Nesting Stage and Nest Defense by Common Terns Author(s): Brian G. Palestis Source: Waterbirds, 28(1):87-94. 2005. Published By: The Waterbird Society DOI: http://dx.doi.org/10.1675/1524-4695(2005)028[0087:NSANDB]2.0.CO;2 URL: http://www.bioone.org/doi/ full/10.1675/1524-4695%282005%29028%5B0087%3ANSANDB%5D2.0.CO %3B2 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/ page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non- commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. Nesting Stage and Nest Defense by Common Terns BRIAN G. PALESTIS Department of Biological Sciences, Wagner College, Staten Island, New York 10301, USA Internet: [email protected] Abstract.—Several hypotheses predict that nest defense should increase as the nesting cycle progresses, but the predicted pattern of increase differs. Previous studies of nest defense in gulls and terns have given conflicting re- sults. Most of these have treated colonies as a unit, thus breeding asynchrony may have obscured temporal patterns. Common Tern (Sterna hirundo) nest defense against Great Black-backed Gulls (Larus marinus) was observed during two years, and responses of individuals or pairs of known nesting stage were recorded. Responses of terns to observ- ers were also recorded. Mobbing of gulls did not increase as incubation progressed, contradicting parental invest- ment models, but did increase as chicks aged. Chicks were defended more strongly by parents than were eggs. Responses to Great Black-backed Gulls were more frequent during the second wave of nesting, when the potential for re-nesting would be very low. There was no association between clutch size and nest defense. Aggression by terns to observers was rare during incubation, but became common immediately after hatching and remained frequent until the chicks fledged. The temporal pattern of mobbing in the semi-precocial Common Tern most closely resem- bles that predicted for species with altricial young. However, interpretation is complicated by the fact that gulls prey more frequently on chicks than eggs. Received 25 July 2004, accepted 30 August 2004. Key words.—Antipredator behavior, breeding phenology, mobbing, parental care, Sterna hirundo, Common Tern. Waterbirds 28(1): 87-94, 2005 Birds are highly flexible in their responses Montgomerie and Weatherhead 1988). Hy- to predators, as interactions with predators in- potheses based on either offspring conspicu- volve not only predator recognition but also as- ousness and value to predators (Harvey and sessment of risk (McLean and Rhodes 1991). Greenwood 1978) or feedback between par- Because mobbing is a potentially costly activity, ents and offspring (McLean and Rhodes nest defense should vary with the threat that 1991) predict a constant level of defense dur- particular species represent (Patterson et al. ing incubation, because eggs do not become 1980; Buitron 1983; Curio et al. 1983; Clode et more conspicuous with age (until pipping). al. 2000). Even within a single species of nest All three categories of hypotheses predict predator, antipredator responses should vary that, at least in altricial species, parents with the context of the encounter (McLean should defend chicks more strongly than and Rhodes 1991; Curio 1993). In the present eggs, because chicks are more valuable and study, the responses of the Common Tern more conspicuous than eggs. Most authors (Sterna hirundo) to the Great Black-backed have predicted that parental defense of Gull (Larus marinus) are compared across stag- chicks in altricial species should increase as es of the tern nesting cycle. chicks age and then decline at fledging Avian nest defense often increases in in- (Barash 1975; Andersson et al. 1980; Mont- tensity as the breeding season progresses gomerie and Weatherhead 1988) or shortly (reviews in Knight and Temple 1986b; thereafter (McLean and Rhodes 1991). Nest Montgomerie and Weatherhead 1988; Re- defense in precocial species, on the other dondo 1989; Brunton 1990; McLean and hand, should peak shortly after hatching Rhodes 1991), and several hypotheses have (Barash 1975; Andersson et al. 1980; Mont- been proposed to explain and predict tem- gomerie and Weatherhead 1988). These de- poral variation in nest defense. Models clines in parental nest defense are expected based on optimal parental investment and because fledglings can escape predation via offspring reproductive value predict an in- flight and precocial chicks can hide from crease in defense throughout incubation, predators, and because predators are less because the value of offspring increases and likely