143 Common Loon Incubates Rocks as Surrogates for Eggs Stephen DeStefano1,*, Kiana K.G. Koenen2, and Jillian W. Pereira3 Abstract - A nesting Gavia immer (Common Loon) was discovered incubating 2 rocks on a floating nest platform on the Quabbin reservoir in central Massachusetts for 43 days, well beyond the typical period of 28 days, before we moved in to investigate. The rocks were likely unearthed in the soil and vegetation used on the platform to create a more natural substrate for the nest. We suggest sifting through soil and vegetation to remove rocks before placing material on nest platforms. Since 1975, up to 17 pairs of Gavia immer Brunnich (Common Loon) have established breeding territories on the Quabbin reservoir in central Massachusetts, with up to 11 pairs (a 5-yr average for 2007–2011) nesting annually, making this the largest concentration of nesting loons in southern New England. The Massachusetts Department of Conservation and Recreation (MDCR) and the Massachusetts Division of Fisheries and Wildlife (MDFW) have been monitoring loon populations on the Quabbin Reservation annually since 1975 and providing up to 10 floating nest platforms since 1986 (DeSorbo et al. 2007, 2008; MDCR 2011; Piper et al. 2002). In July 2011, we monitored a Common Loon exhibiting prolonged incubation on a nest platform on the northern section the reservoir. After 43 days (8 June to 20 July), well after the normal 28-day (range 27.4–30.6 days; Evers et al. 2010) incubation period for this species, we moved in to investigate, believing that the loon had either renested or was attempting to hatch infertile eggs. In the past 5 years, only 1 loon pair attempted to incubate infertile eggs (although there were several instances of loons abandoning infertile eggs). However, in the case reported here, we discovered that this loon was sitting on 2 rocks (Fig. 1). The rocks were brownish in color, roughly similar to one another in size and shape, and smaller than a Common Loon egg (Table 1, Fig. 2). Evers et al. (2010) described Common Loon eggs as being sub-elliptical to oval in shape and olive to brown, with dark brown or black splotches, in color. The larger of the rocks was indeed oval while the smaller rock was more angular; however, both were brownish with faint dark brownto- black highlights. There was no evidence of real eggs (e.g., eggshell fragments) in or around the nest bowl or anywhere on the platform or in the water. The loon sat tight and lowered its neck as 2 observers approached the nest on 20 July 2011. When the boat, which was equipped with an electronic trolling motor, was 1US Geological Survey, Massachusetts Cooperative Fish and Wildlife Research Unit, University of Massachusetts, Amherst, MA 01003. 2Massachusetts Department of Conservation and Recreation, Division of Water Supply Protection, Belchertown, MA 01007. 3Massachusetts Department of Conservation and Recreation, Division of Water Supply Protection, West Boylston, MA 01583. *Corresponding author - sdestef@eco.umass.edu. Notes of the Northeastern Naturalist, Issue 20/1, 2013 Table 1. Length and width (mm) and volume (cm3) of 2 rocks that were incubated by a Common Loon on Quabbin reservoir in central Massachusetts. Measurements for average loon egg for comparison, based on data provided by Evers et al. (2010). Length (SD) Width Volume First rock 66 52 80 Second rock 64 51 60 Average loon egg 90 (2) 57 (1.5) 146 (11) 144 Northeastern Naturalist Vol. 20, No. 1 less than 1 m away, the bird slipped off the platform quietly but then remained within the territory as the nest was examined and the rocks, which were still warm from the loon’s body heat, were retrieved. When the observers returned about 2.5 hours later, the loon Figure 1. Nest of a Common Loon with rock substitutes for eggs (white arrows), found on a floating nest platform on Quabbin reservoir, MA (photo by K.K.G. Koenen, MDCR). Figure 2. Two rocks found in the nest of a Common Loon using an artificial floating platform on Quabbin reservior, MA, and a life-sized wooden replica of a loon egg (from Biodiversity Institute, Gorham, ME) (photo by S. DeStefano, USGS). 2013 Northeastern Naturalist Notes 145 was still present. When the boat was in the area, the loon was often in a raised-neck position in the water and was very alert. At no time did it display any intense disturbance or aggressive behaviors, such as bill dipping, splash-dives, rushing across the water with flapping wings, aggressive upright display, or the penguin dance or posture (Evers et al. 2010, Rummel and Goetzinger 1975, Sjölander and Agren 1972). The bird peered below the water, preened, and rolled over, at which time it was noted that it did not have leg bands. Many nesting loons using Quabbin reservoir have been marked with US Fish and Wildlife Service aluminum and color-coded leg bands; the lack of bands led us to speculate that this was