92136 Issy-les-Moulineaux cedex, France f LTSER “Zone Atelier Plaine & Val de Sèvre”, CNRS, 79360 Villiers-en-Bois, France HIGHLIGHTS GRAPHICAL ABSTRACT • Predation probability in artificial nests decreased with camera trap presence. • Corvids might perceive differently plas- ticine and natural eggs. • Camera trap and plasticine eggs combi- nation are recommendable for identify- ing predator. • Corvid predation increased with the abundance of corvid breeders. • Considering social status of corvids is es- sential when assessing corvid abun- dance impact. article info abstract Article history: Nest predation is a major cause of reproductive failure in birds, but predator identity often remains unknown. Ad- Received 12 May 2020 ditionally, although corvids are considered major nest predators in farmland landscapes, whether breeders or Received in revised form 9 July 2020 floaters are involved remains contentious. In this study, we aimed to identify nest predators using artificial nests, Accepted 9 July 2020 and test whether territorial or non-breeders carrion crow (Corvus corone) and Eurasian magpie (Pica pica) were Available online 17 July 2020 most likely involved. We set up an experiment with artificial ground nests (n = 1429) in farmland landscapes of Editor: Damia Barcelo western France, and assessed how different combinations of egg size and egg material (small plasticine egg, large plasticine egg, quail and natural hen eggs) might influence predation rates and predator species involved. Nest pred- Keywords: ators were identified using remotely triggered cameras and marks left in plasticine eggs. Corvids were by far the Carrion crow predators most involved (almost 80% of all predation events), independent of egg type. Carrion crows alone were Magpie involved in 60% of cases. Probability of predation increased with egg size, and predation rate was higher for natural Floaters than for artificial eggs, suggesting that, in addition to egg size, predators might perceive plasticine and natural eggs Breeders differently. Predation rates of artificial nests by corvids were related significantly to corvid abundance, and far more Camera trap to breeder than floater abundances, for both carrion crows and magpies. This study emphasizes the importance of Plasticine eggs identifying predators at species level, and considering their social status when assessing corvid abundance impact on prey population dynamics. Combining camera traps and plasticine eggs can achieve this objective. Given the high predation rate by carrion crows, a better understanding of landscape-mediated changes in predator diet seems mandatory to design mitigation schemes able to confront ecological challenges raised by generalist predators. © 2020 Elsevier B.V. All rights reserved. ⁎ Corresponding author at: Centre d'Etudes Biologiques de Chizé, UMR 7372, CNRS and La Rochelle Université, F-79360 Beauvoir-sur-Niort, France. E-mail address: [email protected] (C. Bravo). https://doi.org/10.1016/j.scitotenv.2020.140895 0048-9697/© 2020 Elsevier B.V. All rights reserved. 2 C. Bravo et al. / Science of the Total Environment 744 (2020) 140895 1. Introduction When predator identity has been confirmed, corvids were often in- volved and they are regularly claimed to be key drivers of bird abun- Predation is a major selective pressure and driving force acting on dance (Fuller et al., 1995; Luginbuhl et al., 2001; Stoate and Szczur, population dynamics through individual demographic traits, such as 2001), especially in farmland landscapes (Andrén, 1992). While corvid survival (DeCesare et al., 2014) and fecundity (Zanette et al., 2011). and passerines densities or breeding success have been reported as neg- Predation effects have been demonstrated for a wide range of taxa, atively correlated (Dunn et al., 2010; Gabriel et al., 2010; Paradis et al., especially birds (Caro, 2005). Ground-nesting birds, a group that in- 2000), the generality of such patterns has been repeatedly questioned, cludes many farmland, steppe and prairie species, are known to be since corvid populations are not always associated with changes in highly impacted by predation through its effects on nesting success nest predation (Madden et al., 2015; Newson et al., 2010). Predator (Gibbons et al., 2007). Indeed, tree nesting has long been regarded breeding status might also influence prey selection (e.g. Sacks