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Ecology and Management of Cowbirds and Their Hosts

[ Environment/Conservation ]

Ecology and Management of Cowbirds and Their Hosts

By James N. M. Smith, Terry L. Cook, Stephen I. Rothstein, Scott K. Robinson, and Spencer G. Sealy

Forty essays by most of the principal authorities on the biology and management of cowbirds.

2000

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Paperback

8.5 x 11 | 400 pp. | 125 figures, 93 figures

ISBN: 978-0-292-72689-5

In the past two centuries, cowbirds have increased in numbers and extended their range across North America, while many of the native songbird species whose nests they parasitize to raise their young have declined. This timely book collects forty essays by most of the principal authorities on the biology and management of cowbirds. The book's goals are to explore the biology of cowbirds, the threats they pose to host species and populations, and the management programs that are being undertaken to minimize these threats.

The book is organized into five sections, each with an extended editors' introduction that places the contributions in a broad, up-to-date setting. The sections cover:

  • The changing abundance of cowbirds and the ways in which their numbers can be estimated.
  • Host choice by cowbirds, the negative effects of cowbirds on particular host species, and the daily patterns of cowbird behavior.
  • Behavioral interactions between cowbirds and specific host species.
  • Patterns of cowbird abundance and host use across varying landscapes.
  • Management programs designed to control cowbirds and protect threatened songbirds.
  • Foreword: Cowbirds and Bull-hockey (Paul R. Ehrlich)
  • General Introduction: Brown-headed Cowbirds as a Model System for Studies of Behavior, Ecology, Evolution, and Conservation Biology (James N. M. Smith and Stephen I. Rothstein)
  • Part I. Population Trends of Cowbirds and Hosts and Relevant Methodology
    • 1. Introduction (Stephen I. Rothstein and Scott K. Robinson)
    • 2. Temporal and Geographic Patterns in Population Trends of Brown-headed Cowbirds (Bruce G. Peterjohn, John R. Sauer, and Sandra Schwarz)
    • 3. Cowbird Population Changes and Their Relationship to Changes in Some Host Species (David A. Wiedenfeld)
    • 4. The Spread of Shiny and Brown-headed Cowbirds into the Florida Region (Alexander Cruz, John W. Prather, William Post, and James W. Wiley)
    • 5. Brown-headed Cowbird Population Trends at a Large Winter Roost in Southwest Louisiana, 1974-1992 (Brent Ortego)
    • 6. An Evaluation of Point Counts and Playbacks as Techniques for Censusing Brown-headed Cowbirds (R. Kirk Miles and David A. Buehler)
    • 7. The Structure and Function of Cowbird Vocalizations and the Use of Playbacks to Enhance Cowbird Detectability: Relations to Potential Censusing Biases (Stephen I. Rothstein, Chris Farmer, and Jared Verner)
  • Part II. Cowbird Spacing Behavior, Host Selection, and Negative Consequences of Parasitism for Commonly Used Hosts
    • 8. Introduction (James N. M. Smith, Spencer G. Sealy, and Terry L. Cook)
    • 9. Spatial Patterns of Breeding Female Brown-headed Cowbirds on an Illinois Site (Arlo Raim)
    • 10. Differences in Movements, Home Range, and Habitat Preferences of Female Brown-headed Cowbirds in Three Midwestern Landscapes (Frank R. Thompson III and William D. Dijak)
    • 11. The Effects of Host Numbers on Cowbird Parasitism of Red-winged Blackbirds (Christy A. Carello and Gregory K. Snyder)
    • 12. Cowbird Brood Parasitism on a Little-used Host: The Yellow-headed Blackbird (Alfred M. Dufty, Jr.)
    • 13. Host Selection in the Forest Interior: Cowbirds Target Ground-nesting Species (D. Caldwell Hahn and Jeff S. Hatfield)
    • 14. Reproductive Interactions between Brown-headed Cowbirds and Plumbeous Vireos in Colorado (Jameson F. Chace, Alexander Cruz, and Rebecca E. Marvil)
    • 15. Effects of Multiple Parasitism on Cowbird and Wood Thrush Nesting Success (Cheryl L. Trine)
