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 - email@example.com.
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
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. The US Geological Survey’s
Massachusetts Cooperative Fish and Wildlife Research Unit is supported by the
Massachusetts Division of Fisheries and Wildlife, Massachusetts Division of Marine
Fisheries, University of Massachusetts-Amherst, US Fish and Wildlife Service, and
Wildlife Management Institute.
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