Northeastern Naturalist Vol. 26, No. 3 G.J. Riggs, et al. 2019 609 2019 NORTHEASTERN NATURALIST 26(3):609–615 Eviction Notice: Observation of a Sterna hirundo (Common Tern) Usurping an Active Sternula antillarum (Least Tern) Nest Georgia J. Riggs1, Jeffery D. Sullivan2, Kayla M. Harvey3, Dimitri A. Pappas3, Jennifer L. Wall4, Peter C. McGowan5, Carl R. Callahan5, Craig A. Koppie5, and Diann J. Prosser6,* Abstract - Although nest usurpation is common in some species and orders of birds, usurpation has rarely been reported for Sterninae. We observed a Sterna hirundo (Common Tern) egg in an active Sternula antillarum (Least Tern) nest with a complete clutch in a mixedspecies Sterninae colony in Chesapeake Bay, MD, in May 2018. Based on observations from a game camera following usurpation, Common Terns incubated the mixed-species clutch, with no further parental care provided by the usurped Least Tern. The clutch never hatched, as the Common Terns abandoned the nest prior to the hatching. While we suspect that Common Terns usurped the Least Tern nest, alternative scenarios may explain how the Common Tern egg was documented in a Least Tern nest. Introduction Nest usurpation, or nest piracy, occurs when a bird takes over the nest of another individual (Lindell 1996, Shy 1982). However, this terminology encompasses a relatively wide variety of behaviors. Favaloro (1942) described 3 classes of nest usurpation: (1) the non-aggressive occupation of an abandoned nest, (2) the aggressive takeover of an occupied nest in which the overtaking pair destroys previous eggs or chicks, and (3) the aggressive or non-aggressive takeover of a nest with the overtaking pair continuing to care for the original nester’s eggs or chicks. Unlike instances of brood parasitism, when a female deposits her eggs(s) in another individual’s nest and provides no parental support, usurping pairs do not rely on the original nesters for parental support of their offspring (Lindell 1996). Still, the loss of a nest can significantly impact the original nesting pair, and the avoidance of this risk is believed to be a major driving factor for nest-site niche partitioning (Doherty and Grubb 2002, Lindell 1996). Studies have documented nest usurpation across several avian orders including Passeriformes (Haslam et al. 2016, Luchesi and Astie 2017) and Piciformes (Kronland 2007), with frequent documentation in Anseriformes (Gong et al. 2018, 1College of Agricultural and Environmental Sciences, University of California Davis, Davis, CA 95616. 2Natural Systems Analysts, Winter Park, FL 32789. 3College of Agriculture and Natural Resources, University of Maryland, College Park, MD 20742. 4Chesapeake Conservation Corps, Chesapeake Bay Trust, Annapolis, MD 21401. 5US Fish and Wildlife Service, Chesapeake Bay Field Office, Annapolis, MD 21401. 6US Geological Survey, Patuxent Wildlife Research Center, Laurel, MD 20708. *Corresponding author - dprosser@usgs.gov. Manuscript Editor: Peter Paton Northeastern Naturalist 610 G.J. Riggs, et al. 2019 Vol. 26, No. 3 Pratte et al. 2016). While such behavior has been reported for members of Charadriiformes (Dinan et al. 2018, Erwin 1980, Horrocks 2016), there have been comparatively few reports of such behavior in Sterninae. In most instances where nest usurpation behavior has been observed in Sterninae, it was either class 1 (Lake 2004) or the classification could not be confidently determined (Paz and Eshbol 2002, Spendelow et al. 2001). When aggressive nest usurpation has been reported, researchers have assumed it to be a rare or even one-time occurrence (Midura and Beyer 1991). Considering the frequency with which Sterninae live in crowded mixed-species colonies where nesting locations can be at a premium (Nisbet et al. 2017) and their relative willingness to engage in class 1 nest usurpation (Arnold et al. 1998, Quintana and Yorio 1997), we assume that aggressive nest usurpations would be common, but that does not appear to be the case. Here we present an observation of nest usurpation by a Sterna hirundo L. (Common Tern) and discuss potential causes. Field Site Description We conducted fieldwork at Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (38°46'01''N, 76°22'54''W), in Chesapeake Bay, MD. The restoration project uses clean, dredged material from the approach shipping channels leading to the port of Baltimore, MD, to rebuild and restore remote island habitat (Erwin et al. 2007). Approximately 206 pairs of