    • 16. Comparing the Relative Effects of Brood Parasitism and Nest Predation on Seasonal Fecundity in Passerine Birds (Joseph A. Grzybowski and Craig M. Pease)
  • Part III. Host-Cowbird Behavioral Interactions
    • 17. Introduction (James N. M. Smith and Spencer G. Sealy)
    • 18. Morning Nest Arrivals in Cowbird Hosts: Their Role in Aggression, Cowbird Recognition, and Host Response to Parasitism (Dirk E. Burhans)
    • 19. Yellow Warbler Nest Attentiveness before Sunrise: Antiparasite Strategy or Onset of Incubation? (Spencer G. Sealy, D. Glen McMaster, Sharon A. Gill, and Diane L. Neudorf)
    • 20. The Ecology of Egg-puncture Behavior by the Shiny Cowbird in Southwestern Puerto Rico (Tammie K. Nakamura and Alexander Cruz)
    • 21. Why Do Female Brown-headed Cowbirds Remove Host Eggs? A Test of the Incubation Efficiency Hypothesis (Brian D. Peer and Eric K. Bollinger)
  • Part IV. Environmental Correlates of Cowbird Parasitism at Multiple Spatial Scales
    • 22. Introduction (Scott K. Robinson and James N. M. Smith)
    • 23. The Role of Vegetation in Cowbird Parasitism of Yellow Warblers (William H. Howe and Fritz L. Knopf)
    • 24. Association of Cowbird Parasitism and Vegetative Cover in Territories of Southwestern Willow Flycatchers (Jamie C. Uyehara and Mary J. Whitfield)
    • 25. Interhabitat Differences in Parasitism Frequencies by Brown-headed Cowbirds in the Okanagan Valley, British Columbia (David Ward and James N. M. Smith)
    • 26. Cowbird Parasitism and Nest Predation in Fragmented Grasslands of Southwestern Manitoba (Stephen K. Davis and Spencer G. Sealy)
    • 27. Cowbird Parasitism in Grassland and Cropland in the Northern Great Plains (Rolf R. Koford, Bonnie S. Brwn, John T. Lokemoen, and Arnold D. Kruse)
    • 28. Distribution and Habitat Associations of Brown-headed Cowbirds in the Green Mountains of Vermont (Daniel R. Coker and David E. Capen)
    • 29. Impacts of Cowbird Parasitism on Wood Thrushes and Other Neotropical Migrants in Suburban Maryland Forests (Barbara A. Dowell, Jane E. Fallon, Chandler S. Robbins, Deanna K. Dawson, and Frederick W. Fallon)
    • 30. Cowbird Distribution at Different Scales of Fragmentation: Trade-offs between Breeding and Feeding Opportunities (Therese M. Donovan, Frank R. Thompson III, and John R. Faaborg)
    • 31. Brown-headed Cowbird Parasitism of Migratory Birds: Effects of Forest Area and Surrounding Landscape (Lisa J. Petit and Daniel R. Petit)
    • 32. Biogeographic, Landscape, and Local Factors Affecting Cowbird Abundance and Host Parasitism Levels (Frank R. Thompson III, Scott K. Robinson, Therese M. Donovan, John R. Faaborg, Donald R. Whitehead, and David R. Larsen)
    • 33. Cowbird Parasitism in a Fragmented Landscape: Effects of Tract Size, Habitat, and Abundance of Cowbirds and Hosts (Scott K. Robinson, Jeffrey P. Hoover, and James R. Herkert)
    • 34. Within-landscape Variation in Patterns of Cowbird Parasitism in the Forests of South-central Indiana (Donald E. Winslow, Donald R. Whitehead, Carolyn Frazer Whyte, Matthew A. Koukal, Grant M. Greenberg, and Thomas B. Ford)
    • 35. Effects of Land-use and Management Practices on the Presence of Brown-headed Cowbirds in the White Mountains of New Hampshire and Maine (Mariko Yamasaki, Toni M. McLellan, Richard M. DeGraaf and Christine A. Costello)
  • Part V. Cowbird Management, Host Population Limitation, and Efforts to Save Endangered Species
    • 36. Introduction (Stephen I. Rothstein and Terry L. Cook)
    • 37. Brown-headed Cowbird Control on Kirtland's Warbler Nesting Areas in Michigan, 1972-1995 (Michael E. DeCapita)
    • 38. Cowbird Control and the Endangered Least Bell's Vireo: A Management Success Story (John T. Griffith and Jane C. Griffith)
    • 39. Cowbird Control Program at Fort Hood, Texas: Lessons for Mitigation of Cowbird Parasitism on a Landscape Scale (Timothy J. Hayden, David J. Tazik, Robert H. Melton, and John D. Cornelius)
    • 40. Results of a Brown-headed Cowbird Control Program for the Southwestern Willow Flycatcher (Mary J. Whitfield)
  • Contributors
  • Index

