Common Loon Pairs Rear Four-Chick Broods
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Received 8 mally lay a single clutch of two eggs each breeding July 2003, accepted 24 March 2004. season. They occasionally lay one- or three-egg clutch- es, and rarely, four-egg clutches. Participants of the Canadian Lakes Loon Survey provided seven indepen- dent observations of loon pairs rearing four-chick Supernumerary broods, either as a result of broods. Photographic evidence con®rmed two separate nest parasitism by unrelated conspeci®cs, su- instances of adult loon pairs at Anglin Lake, Saskatch- pernumerary clutches, or post-hatch brood ewan, and Kasshabog Lake, Ontario, exhibiting paren- amalgamation, are relatively common among tal behavior toward a four-chick brood. Occurrence of grebes (Storer and Nuechterlein 1992, Cullen four-chick broods may be the result of supernumerary et al. 1999, Muller and Storer 1999, Stout and clutches, nest parasitism, post-hatch brood amalgam- Nuechterlein 1999, Stedman 2000) and water- fowl (Afton and Paulus 1992:90, table 3±21; Sayler 1992). However, there are few docu- 1 Bird Studies Canada, P.O. Box 160, Port Rowan, ON N0E 1M0, Canada. mented instances of supernumerary broods in 2 Corresponding author; e-mail: loons (Barr et al. 2000), including the most [email protected] widely studied species, the Common Loon 98 THE WILSON BULLETIN x Vol. 116, No. 1, March 2004 (Gavia immer; McIntyre 1988:30, McNicholl length with dark gray down feathers), small 1993). young (1/3 to 2/3 adult length with light Common Loons are large, long-lived wa- brown-gray or mottled-gray down feathers) or terbirds that normally lay a single clutch of large young (2/3 adult length or longer with a two eggs each breeding season, although oc- full coat of light and dark gray feathers). Al- casionally they will lay one or, even less fre- though not part of the survey protocol, CLLS quently, three-egg clutches (Peck and James participants often found loon nests during sur- 1983, Croskery 1991, McIntyre and Barr veys and recorded clutch sizes. Periodically, 1997). The frequency of three-egg clutches re- participants also provided photographic re- ported for Common Loons is low and ranges cords of observations during surveys. These from 0.5% (Campbell et al. 1990) to 0.8% records provided the basis for results reported (Peck and James 1983, McIntyre 1988:table here. 2±5). Clutches containing four eggs are rare, but have been noted several times (Nelson RESULTS AND DISCUSSION 1983, Peck and James 1983, Zicus et al. 1983, Since the initiation of the CLLS in 1981, McNicholl 1993). To our knowledge, there is participants have reported 6 of 687 (0.87%) no con®rmed record of Common Loons rear- loon nests containing three-egg clutches and ing four-chick broods. In this paper we report no four-egg clutches. CLLS participants re- seven instances of Common Loon adults ac- ported 45 of 6021 (0.75%) Common Loon companying and rearing four-chick broods, pairs with supernumerary broods (.2 chicks), two of these con®rmed by photographic re- which is similar to values that others have re- cords. ported for this species (see above). In ®ve sep- arate instances, CLLS participants provided METHODS written evidence of four-chick Common Loon Data were gathered by volunteer partici- broods: two on Shepherd Lake, Ontario (448 pants of the Canadian Lakes Loon Survey 399 N, 818 79 W) during 1983 and 1984; one (CLLS), who monitored Common Loon on Oak Lake, Ontario (448 369 N, 778 559 W) breeding pairs on lakes, rivers, and bays during 1984; and two on the Mactaquac River, throughout Canada. Participants selected their New Brunswick (468 019 N, 668 589 W) during own water body or portion of a water body to 1988 and 1993. Participants also provided survey breeding loons and recorded observa- photographic evidence of two separate in- tions of breeding pairs at least once during stances of Common Loon pairs accompanied each of three time periods: nesting (early June by four-chick broods. The ®rst photograph to mid-July), hatching and early brooding was taken on 14 July 1999 at Anglin Lake, (early to late July), and pre-¯edging (mid-Au- Saskatchewan (538 449 N, 1058 569 W; Fig. gust to mid-September). All surveys lasted a 1A). This group of birds was observed inter- minimum of 2 hr and often were supplement- mittently from 14 July to 17 August 1999. ed by incidental observations. During each CLLS participants estimated these chicks to survey, observers recorded the date, survey be 3 weeks of age on 14 July, and adults ex- method (e.g., from a single point on shore, hibited feeding behavior toward all four walking along shoreline, or from a boat or ca- chicks. noe), maximum number of adult loons ob- Photographic evidence was also secured at served (including paired loons), maximum Kasshabog Lake, Ontario (448 389 N, 778 579 number of mated loon pairs present, and max- W; Fig. 1B, C), where a Common Loon pair imum number and age class of young. The was observed attending four chicks from 21 CLLS survey protocol instructed volunteers to July to 30 September 2001. CLLS participants classify chicks as downy young (,1/3 adult estimated this four-chick brood to be 3 weeks → FIG. 1. Photographs of two different pairs of adult Common Loons with four-chick broods: (A) Anglin Lake, Saskatchewan, Canada, 14 July 1999 (photograph by W. R. and E. V. Hoffman), and (B and C) Kasshabog Lake, Ontario, Canada, July 2001 and August 2001, respectively (photographs by P. Grisson). SHORT COMMUNICATIONS 99 100 THE WILSON BULLETIN x Vol. 116, No. 1, March 2004 of age on 21 July, and adults exhibited feeding on a lake by the CLLS. Concentrations of loon behavior toward all four chicks. Observations pairs on Anglin Lake were high on 14 July later that summer showed that one of these 1999, when 36 separate Common Loon pairs chicks was smaller than the other three, and were observed. Size and behavioral differenc- often it was observed farther away from the es in the brood photographed at Kasshabog adults than the other chicks (Fig. 1C). Lake also suggested brood amalgamation. In- Occurrence of four-chick broods in Com- tensive monitoring and/or genetic evidence mon Loons may be the result of supernumer- are required to determine de®nitively whether ary clutches, nest parasitism, post-hatch brood supernumerary broods in Common Loons re- amalgamation, or a combination of these fac- sult from supernumerary clutches, nest para- tors (Nelson 1983, Zicus et al. 1983, Belant sitism, or post-hatch brood amalgamation. and Olson 1991, McNicholl 1993). Both Nel- son (1983) and Zicus et al.