Common Terns nested in the northwest corner of the island along a sandy dike bordering a shallow 36-ha non-tidal impoundment that receives dredge material annually. As of 30 May 2018, the main portion of this colony was ~0.72 ha and had a nest density of ~265 nest/ha (~191 pairs). The south end of this colony abutted a Sternula antillarum L. (Least Tern) colony, which had ~25 pairs encompassing ~0.23 ha, with a density of 130 nests/ha. Although the species’ nesting areas were generally distinct, there was a clearly visible transition zone in which Common and Least Tern nests were intermixed (~ 0.19 ha), but at a lower density than the main portions of either single species colony (Common Terns: 79 nests/ha or ~15 pairs; Least Terns: 26 nests/ha or ~5 pairs; Cumulative: 105 nests/ha). Vegetative cover along the edge of these colonies included Panicum amarum Elliot (Bitter Panicum), P. virgatum L. (Switchgrass), Melilotus alba L. (White Clover), and Setaria spp. (foxtail grasses). Methods We performed colony surveys 1–3 times a week during the period 8 May–6 August 2018 (to ensure coverage of the entire breeding season) as part of an associated project that monitors the breeding success of waterbirds on Poplar Island. Colony surveys consisted of (1) surveying the colony with a spotting scope to identify the approximate number of individuals, and (2) ground surveys (Steinkamp et al. 2003), during which multiple biologists spread out across the width of the colony and walked through it to count nests, check egg status, and capture chicks for banding. We individually marked each nest with a unique Northeastern Naturalist Vol. 26, No. 3 G.J. Riggs, et al. 2019 611 identifier written on a small wooden stake placed into the ground a few inches from the nest (Fig. 1). On 5 June 2018, 5 d after the discovery of the nest usurpation reported in this study, we placed 2 Bushnell Model 119726 game cameras ~3 m away from the nest to investigate any potential interactions between species (placed 5 June 2018). This delay in placement was due to logistical constraints in obtaining cameras. We programmed 1 camera to record 10-sec video bursts and the other to capture still images. To ensure that memory cards would not run out of storage space too quickly, we set the time lag for a minimum of 1 min between recordings. Although cameras were “armed” 24 h a day, they only recorded data when motion was detected. The camera capturing still images functioned properly throughout its deployment, but the camera set to capture video had several battery malfunctions that caused periods of missing data. Due to the time lag between images and the potential for cameras to not detect all movements, our observations reflected an incomplete record of behavior at the nest and should be interpreted Figure 1. A Sternula antillarum (Least Tern) nest after usurpation by Sterna hirundo (Common Tern). Northeastern Naturalist 612 G.J. Riggs, et al. 2019 Vol. 26, No. 3 as such. After we removed cameras on 26 June 2018, we reviewed all images and videos to assess the total number of individuals, by species, incubating the nest or at the nest site (within ~0.3 m). No Common Terns or Least terns were banded at this nest, so we could not recognize individuals. Results On 25 May 2018, we documented 1 Least Tern egg in a scrape in the mixedspecies zone between the Least Tern and Common Tern colonies. During the next survey on 30 May 2018, there were 2 Least Tern eggs and 1 Common Tern egg within the nest scrape. Although we visually observed (via spotting scope) a Least Tern incubating eggs after the survey when the usurpation was first observed (30 May 2018), we manually observed no adults incubating at this location prior to subsequent nest surveys. Starting 5 June 2018 when we placed cameras near the nest, we documented only Common Terns incubating the eggs; up to 2 Common Terns attended the nest. We observed 1 unmarked Least Tern at the nest site repeatedly until 19 June 2018, but a Least Tern was never documented incubating eggs or at the nest concurrent with a Common Tern after 5 June. During the colony survey on 20 June 2018, we noted that the clutch was partially buried by windblown sand and continued to appear unattended during the next survey on 26 June 2018, leading us to declare the nest abandoned. A review of camera footage indicated that a Common Tern last incubated the nest on 12 June 2018, and then sporadically visited the nest site until 