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    The Condor 105:8094q The Cooper Ornithological Society 2003


    GARY L. NUECHTERLEIN1,3, DEBORAH BUITRON1, JOEL L. SACHS2,4 AND COLIN R. HUGHES2,51Department of Biological Sciences, North Dakota State University, Fargo, ND 58105

    2Department of Biology, University of North Dakota, Grand Forks, ND 58202

    Abstract. During 19951998 we studied the dynamics of nesting dispersion in a popu-lation of 160200 Red-necked Grebes (Podiceps grisegena) on Lake Osakis, a large, wind-blown lake in central Minnesota. Breeding grebe pairs arriving on the lake were highlyattracted to several large floating cattail (Typha angustifolia) mats that had broken off anddrifted away from the mainland to become islands snagged within rooted emergent beds.These mats provided prime nesting habitat protected from waves and predators that waschosen by nearly 40% of the breeding population. Most pairs establishing nests on theislands averaged 510 simultaneously active neighbors within 50 m, compared to pairs thatestablished territories along the shoreline, most of which had no neighbors within 50 m.The overall success of the two strategies varied widely from year to year, depending on thewater levels, nesting habitat availability, timing of windstorms, and predator activity. Duringmost years, however, pairs that established nests on the mats hatched earlier in the seasonand had larger clutches and broods. The dense mats appeared to provide protection fromwind and wave action, the primary cause of early nest loss. Our blood analysis work showedno genetic structure of the population and no tendency for neighbors to be closely related.However, pairs often returned to the same area and type of nesting situation in subsequentyears. The behavioral flexibility of Red-necked Grebe pairs to nest in groups within primehabitat areas may enable them to breed successfully within unpredictable emergent wetlandhabitats.

    Key words: coloniality, intraspecific variability, nesting dispersion, Podiceps grisegena,Red-necked Grebe, spacing, territoriality.

    Los Zambullidores Podiceps grisegena Nidifican Semi-Colonialmente Cuando hay Disponibilidadde Buen Sustrato de Nidificacion

    Resumen. Entre 1995 y 1998, estudiamos la dinamica de dispersion de nidos en unapoblacion de 160200 zambullidores Podiceps grisegena en Lake Osakis, un lago grandeafectado por el viento ubicado en Minnesota central. Las parejas de zambullidores quearribaron al lago fueron muy atraidas a varios parches flotantes de Typha angustifolia quese haban separado de la costa convirtiendose en islas. Estos parches representaron el mejorhabitat de nidificacion (protegido de las olas y los depredadores), y fueron escogidos porcerca del 40% de la poblacion reproductiva. La mayora de las parejas que establecieronsus nidos en las islas tuvieron en promedio 510 vecinos activos simultaneamente a menosde 50 m, en comparacion con las parejas que ubicaron sus nidos a lo largo de la costa, lamayora de las cuales no tuvieron ningun vecino a menos de 50 m. El exito general de lasdos estrategias vario ano a ano, dependiendo de los niveles de agua, la disponibilidad dehabitat de nidificacion, el momento de ocurrencia de tormentas de viento y la actividad delos depredadores. Sin embargo, durante la mayora de los anos, las parejas que ubicaron losnidos en los parches flotantes tuvieron nidadas que eclosionaron mas temprano, nidadas masgrandes y un mayor numero de nidadas. Estos parches densos parecieron conferir proteccionante la accion del viento y las olas, las causas principales de la perdida temprana de nidos.Nuestros analisis de muestras de sangre no mostraron estructura genetica en la poblacion,ni una tendencia a que los individuos vecinos estuvieran estrechamente relacionados. Sinembargo, las parejas a menudo regresaron a la misma area y la misma situacion de anidacionen anos subsecuentes. La flexibilidad comportamental de las parejas de P. grisegena paraanidar en grupos en las areas con mejor habitat les podra permitir reproducirse exitosamenteen habitats de humedales emergentes e impredecibles.

    Manuscript received 15 February 2002; accepted 5 September 2002.3 E-mail: gary.nuechterlein@ndsu.nodak.edu4 Present address: Integrative Biology, University of Texas, Austin, TX 78712.5 Present address: Department of Biology, University of Miami, Box 249118, Coral Gables, FL 33124.



