2007 NORTHEASTERN NATURALIST 14(4):637–642 Short Call-broadcasts Fail to Detect Nesting Least Bitterns (Ixobrychus exilis) Douglas C. Tozer1,*, Kenneth F. Abraham2, and Erica Nol3 Abstract - We conducted 11 call-broadcast surveys at one location in each of four wetlands, and detected an Ixobrychus exilis (Least Bittern) on only 9 of the 44 (20%) surveys, while the observer was located on average 50.1 m ± 19.7 SD (range = 25–75 m, n = 11 nests) from at least one active bittern nest during each survey. For 8 of 9 (89%) detections, at least one bittern was already vocalizing at the beginning of the survey, before the Least Bittern call was played. We show that it is possible for this species to remain undetected with the use of call-broadcasts that are less than 30 seconds, even though the species may be nesting as close as 25 m to the location from which the calls are broadcasted. We suggest that systematic nest searches are more reliable than call-broadcast surveys for detecting nesting Least Bitterns. Introduction Call-broadcast surveys (Conway and Gibbs 2005) increase detections of several secretive marsh-bird species (Ribic et al. 1999). When compared with passive listening, call-broadcasts increase detections by 78–1675% for Porzana carolina Linnaeus (Sora), Rallus limicola Viellot (Virginia Rail), Rallus elegans Audubon (King Rail), Rallus longirostris Boddaert (Clapper Rail), and Gallinula chloropus Linnaeus (Common Moorhen) (Conway and Gibbs 2005, Gibbs and Melvin 1993, Hinojosa-Huerta et al. 2002, Manci and Rusch 1988). As a result, call-broadcasts are commonly used during marsh-bird surveys (e.g., Conway and Gibbs 2005, Crewe et al. 2006, Krzys et al. 2002). Ixobrychus exilis Gmelin (Least Bittern) is a small, secretive, wetland- dependent heron of conservation concern (Gibbs et al. 1992). It is commonly assumed that detections of this species will increase with the use of call-broadcasts (Conway and Gibbs 2005). However, studies that test the influence of call-broadcasts on detection of Least Bitterns report inconsistent results: some show increases (Bogner and Baldassarre 2002, Gibbs and Melvin 1993, Manci and Rusch 1988, Swift et al. 1988) while others show decreases (Lor and Malecki 2002, Paine 1997). Here, we report on detections of Least Bitterns during multiple call-broadcast surveys at each of 4 wetlands while the observer was located within 75 m 1Watershed Ecosystems Graduate Program, Environmental Science Centre, 1600 West Bank Drive, Trent University, Peterborough, ON K9J 7B8 Canada. 2Ontario Ministry of Natural Resources, DNA Building, 2140 East Bank Drive, Trent University, Peterborough, ON K9J 7B8 Canada. 3Biology Department, Environmental Science Centre, 1600 West Bank Drive, Trent University, Peterborough, ON K9J 7B8 Canada. *Corresponding author - dtozer@trentu.ca. 638 Northeastern Naturalist Vol. 14, No. 4 of at least one active Least Bittern nest, using Bird Studies Canada’s Marsh Monitoring Program (MMP) bird point-count protocol (Crewe et al. 2006, Weeber and Vallianatos 2000). These data are useful for assessing the effectiveness of call-broadcasts for detecting nesting Least Bitterns. Study Area and Methods Our study took place in four temperate wetlands (National Wetlands Working Group 1988) near Peterborough, ON, Canada (44°19'N, 78°23'W). Wetlands were: 7, 23, 24, and 131 ha in area, had at least 60% cattail (Typha sp.)-dominated cover, and had average water depths between 40 and 65 cm. We established one 100-m radius half-circle at a random location along the shore of each wetland, with the straight edge parallel to the shore. The same observer, standing at the midpoint of the straight edge of the half-circle, conducted one 10-minute survey at each location on 11 separate days (11 surveys x 4 locations = 44 surveys). Surveys were conducted three to five days apart between 21 May and 4 July 2001 following MMP protocol (Crewe et al. 2006, Weeber and Vallianatos 2000). Surveys were conducted between 1800 h and sunset when the air temperature