et al., as an anti-nest predation adaption in birds (Collias, 1997), since 1999). Indeed, breeding status in corvids shapes social structure, with ground-based nests, and the offspring therein, are more exposed to monogamous pairs defending breeding territories while non-breeding predators. individuals (floaters) are gregarious (Clayton and Emery, 2007). Despite Many studies have investigated predation rate on ground-nesting increased predation risk near corvid nests (Roos and Pärt, 2004), and birds using experimental dummy eggs or nests (Major and Kendal, the known effect of breeding status on predation rate (Bui et al., 2010; 1996). However, such set-ups have limitation as they may not assess Capstick et al., 2019), whether predation by corvids is mainly attribut- natural predation rates accurately, and predator-specific patterns are able to breeding individuals remains uncertain. difficult to analyse (Moore and Robinson, 2004). For instance, artificial In the current study, we experimentally quantify ground nest preda- nests might be more attractive to predators than those in natural situa- tion in farmland landscapes, asking whether corvid species, especially tions. Moreover, the lack of parental activity at artificial nests could ei- the carrion crow (Corvus corone) and Eurasian magpie (Pica pica), ther decrease predation rate by removing cues for predators, or were the main predators involved. Using a large data set of 1429 artifi- increase it by parent-mediated removing egg concealment (Major and cial nests associated in part with camera traps, we established predator Kendal, 1996; Moore and Robinson, 2004). Despite these limitations, ar- identities and assessed their relative contribution to observed predation tificial nests have been extensively used to explore patterns of nest pre- rates. More specifically, we: (1) compared predation rates between ar- dation, mainly because of their non-intrusiveness and the ease with tificial nests with plasticine or natural eggs of different sizes, (2) com- which a large number of replicates can be deployed, allowing investiga- pared predation rates between artificial eggs with, or without, camera tion of the impacts of various environmental factors on nest predation. traps; (3) assessed the accuracy of predator identification from plasti- Several parameters have been shown to affect observed predation cine eggs versus camera traps; and (4) quantified relationships between rates. Egg size matters, with large eggs generally preventing predation corvid abundance according to their social status (breeders vs. floaters) (Latorre et al., 2013), though not always (Vazquez et al., 2018). Egg ma- and the likelihood of predation. terial, in particular whether eggs are natural or not (i.e. plasticine or clay eggs), also affect predation rates (Major and Kendal, 1996). The smell of 2. Methods plasticine eggs may attract mammalian predators, and enhance preda- tion rates compared to natural eggs (Rangen et al., 2000), whereas the 2.1. Study area eggshell of quail or hen eggs may not be crushed by small mammals (Bayne et al., 1997). Thus, it is crucial to recognize that artificial nest The study was carried out in 2018 and 2019, in the LTSER “Zone Ate- set-ups aim to assess relative rather than actual predation rates lier Plaine & Val de Sèvre” (46°15N, 0°30W), a site which covers (Moore and Robinson, 2004). 450 km2 of intensive agriculture in Western France (Fig. 1a), mostly If predation on ground-nesting birds has long been studied, predator dedicated to cereal production (see Bretagnolle et al., 2018a for general identification remains a major challenge (Benson et al., 2010). Several site description). The study area is mostly open, flat (altitude is 40 to methods have been used to identify predators, including nest cues, 100 m asl) with a temperate Atlantic oceanic climate, sparsely popu- hair catchers, tracking boards and bait marker chemicals (Jones, 2007; lated (62 ind/km2) and managed almost exclusively for arable and Staller et al., 2005). However, all have their pitfalls (Major, 1991; mixed farming. The most common crops are wheat (33.8%), meadows Williams and Wood, 2002). Marks left in plasticine eggs is one of the (13.5%, including both permanent grasslands and temporary hay, such most powerful method to identify predator species (Bayne et al., as alfalfa), sunflower (10.4%),