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In this introduction, we present background information on the behavior, ecology, and evolution of brood parasites, with special reference to the Brown-headed Cowbird. Our intent here is to highlight the considerable breadth of biological questions and conservation issues that arise from studying brood parasites. We also describe the genesis of this book and briefly comment on how cowbirds may affect the population levels and even survival of other species. The connection between cowbirds and conservation biology is especially interesting for two reasons. Cowbirds can affect numerous other species, and the anthropogenic changes to landscapes that have benefited the cowbird may simultaneously have harmed other bird species. This intertwining of anthropogenic and natural biotic effects in turn raises the question of whether declines of songbirds are due to parasitism or habitat loss, or some combination of these two potential impacts.

Three species of cowbird now occur in North America, the Brown-headed Cowbird (Molothrus ater), the Bronzed Cowbird (Molothrus aeneus), and the Shiny Cowbird (M. bonariensis). Informative accounts of the general biology of the native Brown-headed and Bronzed Cowbirds are given by Lowther (1993, 1995). The third species, the Shiny Cowbird, is a recent immigrant from the south (see Cruz et al. Chapter 4). A recent and detailed review of the behavior and ecology of cowbirds is given by Robinson et al. (1995a). Much additional information on the biology of avian brood parasites will be elaborated in a forthcoming book (Rothstein and Robinson 1998). That information is complementary with the more applied work that is the main, but not sole, focus of this book. Together, we hope that these two books will help to bring the many unsolved basic and applied questions about brood parasites to a wider audience and into sharper relief.

The Brood Parasitic Life History

Brood parasitism is an unusual and fascinating life history found in some birds, insects, and fishes that share the common habit of rearing their young in a fixed nest (Clutton-Brock 1991). Brood parasites lay their eggs in the nests of other individuals or colonies, thus duping the unsuspecting foster parents into providing parental care. Some brood parasites, like the cowbirds discussed in this book, are obligate, in that they never rear their own young and lay only in the nests of other species. Other animals, like many species of colonial waterfowl, are facultative brood parasites, in which an individual may lay some eggs in the nests of conspecifics or other similar species while incubating the rest of its eggs in its own nest (Lyon and Eadie 1991).

Because they depart so strongly from the usual avian model of strong parental care, brood parasitic birds have always attracted much attention, and numerous authors over the last century have collected data on cowbirds in particular. Much of these data were collected incidentally in the course of nesting studies focused on various host species. There was little attempt to synthesize what was known, with the notable exception of Herbert Friedmann's studies from the 1920s to the 1980s, and the reviews by Lack (1968) and Payne (1977). Indeed, most research that has focused on brood parasitism itself has appeared only in the last two decades. It has recently become apparent that brood parasites and their hosts raise many fascinating problems in five major biological disciplines: behavior (Davies and Brooke 1988, Lotem et al. 1995), population ecology (May and Robinson 1985, Robinson et al. 1995b), evolution (Rothstein 1975a, 1990), neurobiology (Sherry et al. 1993, Clayton et al. 1997), and conservation biology (Robinson et al. 1995a, b). A brief account of the general biology of brood parasites and of cowbirds in particular follows as an introduction to this book.