was greater than 16 °C, there was no precipitation, and wind speed was less than 20 kph. At the beginning of each survey, the MMP bird call-broadcast (Crewe et al. 2006) was played with a cassette recorder (Radio Shack Optimus Intertan portable AM/FM cassette recorder, CTR-110, Model #14-733) from 1 m above the substrate, with 80-dB maximum sound pressure measured with a sound-level meter (Radio Shack, Model #33-2050) 1 m from the source. The call-broadcast contained approximately 30 seconds of calls of each of 5 species (original recordings from: Monte Brigham, Lang Elliot, and the Cornell Library of Natural Sounds; Crewe et al. 2006) separated by 30 seconds of silence, in the following order (call type in quotes): Virginia Rail (“grunt;” Conway 1995), Sora (“whinny;” Melvin and Gibbs 1996), Least Bittern (“cooing;” Gibbs et al. 1992), Fulica americana Gmelin (American Coot) (“grunt, croak, squawk, etc.” Bent 1926), and Podilymbus podiceps (L.) (Pied-billed Grebe) (“song;” Muller and Storer 1999). The entire area of each half-circle was systematically searched for Least Bittern nests between 1 May and 1 August 2001. To relate timing of Least Bittern detections with nesting chronology, we estimated the dates of egg and nestling stages for each nest following Manolis et al. (2000), based on a 19-day incubation and a 14-day nestling period (Fragnier 1996, Gibbs et al. 1992). Each nest was visited every 3–5 days to determine status until the nest failed or all of the nestlings fledged. Because we were interested in comparing the number of nesting pairs with the number of Least Bitterns detected within a specific area, we restricted our analysis to bitterns detected within the half-circles only. 2007 D.C. Tozer, K.F. Abraham, and E. Nol 639 Results We detected at least one Least Bittern during 9 of 44 (20%) surveys. In eight of nine (89%) detections, at least one Least Bittern was already actively calling when the observer arrived at the location, and calling did not stop during or after call-broadcasts. All but one of the nine detections coincided with the nest-initiation or early egg stage of at least one nest within the halfcircle (Fig. 1). The number of calling males matched or exceeded the number of known active nests within the half-circle for only two of the nine (22%) detections. We found 11 active nests (range of density per half-circle: 0.6– 1.9 per ha) in 64 hours of searching. Least Bitterns were not detected at two locations, each with three nests less than 50 m from the broadcast location. At the remaining two locations, one of which had two nests and the other three nests, at least one Least Bittern was detected on 3 and 6 of 11 surveys, respectively (Fig. 1). The mean distance from the broadcast location to a nest was 50.1 m ± 19.7 SD (range = 25–75 m, n = 11 nests). Discussion Figure 1. Detections of Least Bitterns using Bird Studies Canada’s Marsh Monitoring Program point-count protocol in relation to nest stage of active Least Bittern nests located within 75 m of the observer, May to July 2001, at four wetlands (site name and wetland area indicated at top left of each graph) near Peterborough, ON, Canada. Horizontal bars represent individual nests: grey indicates egg stage and white indicates nestling stage. Vertical lines represent individual point counts: dashed indicates no detection of bitterns, solid indicates detection of at least one bittern prior to the call-broadcast, and dotted indicates detection of at least one bittern after the beginning of the Least Bittern call-broadcast. Numbers above solid and dotted vertical lines indicate the number of Least Bitterns that were detected. The x-axis is Julian date (e.g., 130 = 10 May). 