    Birds nest in a wide variety of spatial patterns,ranging from highly dispersed to tightly packed.The nesting pattern typical of a species dependson many ecological and social factors, includingthe availability and dispersion of good feedingareas or safe nesting sites and the social costsand benefits of defending space around a nest.Although only 13% of all bird species nest co-lonially (Lack 1968), much attention has beenpaid to the many costs and benefits of colonialversus solitary nesting (Alexander 1974, Witten-berger and Hunt 1985, Danchin and Wagner1997, Rolland et al. 1998). Most field studieshave compared single-species colonies of differ-ent sizes (Hoogland and Sherman 1976, Brownand Brown 1996, Hill et al. 1997). Less commonare studies of a population that nests both soli-tarily and colonially on a regular basis (Snapp1976, Andersson and Wiklund 1978, Shields etal. 1988, Sasvari 1993). Intraspecific variation inspacing behavior, its causes, and its influence onother behavior are of great interest in examiningcoloniality and its possible evolution (Mller1987, Siegel-Causey and Kharitonov 1990, Fos-ter 1999).

    Nesting behavior ranging from solitary tohighly colonial is found among the 21 speciesof grebes (order Podicipediformes). In NorthAmerica, Western Grebes (Aechmophorus occi-dentalis), Clarks Grebes (A. clarkii), and EaredGrebes (Podiceps nigricollis) are highly colo-nial. These species often nest in colonies of hun-dreds of pairs spaced only a few meters apart(Storer and Nuechterlein 1992, Boe 1993, Cul-len et al. 1999). At the other end of the spec-trum, Red-necked Grebes (Podiceps grisegena),Horned Grebes (P. auritus), Least Grebes(Tachybaptus dominicus), and Pied-billedGrebes (Podilymbus podiceps) typically nest inwidely spaced, all-purpose territories, often withonly one pair to a pond (Cramp and Simmons1977, Muller and Storer 1999, Stout and Nuech-terlein 1999). A similar range in sociality isfound among the grebe species of other conti-nents (Cramp and Simmons 1977, ODonnel andFjeldsa 1997).

    Red-necked Grebes are renowned for beingextremely aggressive toward each other as wellas toward other waterbirds. Pairs may defendterritories of 1 ha or more, attacking intrudersfrom underwater (submarine attacks, Wobus

    1964, Garner 1991, Stout and Nuechterlein1999). Yet in some populations, Red-neckedGrebe pairs regularly form semicolonial groupsof nests spaced within 1020 m of each other(Munro 1941, Cramp and Simmons 1977, Stoutand Nuechterlein 1999).

    In this study, we examined the dynamics ofnesting dispersion in a population of Red-neckedGrebes on Lake Osakis, a large, windblown lakein central Minnesota. This single populationbridged the spectrum from intensely territorial tosemicolonial, providing an opportunity to inves-tigate ecological, behavioral, and genetic factorscontributing to sociality in this species. We ex-amine the hypothesis that Red-necked Grebenesting aggregations on the lake are associatedwith exploitation of temporary, ideal nestinghabitats formed by large floating mats of na-rrowleaf cattail (Typha angustifolia) that occa-sionally break loose from the mainland and be-come snagged offshore. We compare nest spac-ing, nest initiation dates, clutch size, egg size,hatching success, brood size, and genetic relat-edness for grebes nesting in these floating matsto those nesting in nearby rooted cattails.



    Lake Osakis covers 2537 ha and is located incentral Minnesota (458539N, 958099W), wherewinds typically reach 4050 km per hr severaltimes during a typical 4-week incubation period.The lake is frozen over from November to mid-April, when the ice melts and breaks up. Shortlyafter ice-out, when open water appears, 80100 breeding pairs of Red-necked Grebes returnto nest in emergent vegetation, primarily na-rrowleaf cattail, but also bulrush (Scirpus acu-tus) and phragmites (Phragmites communis).Nest sites are usually located where these plantsemerge from water that is 0.52.0 m deep. Bothsexes help build and defend the floating nests,constructed of primarily emergent vegetation de-bris. Both sexes incubate the eggs, defend, feed,and back-brood the young (Stout and Nuechter-lein 1999). After settling on their territories,pairs rarely fly except for brief, low patter-flightsof 520 m during aggressive interactions. Red-necked Grebes on Lake Osakis feed inverte-brates to newly hatched chicks but primarilyfeed minnows to young that are over two weeksold. Fish of an appropriate size, such as yellow


    FIGURE 1. The location of solitary and aggregated nesting areas of Red-necked Grebes within Lake Osakis,Minnesota. The detailed maps show the changes in floating cattail mat nesting areas (dark-shaded) between 1995and 1998. Lighter shading indicates shoreline areas accessible to predators without swimming. Floating mathabitat was highly dynamic compared to rooted cattails and decreased greatly from 1995 to 1998.

    perch (Perca flavescens) and shiners (Notropisspp.), appeared to be abundant and well dis-persed on Lake Osakis during the duration ofour study (Fraser 1997). Adult grebes spent littletime feeding, requiring only 1530 min to catchseveral 1015 cm fish. About 80% of the lakeis between 1 and 10 m in depth and easily avail-able to feeding grebes.