The principal life history trade-off involved in brood parasitism is that by foregoing parental care, brood parasites greatly increase their annual fecundity. Most workers are agreed that wild Brown-headed Cowbirds lay eggs on 70-80% of days during a two- to four-month breeding season, for a total of at least 40 and perhaps as many as 100 eggs per year (Scott and Ankney 1980, 1983; Fleischer et al. 1987; Smith and Arcese 1994). In captivity, one female laid more than 70 eggs, and fecundity depended strongly on female age (Holford and Roby 1993). Kaftan (1997) reports fecundities of 120 eggs per year in wild Shiny Cowbirds. However, recent estimates using molecular methods to track egg laying by individual females (e.g., Gibbs et al. 1997) have revealed that individual fecundities can also be much lower, perhaps 5 to 15 eggs per year (S. G. Sealy pers. comm., D. C. Hahn pers. comm.). In contrast, the average female in two short-lived, open-nesting songbirds, the Song Sparrow (Melospiza melodia) and the Meadow Pipit (Anthus pratensis), lays 7-11 eggs per year (Smith 1988, Hotker 1989). Thus, the fecundity advantage gained by brood parasitism is at least two- to three-fold and may exceed ten-fold. High fecundity gives cowbirds a high potential intrinsic rate of population growth when they colonize new areas, as they have done almost continuously in the past 200 years (Rothstein 1994).

Reproductive success depends on more than high fecundity, however, and survival of eggs and young in brood parasites is presumably lower than in parental species. In many cases, cowbirds lay eggs in nests of inappropriate host species or lay at inappropriate times (Robinson et al. 1995b, Kaftan 1997). A review of studies on a range of host species led Scott and Ankney (1980) to conclude that only about 15% of cowbird eggs result in fledglings. Cowbirds can, however, achieve much higher survival from laying to fledging and may even match the fledging success of especially suitable hosts such as the Song Sparrow (Smith and Arcese 1994). It is not known how the reproductive strategy of cowbirds influences their life span, but there is little reason to believe that they are more long-lived than other similarsized songbirds.

One noteworthy aspect of the life history of cowbirds is the strongly male-biased sex ratios, which contrast with the female-biased sex ratios typical of nonparasitic blackbirds (Orians 1980). This bias implies that there is a survival cost to the high fecundity of female cowbirds. The survival cost is further indicated by Darley's (1971) discovery that males and females have similar survival rates in their first year of life, when females have not yet laid eggs, but that survivorship among mature females is lower than for males. Keys et al. (1986) documented a possible physiological cost, lowered volumes of red blood cells, that females may incur because of their extended egg laying (but see Ankney and Scott 1980).

The Evolution of Brood Parasitism

The evolution of obligate brood parasitism as a reproductive strategy is not well understood. One possibility is that it evolved via intraspecific nest parasitism (Nolan and Thompson 1975). Two facts, however, suggest that this explanation is unlikely. First, obligate parasitism is rare in waterfowl, the group of birds with the highest levels of intraspecific parasitism. Second, intraspecific parasitism is rare in blackbirds closely related to cowbirds (Lyon et al. 1992, Rothstein 1993). Another hypothesis is that it arose via the habit of stealing the nests of other birds (Friedmann 1929). Nest stealing by the so-called Bay-winged Cowbird (Molothrus badius) is common (Fraga 1998), but this nonparasitic species is actually not closely related to other cowbirds (Lanyon 1992).

