Usurpation and Brooding of Least Tern (Sternula antillarum) Chicks by Common Terns (Sterna hirundo)

Abstract

While nest usurpation and subsequent incubation of eggs and even brooding of chicks from other species has been reported for Common Terns (Sterna hirundo), such behavior is considered rare. We report an observation of a Common Tern pair usurping the Least Tern (Sternula antillarum) nest and brooding the Least Tern chicks. While the Least Tern pair attempted to provide care for the chicks, the Common Terns displayed aggressive behavior and defended the nest. Though both species attempted to feed the chicks, no feeding events were observed due to harassment from the other species. Neither pair was observed nesting prior to or following this event, and all chicks are believed to have been lost to predation. We discuss the possible scenarios leading to the observed usurpation event, the possibility that usurpations are more common than previously believed, and the need for different monitoring methods to elucidate the causes of usurpations.

1. Introduction

While lifetime reproductive success is clearly advantaged by avian breeding pairs raising their own chicks, it is well established in the literature that numerous Sterninae species will willingly incubate the eggs of conspecifics [1,2,3]. Furthermore, species such as Common (Sterna hirundo) and Little Terns (Sternula albifrons) have been found to regularly adopt young conspecific chicks from other pairs into their own broods [4,5,6], though interactions between adults and young from other broods can also be aggressive [5,7]. Both caring for eggs andyoung of conspecific pairs are believed to be driven by an inability to differentiate between their own eggs/chicks and those around them [8].

Interestingly, it appears as though some Sterninae species may also be willing to incubate eggs and raise chicks from other species, not just conspecifics. An example of this was seen when a pair of Caspian Terns (Hydroprogne caspia) adopted two Ring-billed Gull (Larus delawarensis) chicks, one of which they successfully fledged [9]. Similarly, there is a single report of a Least Tern (Sternula antillarum) pair usurping a Piping Plover (Charadrius melodus) nest and successfully hatching chicks from both species [10], as well as of Roseate Terns (Sterna dougallii) fledging a Sandwich Tern (Thalasseus sandvicensis) which they incubated and brooded [11]. However, the largest number of such cases has been reported in Common Terns. Common Terns have been reported as forcibly taking over still-empty nests (eggs had not yet been laid) of several other waterbird species [12], as well as usurping nests already containing eggs from species such as Least Terns [13], and Spotted Sandpiper (Actitis macularius) [14] prior to laying their own eggs and incubating the mixed clutch. Common Terns have even been observed brooding Black-winged Stilt chicks (Himantopus himantopus) [12]. Despite the examples in the literature of tern species usurping nests and caring for the eggs and chicks of other species, such behavior is generally believed to be rare [10]. Thus, it is important to document instances of such behavior to improve our understanding of the frequency and scope of this apparently maladaptive behavior.

Here, we present an observation of a pair of Common Terns usurping a Least Tern nest, brooding the young chicks, and incubating the remaining egg. We present the history of the usurped nest and details on the age of the usurping Common Terns to provide limited insight into possible driving factors behind this incident.

2. Materials and Methods

2.1. Study Area

This observation took place on the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (hereafter. Poplar Island; 38°46′01″ N, 76°22′54″ W), a restoration site located in the Maryland (MD), USA portion of the Chesapeake Bay that uses dredged material from the approach shipping channels leading to the port of Baltimore, MD, to rebuild and restore remote island habitat [15]. There are numerous sub-colonies of both Common and Least Terns across Poplar Island, but this observation occurred in what is referred to as Cell 7, a development unit located along the island’s northwestern side. As of 7 July 2023, when this observation occurred (See Results), there were 70 and 33 active Common and Least Tern nests, respectively, in this sub-colony. Common Terns were located predominately on the western side of a sandy shelf, and Least Terns were located in the middle of this same shelf (Figure 1).

2.2. Surveys

This observation was made during routine colony monitoring as part of a larger study focusing on the reproductive success of terns on Poplar Island. Colony surveys were performed one to three times per week from 16 May until 11 August 2023. Upon arriving at a sub-colony, a single researcher conducted a visual survey with a spotting scope to count the number of visible adults and chicks. Following the completion of the scope count, researchers walked through the sub-colony in a line-abreast formation, identifying and marking new nests with wooden markers, recording the number of eggs and their condition by the nest, and capturing juveniles for banding [16,17]. During these surveys, special care is taken to identify any nests that appear to have been abandoned or are otherwise non-viable. All detected juveniles old enough for amniotic fluid to have dried post-hatch were aged to the nearest day [18] and fitted with stainless steel or Incoloy U.S. Geological Survey Bird Banding Lab (BBL) and plastic field readable (PFR) bands. When we encountered previously banded juveniles during colony surveys, they were recaptured by hand to confirm identity and released.

In an effort to identify the parents at every Common Tern nest, a single researcher conducted resighting surveys sporadically in all Common Tern colonies on Poplar Island. Surveys occurred from 19 May through 14 July 2023 and were variable in length (30–325 min), time of day (morning or afternoon), and the number of surveys per sub-colony as dictated by the presence of observable individuals, sub-colony size, and current weather conditions. Despite the variability in survey location and duration, all resighting surveys were conducted in the same manner, with the observer moving slowly through the sub-colony and observing and noting PFR and BBL bands (or lack thereof) at each nest via a spotting scope. Observations were made from a seated position to limit disturbance to breeding pairs, and the observer withdrew from the sub-colony if pairs were showing signs of heat stress or an unwillingness to attend the nest in their presence. Finally, to inform fledging success estimates for juveniles and determine adult return rates, we conducted resighting surveys for adult and juvenile terns one to four times per week using spotting scopes within the colonies and at nearby staging areas on Poplar Island.