25 June 2018. Discussion The usurpation of a Least Tern nest by a Common Tern(s) reported in this study adds to the limited body of literature on such occurrences within terns (Sterninae). Our observation is similar to Dinan et al. (2018), as we observed only 1 instance despite monitoring of several hundred pairs. However, we believe nest usurpation could be under-reported due to the cryptic nature of this behavior. For instance, nest usurpation would be extremely difficult to identify when eggs of different species are similar or when intraspecific usurpation occurs. Although we cannot conclusively demonstrate that the usurpation event we observed was an aggressive takeover of an active nest, we believe that this is a reasonable assumption because (1) we observed a Least Tern incubating the nest after the Common Tern egg had been deposited, and (2) at least 1 Least Tern visited the nest regularly after usurpation. While we observed no antagonistic interactions between Common and Least Terns, this behavior may have occurred prior to game-camera placement. Additionally, if the Least Tern nest was truly only being tended by a single individual it may have facilitated this usurpation. Although the cause of aggressive nest usurpation is not known, some biologists have suggested that limited nesting locations within a colony could be a potential factor (Lake 2004, Midura 1991, Paz and Eshbol 2002). However, this observation took place in an area with relatively low nest density, suggesting such behavior Northeastern Naturalist Vol. 26, No. 3 G.J. Riggs, et al. 2019 613 would not be necessary. Furthermore, though taking over a nest at a “better” location within the colony may improve fitness by reducing the risk of depredation or by providing offspring with access to better microclimates (Lindell 1996), there were no obvious differences between the location of the usurped nest and the surrounding habitat. These factors do not necessarily preclude competition for optimal nest sites as motivation for this behavior, but they do make this explanation appear less likely. One particularly interesting aspect of this observation is that we found only 1 Common Tern egg in the usurped nest. This finding is unusual because Common Terns typically deposited 3-egg clutches at this study area in 2018, and Least Terns average a 2-egg clutch at this location. One possible explanation is that following usurpation of the nest the female recognized the other eggs and adjusted her own reproduction assuming the eggs were her own (Heaney and Monaghan 1995). Alternatively, the Common Tern(s)’ previous nest may have had 2 deposited eggs but was depredated or destroyed, forcing the female to relocate prior to depositing a third egg that was already in development (Arnold et al. 1998). Finally, the usurping female could have deposited additional eggs in another location and was attempting to care for 2 nests, or the single egg had become displaced from a nearby Common Tern nest and only coincidentally entered the Least Tern nest. However, this latter possibility seems unlikely as the nearest known Common Tern nest was located ~10.5 m downhill of the usurped nest, with no known Common Tern nests located uphill. Although we are unable to conclude the motivations behind this usurpation, future work within mixed species colonies could consider the use of fixed monitoring techniques (Wall et al. 2018) to help elucidate this and other cryptic or underreported behaviors. Acknowledgments All data reported in this manuscript were collected in accordance with protocol approved by the Patuxent Wildlife Research Center Animal Care and Use Committee. This work was supported by the US Army Corps of Engineers (Baltimore District), US Geological Survey (Patuxent Wildlife Research Center), the US Fish and Wildlife Service (Chesapeake Bay Field Office), and the Maryland Environmental Service. We would like to thank our internal reviewer, Ian Dwight, and 2 anonymous peer reviewers for their thoughtful revisions during the publication process. The use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government. All authors provided significant contributions to the preparation of this manuscript. Literature Cited Arnold, J.M., I.C. Nisbet, and J.J. Hatch. 1998. Are Common Terns really indeterminate layers? Responses to experimental egg removal. Colonial Waterbirds 21:81–86. Dinan, L.R., A. Halpin, A. 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