Present-day classifications indicate that obligate brood parasitism has evolved seven times, as it occurs in seven different taxa: Old World cuckoos (family Cuculidae, subfamily Cuculinae), New World cuckoos (Cuculidae, subfamily Neomorphinae), honeyguides (Indicatoridae), blackbirds (Emberizidae, subfamily Icterinae), widow birds (Estrildidae, subfamily Viduinae), Old World sparrows (Ploceidae), and ducks (Anatidae). The last two taxa each contain a single obligately parasitic species and numerous nonparasitic ones. Recent DNA studies suggest that the single parasitic Ploceid, the Cuckoo Finch (Anomalospiza imberbis), is related to the widow birds rather than to Ploceids (R. Payne pers. comm.), which could mean that obligate parasitism has evolved only six times.

Obligate brood parasites differ greatly in their degree of specialization on host species. Most African widow birds parasitize only a single host species and show remarkable mimicry of the host's song, nestling behavior, and even the gape markings inside the mouth of the nestling (Nicolai 1964, Payne 1982). Other brood parasites, like the wellstudied Common Cuckoo (Cuculus canorus), usually specialize on a single host species in any particular part of Europe (Brooke and Davies 1987). The degree of local specialization, however, is only moderate (Moksnes and Røskaft 1995), and there seems to be little genetic differentiation between cuckoos with different mimetic egg types (Gibbs et al. 1996). Further flexibility in host use is shown by cuckoos in Japan, where the same cuckoo commonly parasitizes several hosts in a single region and often does so at much higher rates than seen in Europe (Nakamura et al. 1998). When several Old World cuckoo species coexist locally, they tend to parasitize different host species (Friedmann 1948, Higuchi and Sato 1984).

The five species of parasitic cowbirds are remarkable because they include both the most generalized and nearly the most specialized among the world's 80 or so known obligate brood parasites. The most specialized is the Screaming Cowbird (M. rufoaxillaris), which was known to parasitize only one host species until two other hosts were noted in the last decade (Fraga 1996, 1998; Rothstein et al. in prep.) The two most generalized cowbirds, the Brown-headed and the Shiny, have by far the largest number of known host species among brood parasites. Each has been recorded laying in the nests of more than 200 other species (Friedmann and Kiff 1985, Friedmann et al. 1977), and individuals are known to lay in the nests of several hosts (Fleischer 1985, S. G. Sealy pers. comm.). Host specialization has the benefit of increasing the chance of choosing a suitable host, but it decreases the number of potential hosts and therefore the growth potential of a parasitic population. Also, there is a risk that a host will evolve a successful defense against a specialized parasite, if the parasite exerts a strong selection pressure on its host. This defense could, in turn, select for a host shift on the part of the parasite (Rothstein 1990).

Reviews of coevolution between parasitic birds and their hosts have suggested that generalized parasites will become more specialized over time as more and more host species evolve defenses such as egg recognition (Davies and Brooke 1989a, Rothstein 1990). According to these arguments, increasing specialization is likely because the most effective adaptation by the parasite to host egg recognition is mimicry of the host egg. Genetic constraints (the need for assortative mating among genotypes) mean that a parasitic population cannot be subdivided to produce many different genetically determined egg types at any one time.

In contrast to previous work, Lanyon (1992) argued that cowbirds have become progressively more specialized over time because the first species to branch off in the cowbird clade is the most specialized, while the two most recent species are the most generalized. Rothstein et al. (in prep.), however, have argued that host number is too labile for one to assume that current host numbers reflect numbers when each cowbird species appeared (see also Freeman and Zink 1995). They have also pointed out that the relationship between branching order and host number is consistent with the hypothesis of increasing specialization, because the potential hosts of the oldest cowbird species have had the greatest amount of time to develop defenses. Neither hypothesis, that of increasing versus decreasing specialization over time, can be invalidated at present.