a younger individual. The loon did not vocalize while the observers were in the area, and only one individual was seen in the territory. There were no further attempts at nesting on this platform for the remainder of the breeding season. To our knowledge, rocks in the nests of Common Loons have not been reported previously. Rocks and other foreign objects (e.g., pine cones) have been found in nests of other species of birds, including gulls and terns (Conover 1985, Coulter 1980), Branta canadensis L. (Canada Goose) (Knight and Erickson 1977), Phalacrocorax auritus Lesson (Double-crested Cormorant) (Hobson 1989), Recurvirostra americana Gmelin (American Avocet) (Sugden 1947), and albatrosses (P. Sievert, USGS, Amherst, MA, pers. comm.), and have been purported to stimulate breeding or incubation. For example, Coulter (1980) speculated that Larus occidentalis Audubon (Western Gull) and Sterna hirundo L. (Common Tern) occasionally rolled rocks into their nests to make up for missing eggs in their preferred 3-egg clutches. However, Conover (1985) believed that rocks were mistaken for eggs, rather than served as stimuli for incubation, by Larus delawarensis Ord (Ring-billed Gull) and L. californicus Lawrence (California Gull). Conover also rejected the mistaken-food hypothesis (i.e., rocks were mistakenly ingested as food and regurgitated into the nest) (Sugden 1947, Twomey 1948) in favor of the mistaken-egg hypothesis. Hobson (1989) and other authors speculated that construction of mock or ephemeral nests may help to develop nesting skills in younger birds, and inclusion of pebbles or rocks may be an extension of that behavior. The fact that this loon was not banded could indicate that it was a younger, novice individual. It has long been known that plastic eggs and other egg substitutes can prevent or stop egg laying and alleviate chronic egg laying in pet birds (Mitchell 2005, Romagnano 1996). Common Loons will often re-nest 8–19 days after an entire clutch is lost (Evers et al. 2010). The presence of egg-like rocks in the nest may prevent a female from laying, or discovery of egg-like rocks in the nest after an initial clutch of eggs is lost may prevent loons from successfully re-nesting. Sutcliffe (1982) reported prolonged incubation of infertile eggs of 68 and 74 days for two loons in New Hampshire. Prolonged incubation and the possible negative effects it can cause to parental fitness, such as additional costs of incubation and decreased chances of re-nesting, has been noted for Common Loons (Sutcliffe 1982) and Podiceps grisegena Boddaert (Red-necked Grebe) (Kloskowski 1999). Prolonged incubation is energetically demanding, can contribute to weight loss and decreased body condition, and puts the adult at increased risk of predation or some other cause of mortality. We could think of 3 possibilities as to how the rocks got into the nest. The first was that the loon or loons brought them into the nest. Common Loons will handle and ingest small stones between 4.75–8.00 mm as grit (Franson et al. 2001), but we believe that transporting these rocks was improbable because of their size and weight. The 146 Northeastern Naturalist Vol. 20, No. 1 second was that a boater approached the nest platform and placed the 2 egg-like rocks in the nest. On rare occasions, we have seen boaters nearing the platforms to take a closer look but, even so, we believe this explanation is possible but not likely because a widespread information and education program has created a positive relationship between wildlife and recreationists within the Quabbin boating community. The most plausible explanation is that rain or the loon itself uncovered the rocks in the soil and vegetation that was placed on the nesting platform, which was added to provide a substrate and material to form the nest bowl. Rocks of this size could have easily been incorporated into the soil, and later uncovered by weather or the activities of the adult loons. We also found three additional rocks (all under 47 mm in length) on the platform and just outside the nest bowl. These rocks were not incorporated into the nest, but their presence does provide more evidence that rocks present were in the soil and later became exposed. Piper et al. (2002) placed soil and moist vegetation on floating platforms to be used as nesting material, and DeSorbo et al. (2008) recommended that natural material (soil and plants) be placed on platforms for nesting loons. We suggest sifting the soil, or at least raking or probing through it and any vegetation to find and eliminate egg-sized rocks, as a precaution to eliminate the possibility, however small, of loons attempting to incubate rocks as surrogates for eggs. Acknowledgments. We thank Daniel Clark, Kyle McCarthy, Jean-Pierre L. Savard, and two anonymous referees for reviewing the manuscript. 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