to remove an entire brood at once if the ability to re-nest decreases as offspring chicks have dispersed from the nest site. All age (Barash 1975; Andersson et al. 1980; of these models predict that large clutches or 87 88 WATERBIRDS broods should be defended more strongly This study was observational, involving natural en- than smaller clutches or broods, although counters with nest predators. Field observations were made from a small boat anchored approximately 20 to very few empirical studies have supported 30 m from the island. Most observations were made this prediction (Rytkönen 2002). alone, but occasionally with the help of an assistant. Ex- In the semi-precocial gulls and terns (Lar- cluding terns in the center of the island, which were dif- ficult to see from the boat, terns tended to nest along idae), seasonal patterns should be intermedi- three edges of the island on wrack (mainly dead eel- ate to those predicted for altricial and grass, Zostera). A typical observation session consisted of precocial species, since their young are mo- 3 h of observation, one at each of these edges, and typi- cally four such sessions were performed per week at var- bile but typically remain near the nest site ious times of day. Observations totaled 124 h in 2001 (see Nisbet 2002 for Common Tern). The re- and 163 h in 2002. sults of previous studies of larid nest defense Whenever a nest predator approached or entered the colony, the responses of the visible individuals were have often contradicted one another (see recorded. The variable analyzed was the proportion of Discussion), and thus no clear pattern has encounters that elicited a response, called a “tern fly- emerged. One weakness of most previous ing up”. Only encounters with predators that flew di- rectly over or within approximately 20m of the island studies of nest defense in larids and other co- were included, thus excluding encounters with preda- lonial birds (including my own work; Palestis tors landing or attempting to land on the island, which 2000) is that researchers did not examine the were rare and more likely to elicit a response from terns (Hatch 1970; Becker 1984; Cavanagh and Griffin responses of individuals or breeding pairs, 1993). Although the responses of terns to several spe- but instead treated the entire colony as one cies of nest predator were recorded, only responses to unit (for a recent exception see Meehan and Great Black-backed Gulls (hereafter, “gulls”) are ana- lyzed here, for three reasons: 1) This species was ob- Nisbet 2002). Because the behavior of many served frequently (relative to other predators) and individuals was combined, reproductive asyn- consistently throughout the study: on average, approx- chrony may have obscured seasonal patterns imately 3-4 encounters per hour in both years, with no trend across nesting stages. 2) At Pettit Island, terns of nest defense, as not every pair in a colony mob this species more frequently than other species would be at the same stage of the nesting cy- (Palestis and Burger 1997). 3) This was the only spe- cle at a given point in time. Although mob- cies observed preying on tern young, although such events were very rare (only one in the two years of the bing may spread among neighbors in a tern present study). colony (Burger and Gochfeld 1991), not all Occasionally, terns nesting close to the edge of the members of a colony or subcolony will re- island flew toward the observers in the boat. This aggres- sion usually occurred immediately after arrival, when I spond to the same encounter with a nest was preparing to enter the colony, or soon after I left the predator. For example, Burger and Gochfeld colony. In addition to responses to gulls, the frequency (1991) report that in colonies with 100 to 250 of aggression by terns toward the observers in the boat is also quantified. Mobbing while observers were on the Common Terns, the average number of terns island was not quantified. flying up in response to a predator was only All nest sites that were potentially visible from the 23.4 and was highly variable (SD ± 22.0). Dif- boat were marked with numbered tongue depressors. Typically four censuses were performed per week, which ferences in nesting stage may explain some of included tracking the contents of marked nests and this variation. This study was conducted to ex- searching for and marking new nests. In 2001, 15 incu- amine temporal changes in nest defense by bating adults were captured in nest traps and uniquely marked in combinations of blue, red and green on their Common Terns of known nesting stage.