640 Northeastern Naturalist Vol. 14, No. 4 Our observations with few detections are inconsistent with the common assumption that call-broadcasts increase detections of Least Bitterns. Others have reported improvements of 13 to 750% in numbers of Least Bitterns detected by using call-broadcasts over passive listening (Bogner and Baldassarre 2002, Conway and Gibbs 2005, Gibbs and Melvin 1993, Swift et al. 1988), while some have reported decreases of 11 to 70% (Lor and Malecki 2002, Paine 1997). Call-broadcasts dramatically and consistently increase detections in wetland species that use one type of vocalization prior to pairing and another type after pairing (Gibbs and Melvin 1993, Johnson and Dinsmore 1986). For example, the “ker-wee” (Melvin and Gibbs 1996) and “tick-it” (Conway 1995) calls of the Sora and Virginia Rail, respectively, are used primarily by males to attract mates. Once nesting is underway, these species switch to the “descending whinny” and the “grunt,” respectively, for territory defense and pair-bond maintenance (Conway 1995, Melvin and Gibbs 1996). Response rates to call-broadcasts by Least Bitterns may be lower than those by Sora and Virginia Rail because Least Bitterns lack these territory defense and pair-bond maintenance calls, and are quite silent once pairs are formed and courtship ceases (Bogner and Baldassarre 2002). Although Least Bitterns are thought to be territorial in that they exclude conspecifi cs from the immediate nest area (Bogner and Baldassarre 2002, Lor and Malecki 2002, Sandilands 2005, Weller 1961), very few instances of defense and chase have been reported (Gibbs et al. 1992, Weller 1961). For example, in June 2000 at Matchedash Bay, ON, D.C. Tozer broadcast the Least Bittern portion of the MMP call-broadcast in the presence of an adult male (as indicated by its black, rather than brown, crown) Least Bittern, located 45 m away. This individual never vocalized, but did show interest by bobbing up and down repeatedly and facing the source of the call-broadcast. Figure 2. Ixobrychus exilis Gmelin (Least Bittern) in typical wetland habitat. Photograph © Warren Williams. 2007 D.C. Tozer, K.F. Abraham, and E. Nol 641 Shortly after, D.C. Tozer followed the male to a nearby nest (40 m from the call-broadcast location) where it switched with its mate that was incubating 6 eggs. This male probably remained silent during the call-broadcast because it was already mated, and because D.C. Tozer was located well beyond the immediate nest area when he played the call-broadcast. Vocalizations are not used during presumed territorial defense by Least Bitterns (Davidson 1944, Lor and Malecki 2002), although males have been observed silently approaching the source of call-broadcasts while remaining concealed in dense vegetation (Gibbs and Melvin 1993, Lor and Malecki 2002, Swift et al. 1988). The habit of remaining concealed when approaching call-broadcasts probably contributes to the low response rates in this species. Our call-broadcasts were short (30 sec). Others have used call-broadcasts of up to 5 minutes (Swift et al. 1988) within 30 m of a radio-tracked Least Bittern (Bogner and Baldassarre 2002) and increased detections from 10 to 26%, still low for determining presence or absence at a wetland, particularly if only one visit is made. These studies combined with our results suggest that Least Bittern call-broadcasts of 30 seconds or less are too short to consistently increase detections. Despite our small sample of wetlands, our results show that it is possible to miss Least Bitterns using short callbroadcasts even though the species is nesting near the broadcast location. Systematic nest searching, although time consuming, detected more nesting Least Bitterns than call-broadcasts. Acknowledgments We thank Melanie Radder, Andrea Stewart, and Mary Van Sleeuwen for conducting point counts and/or searching for nests. Daniel Keppie, an anonymous reviewer, Ron Tozer and Don Sutherland provided useful comments on an earlier draft. This research was made possible by support from Bird Studies Canada in partnership with Ontario Ministry of Natural Resources, Trent University, United States Geological Survey, Canadian Wildlife Service—Ontario Region (Science Division), Ducks Unlimited Canada, and The Ontario Trillium Foundation. 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