Nest Distance Mediates the Costs of Coloniality in Eared Grebes

Abstract

We examined the behaviors and reproductive characteristics of 4 Eared Grebe (Podiceps nigricollis) colonies located at the Malheur National Wildlife Refuge in eastern Oregon that varied in nesting density. In 1993, we studied 2 colonies found on 2 different lakes, whereas in 1994 we conducted a within-lake comparison of 2 subcolonies. During both years, nearest neighbor distances differed significantly between the colonies investigated. Breeding pairs were significantly more aggressive at those colonies with smaller distances among nests. Within each year, there was a greater rate of intraspecific brood parasitism and egg loss at the higher density colony. Infanticide occurred at high rates when nests were very densely packed, whereas a relatively high rate of predation by American Coots (Fulica americana) resulted when nests were quite dispersed. Thus, in spite of the distinct breeding characteristics of Eared Grebes, many disadvantages associated with nesting density in seabirds also have an impact on these colonially breeding waterbirds.

Effects of management practices on wetland birds: Eared Grebe

“…Vegetation such as algae, dead cattail (Typha spp. ), cottonwood (Populus deltoides)/willow (Salix amygdaloides, S. interior), common bladderwort (Utricularia vulgaris), coontail (Ceratophyllum demersum), sago pondweed, goosefoot (Chenopodium rubrum), or hardstem bulrush are used as nest material (Bryant 1983, See et al 1992, Boe 1993, Hill et al 1997. Nests may be anchored to emergent plants such as hardstem bulrush, alkali bulrush (Scirpus maritimus), sprangletop (Scolochloa festucacea), and golden dock (Rumex maritimus), or built on dense, floating mats of submergent vegetation or algae (Friley and Hendrickson 1937, Stewart 1975, See et al 1992, Boe 1993.…”

“…Distances between nests within a colony have been reported as 0 to 9.5 m, and depend on habitat conditions (Job 1902, Friley and Hendrickson 1937, McAllister 1958, Buresh 1971, Ray and Kruse 1989, Lyon and Everding 1996, Hill et al 1997. Hill et al (1997) reported that nests in a low-density colony were farther apart (3.3 versus 1.4 m, respectively) than in a high-density colony (numbers of nests within low-and high-density colonies were not given). Depredation rate was higher in the low-density colony than in the high-density colony.…”

“…Pied-billed Grebes may harass Eared Grebes, although young Pied-billed Grebes may associate with postbreeding Eared Grebes without aggression (Cullen et al 1999). American Coots may depredate Eared Grebe nests (Breault 1990, Boe 1994, Hill et al 1997. A table near the end of the account lists the specific habitat characteristics for Eared Grebes by study.…”

“…Five nests were multiply parasitized, and most cases of multiple laying were by different females. Of 21 conspecific brood parasitism events reported by Hill et al (1997), three nests each contained two parasitic eggs and 18 nests each contained one parasitic egg. No records of brood parasitism by Brown-headed Cowbirds (Molothrus ater) are known.…”

“…In cases where power lines must cross flyways, an attempt should be made to mask the lines with structures such as bridges (McKenna 1976). Power lines should be buried where possible and corridors established where power lines can be congregated to reduce their proliferation (McKenna 1976 with nesting areas close to shore than on wetlands with nesting areas far from shore; nests close to shore were more concealed but were more susceptible to nest depredation, whereas nests far from shore were less concealed but were less affected by nest depredation Hill et al 1997 Oregon Lake Nests were constructed of goosefoot or hardstem bulrush; nests were significantly closer (0.68 m apart) to one another at one of the study lakes than at the other study lake (1.55 m); distances between nests within two colonies in the same lake also differed significantly; nests in the low-density colony were 2.5 times farther apart than in the high-density colony …”