Nest Site Selection by Eared Grebes in Minnesota’
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The Condor 96: 19-35 8 The Cooper Ornithological Society 1994 NEST SITE SELECTION BY EARED GREBES IN MINNESOTA’ JANET S. BOE* Department of Zoology, North Dakota State University,Fargo, ND 58105 Abstract. This study examined nest site selection within Eared Grebe (Podicepsnigri- collis) colonies in terms of nesting synchrony, nearest-neighbordistances, vegetation, ex- posure,and nest success.Nests in 11 Eared Grebe coloniesin Minnesota were initiated over a span of 1 l-45 days, with most nests in each colony initiated in < 11 days. In each of six colonies studied, mean nearest-neighbordistance of the earliest nestswas greater than that of the complete colony. Nearest-neighbor distance tended to decreasewith an increase in emergent vegetation density. Early nests seemed to form the skeleton of a neighborhood, with later nests filling in or establishingother neighborhoods.In five of six colonies, clutch sizes were larger in earlier nests. No significantdifference in egg volume between early and late nests was apparent in the two colonies studied. Early nests were more successfulthan were late nests in two of four colonies; center nests were more successfulthan were edge nestsat only one of four colonies. Most nest destructionin this study was causedby waves generatedby high winds; ~2% of nests showed evidence of predation. Key words: Eared Grebe;Podiceps nigricollis; Podicipediformes;habitat selection:nest- site selection;nesting synchrony. - INTRODUCTION result in rapidly changing vegetation patterns, Selection of a breeding habitat that meets the which probably contribute to the peripatetic na- reproductive requirements of an animal is crit- ture of grebe colony formation (Cramp and Sim- ical to its evolutionary fitness (Partridge 1978). mons 1977). Breedinghabitat selectionby birds probably takes The objective ofthis study was to examine nest place on several levels (Burger 1985). Among site selection within Eared Grebe colonies in colonial birds, colony site selection and nest site terms of nesting synchrony, nearest-neighbor selection are two major levels that are frequently distances,vegetation, exposure,and nest success. studied (Birkhead and Harris 1985). STUDY AREA AND METHODS Among the colonial grebes,quantitative stud- ies of nest site selection have been conducted on I studied nest site selection by Eared Grebes in Western Grebes (Aechmophorus occidentalis) 11 colonies on nine wetlands in western and (Nuechterlein 197 5) and on Silver (Podiceps oc- southern Minnesota (Fig. 1) from May-August cipitalis) and Rolland’s (Rollandia rolland) in 1987-1989. These wetlands mark the eastern Grebes (Burger 1974b). Studies of Eared Grebe extent of the Eared Grebe breeding rangein North (Podiceps nigricollis) habitat selection include a America (Robbins et al. 1983) and the eastern study of wetland selection by three grebe species edge of the prairie pothole region of the conti- in North Dakota (Faaborg 1976) and recently nent. Most of the wetlands are shallow and hy- completed studies of wetland and colony site se- pereutrophic (Heiskary and Wilson 1989) and lection by Eared Grebes in British Columbia frequent, high winds and high summer temper- (Breault 1990) and in Minnesota (Boe 199 1). atures contribute to rapid evaporation rates (Ad- Eared Grebes nest over water on shallow, eu- ams 1988). trophic wetlands that are particularly vulnerable Terminology largely follows Gochfeld (1980) to yearly fluctuations in water levels, including and Kushlan (1986): a colony site is the place periodic natural lowering due to drought (Kan- where colonial nesting takes place, a colony is trud et al. 1989). These dynamic water regimes the collection of birds using the site, a subcolony is a cluster of birds separatedfrom adjacent clus- ters by space or habitat variation, and a neigh- I Received 16 February 1993. Accepted 7 September borhood is a cluster of birds whose members 1993. 2 Presentaddress: Minnesota County BiologicalSur- interact frequently with each other. I use the term vey, Minnesota Department of Natural Resources,Deer colony to refer to either the nests or the birds River RangerDistrict, Box 308, Deer River, MN 56636. present. A colony site is the location of a group [I91 20 JANET S. BOE t-------l100 km 1. Thief Lake 2. Ash Lake 3. Mud Lake 4. Harstad Slough 5. Lane Lake 6. W. Toqua Lake 7. Swenson Lake 8. Salt Lake 9. Swan Lake FIGURE 1. Map showing location of Eared Grebe breeding wetlands in Minnesota with colonies marked in 1987-1989. of nestson a breeding wetland, and nest sites are which nested at a different location after the orig- the locations of nests at the colony site. inal colony was destroyedby wind, were marked. Colonies are identified by the name of the Eggswere numbered uniquely using a felt-tip breeding wetland and the year. All nests in each marker. They were then floated (Westerkov 1950, colony were marked with uniquely