During one of our colony surveys (pair count prior to entering the sub-colony), it was observed that a single Least Tern nest containing eggs was usurped by a Common Tern pair (see results). Following the identification of the usurped nest, a single observer monitored the nest via a spotting scope from 10:17 until 10:31, when a game camera was erected ~2.5 m from the nest. The camera was programmed to record 10 s videos every 30 s, but a malfunction resulted in the capture of 30 s videos with a 10 s lag between videos. The observer first sought to identify the PFR bands of all involved birds and then began documenting the behaviors of individuals at the nest. Game camera footage was later reviewed to assess for any changes in behaviors.

3. Results

The usurped Least Tern nest, identified as LETE 17, was first identified as a three-egg clutch on the outskirts of a sub-colony in Cell 7 on 16 June 2023 (Figure 1). During the subsequent seven colony surveys (20, 22, 26, 28, 30 June and 3, 5 July 2023), no unusual events or conditions were observed at this nest. However, during a visual scope count on 7 July 2023, we observed two adult Common Terns sitting at the nest site while two adult Least Terns displayed aggressive behavior overhead. During the colony survey, we found two Least Tern chicks (both 1 day old) and one unhatched Least Tern egg inside the nest bowl, along with a new scrape ~8 cm from the nest, which had not been present during previous surveys. After banding the chicks, researchers continued surveying the sub-colony, leaving one person to monitor the nest from a distance with a spotting scope.

During the survey period on 7 July, the observer noted that both the Common Tern (WC6; BBL band only) and Least Tern paired adults (D17, E80) were previously banded. This indicates that the known age Common Tern was banded as a chick in 2021, and both Least Terns were banded as chicks in 2019. The behaviors during the manual observation period and the review of game camera footage were the same, with the Common Tern pair taking turns brooding the chicks and incubating the egg. While one of the Least Terns would occasionally return to the nest first following a disturbance that caused the full sub-colony to flush, the Common Terns returned a few seconds thereafter and displaced the Least Tern. Despite the continued presence of Common Terns, Least Tern adults continuously made efforts to reclaim their chicks/eggs by dive-bombing the Common Tern on the nest and making short, crouched scampers towards the nest. They also made regular, unsuccessful vocalizations in an apparent attempt to draw the chicks from the nest site. Both Common and Least Tern adults were observed returning to the nest site with fish, but no feeding events were observed, as harassment from the other species led to the fish being dropped. Unfortunately, the game camera footage ended at 13:12 due to a camera malfunction. When we returned on 10 July 2023, the nest site was abandoned following what appeared to be a large-scale predation event within the subcolony (heron predation suspected but not confirmed). This was the first observed nesting attempt for all reported individuals (banded Common Tern and Least Terns). Similarly, neither the banded chicks nor the adults were recaptured or resighted for the remainder of the season. It should be noted that levels of predation were high in all colonies on Poplar Island this year, and such predation was not exclusive to the sub-colony in which this observation occurred.

All game camera footage collected from this nest is available on the USGS ScienceBase Repository: https://doi.org/10.5066/P97ELOAW.

4. Discussion

To our knowledge, this observation represents the first report of a Common Tern brooding the chicks of another tern species. Though such events have historically been considered rare [10], the increasing number of aggressive usurpation events followed by incubating or brooding the eggs and young of other species reported in the literature [12,13,14] paired with the frequency with which Common Terns nest near other waterbird species [6] suggests such behavior may be under-reported. Thus, researchers may be well served to remain vigilant for such instances within their colonies.

Given that the Common Tern scrape, within centimeters of the usurped nest, was first observed by researchers concurrent with the usurpation, we suspect that the Common Tern pair established their scrape prior to hatching the Least Tern eggs. Given the proximity of the two nests, Common Terns may have displaced the Least Terns (though they did not abandon the area) and, when the first Least Tern egg hatched, mistaking it for their own young despite not yet having laid any eggs. However, we have no observations to support this explanation of events, and the Common Tern pair may have usurped the nest before any chick hatched. It should also be noted that the usurpation may have been influenced by the young age of the Common Tern pair, given the known age bird was two years old. Though Common Terns have been found to breed as young as one year old [19], most do not breed until three years of age [20,21]. Given the age of this Common Tern pair, this late-season nesting was likely their first breeding attempt of the season (no previous attempts were observed), though this cannot be confirmed. The cumulative evidence from this observation leads us to suggest inexperience may be at least a partial driver behind this usurpation event. Unfortunately, the age of usurping birds is not available for other reported instances to compare this finding.

An alternative explanation for this usurpation event is competition for nesting space, which has often been reported as a suspected driver of similar observations [12,14,22]. However, we feel this explanation is unlikely in this case, given the adequate habitat in this sub-colony (Figure 1) and across Poplar Island as a whole. Still, this possibility cannot be completely dismissed given that the usurped Least Tern nest was positioned far away from conspecifics and in relative proximity to Common Terns, with the factors that drove each pair to choose their specific nesting site unknown. However, we would posit that this event occurred partly because the Common Terns established a territory in such close proximity to the Least Terns but not because they had nowhere else to establish a nest.

The ability to identify the driving factors behind usurpation events like the one described here appears limited by the fact that intensive monitoring of the usurped nest does not begin until after the unique situation is identified, by which time it is too late to capture data on how interactions at the initial stage of usurpation unfolded. While traditional colony monitoring provides some insight into the timing and potential correlated issues, such as proximity and nest density, researchers have been unable to assert evidence-based claims for causation. Thus, researchers may consider deploying fixed monitoring techniques [23] in mixed species colonies in the future to improve the likelihood of gathering information at the beginning stages of both usurpation events and other cryptic behaviors.