numbered 2 Nuechterlein 1975) to determine stage of incu- m x 1 cm diameter steel posts placed immedi- bation, and their color was recorded. Lengths of ately north of eachnest. When nestswere marked, flotation categories(Fig. 2) were determined us- water depth was measured on the west side of ing frequent checks (usually 1-3 day intervals) the nest with a calibrated wooden stick. At Har- of 100 eggsfound on the day of laying (chalky stad Slough in 1989, nests in both an original blue, unstained eggs).Length and breadth were colony (I) and a probable renesting colony (II), measured to the nearest 0.1 mm with plastic cal- EARED GREBE NEST SITE SELECTION 2 1 Stage Days Description 6 19+ eggfloats at about 45 ’ 16-21 egg floats vertically 14-19 4 egg suspends vertically in water lo-16 0 egg sinks vertically 4-l 0 3 egg sinks at about 45 ’ 2 1 egg sinks horizontally l-5 @b FIGURE 2. Sta :s of incubation of Eared Grebe eggsdetermined from flotation of 100 fresh eggs. ipers at all or most nests in the two West Toqua were marked. Once nest numbers were matched colonies. with bottles on the photo, I determined the scale Intervals between nest checks varied and de- using ground distances measured between se- pended upon the distance from other colonies, lected nests. size of the colony, and status of other marked Map Image Processing Software (MIPS) colonies.Number of checksranged from two (one (MicroImages, Inc. 1990) a geographic infor- after discovery of the colony, one near hatching) mation and computer-assisteddesign system, was to six (Table 1). Number ofnests, vegetation type used to determine x-y coordinates for each at the colony site, and phase of incubation when marked nest in 10 colonies. Then, a program the colony was discovered varied (Table 1). written by D. H. Johnson using SAS (SAS In- Nest sites were marked photogenically and stitute 1987) statistical software was used to cal- photographed from the air to determine nest culate nearest-neighbor distances. Nearest- placement. After nest abandonment, nest mark- neighbor distances were calculated for several ing postswere fitted with gallon plastic milk bot- stages of colony development and were com- tles that had been spray-painted fluorescent or- pared for colonies in several vegetation types. I ange. Low level (7 1 to 630 m above ground level; tested further for the effects of social attraction x = 179 m) 35 mm aerial photographs of these in nest site selectionby comparing the incubation “bottle colonies” were taken from a small, fixed- stageof the oldest eggin each nest to that in its wing aircraft with a belly camera mount. Prints nearest neighbor in three colonies with late stage or Mylar overlays of nest sites were enlarged to incubation data available for them. Becauselate scalesthat allowed labelling with nest numbers. stagesare shorter, they give better resolution of Bottles on the aerial photos were matched with age differencesbetween nests. A Spearman rank their nest numbers by taking the aerial photo to correlation was calculated for nests in each of the colony site or by using nearest-neighbor dis- those colonies. tances and directions recorded when the nests Eggvolume was calculatedusing Hoyt’s (1979) 22 JANET S. BOE TABLE 1. Number of nests,vegetation at colony site, Perimeter nests were defined as those with no phase of incubation (E[arly] = oldest egg in stagesl- other nests within a 120” arc around the nest 2, M[iddle] = stage3, L[ate] = stages4+) ofmost nests (AppendixesA and B). Once perimeter nestswere when colony marked, and number of nest checks in marked Eared Grebe colonies. defined, I used MIPS to calculate the area of the colony. Large colonies (2 50 nests) were then di- Colony Nests Vegetatmn Phase Checks vided into three concentric groupsof nests(edge, intermediate, and center) with approximately Swenson 87 19 algal mat E 4 equal numbers of nests in each group (Appendix W. Toaua 87 73 dead cattail E 4 (TYPhUSPP.) A). To maximize contrasts,I used only edge and Salt 88 15 pondweed mat E 4 center nests in comparisons. In small colonies (Potamogetonspp.) (< 50 nests;Appendix B) and in one linear, large Swan 88 325 bulrush island E 2 colony (West Toqua 87, Appendix A), only edge (Scirpus acutus) Thief 88 583’ bulrush islands/ E 3 and center nests were designated. An unusual pondweed mats pattern of nest placement in one large, nonlinear W. Toqua 88 121 dead cattail E 5 colony (Ash 89, Appendix A) led to a colony map Ash 89 97 cottonwood (Populus M 2 in which all edge nests were also perimeter nests deltoides)lwillow and most remaining nests were designated as (Salix amygdaloides and S. interior) center nests to equal approximately the number Harstad I 29 dead cattail E 5 of edge nests. At West Toqua 87, two discrete 89 subcolonies were formed and were considered Harstad II 88 dead cattail/coontail E 6 separatelywhen designatingedge and center nests, 89 mat (Ceratophyllum demersum) but for other analyses, subcolonies were com- Lane 89 18 bladderwort L 4 bined.