Behavioral Evolution in Brood Parasites and Their Hosts

The best-studied behavioral traits that affect the success of brood parasites are egg ejection and nest desertion by hosts. In Common Cuckoos, which show strong host specialization, egg ejection is a population-specific trait in at least some hosts, and its expression depends on the historical association of the population with cuckoos (Davies and Brooke 1989a) and even on the present levels of parasitism (Davies et al. 1996). In some populations of cuckoo hosts, egg ejection or nest desertion is maintained at intermediate levels by the costs of egg recognition (Davies and Brooke 1989b, Davies et al. 1996) or by the need for young birds to learn to recognize their own eggs (Lotem et al. 1995).

Hosts eject Brown-headed Cowbird eggs in two ways. In most species, the female host grasps the cowbird egg with her open beak and removes it, something that is easier for larger hosts to do (Rothstein 1975a). In a few species, females spike the cowbird egg with their beak to remove it (e.g., Rothstein 1977, Sealy 1996).

Nearly all actual and potential hosts of Brown-headed and Shiny Cowbirds are apparently made up entirely of acceptor or rejector individuals (Rothstein 1975a,b; Mason 1986; but see Dufty 1994, Sealy and Bazin 1995). That is, close to 100% of individuals eject cowbird eggs placed in their nests, or close to 100% accept them.

Many studies have shown that variable numbers of individuals within acceptor species desert naturally parasitized nests and renest (Rothstein 1975a, b; Graham 1988; Hill and Sealy 1994; Goguen and Matthews 1996). Such desertions seem not to be caused by the cowbird eggs themselves (Rothstein 1975a, 1982, 1986) but rather by the presence of cowbirds near the host nest (Rothstein 1975a, Burhans Chapter 18) or by clutch reduction when eggs are removed by cowbirds (Rothstein 1982, Hill and Sealy 1994, Sealy 1995). Grzybowski and Pease (Chapter 16) use simulation models to explore how the effects of nest desertion interact with varying frequencies of parasitism.

A puzzling fact is that most cowbird host species do not eject cowbird eggs, even when their eggs differ greatly from the nonmimetic cowbird egg. The absence of egg ejection in many frequent cowbird hosts is often explained under the evolutionary lag hypothesis (Rothstein 1990, Ward et al. 1996), where ejection is assumed to be adaptive but has not evolved because of the absence of the appropriate genetic variation in host populations. Alternatively, ejection behavior may not have evolved because it inflicts greater costs than rearing the parasite, the evolutionary equilibrium hypothesis (Davies and Brooke 1989a, Petit 1991, Rohwer et al. 1989, Brooker and Brooker 1996). Finally, rejection of brood parasite eggs is unlikely to evolve in species with low historical or present levels of parasitism (Brooke and Davies 1987). Rejection costs can arise from errors in egg recognition (Davies and Brooke 1988, Lotem et al. 1995), which can lead hosts to reject their own eggs. Most cowbird hosts, however, have eggs that are easily distinguished from the egg of the cowbird. If a host is too small to eject a parasitic egg easily, it might break some of its own eggs during ejection attempts (Rothstein 1976a, Rohwer et al. 1989). This cost of rejection is probably low for most acceptor species, especially given Sealy's (1996) demonstration that a small host can eject cowbird eggs with minimal cost. A host might have to pay the costs of renesting if desertion is its only option (Rohwer and Spaw 1988). Nest site limitation in the Prothonotary Warbler (Protonotaria citrea) may make renesting difficult, thus favoring acceptance of cowbird eggs (Petit 1991).

Although egg ejection is lacking in many species in which it would seem to be adaptive, it is still a widespread response to brood parasitism relative to the rarity of discrimination against parasitic chicks by the foster parents. In fact, although there are some cases of clear nestling mimicry among parasitic birds, no bird is known to reject nonmimetic nestlings. Lotem (1993, Lotem et al. 1995) has attributed the absence of nestling rejection to the risk that a host individual may reject its own nestlings and accept only parasites if it learns to recognize its own young from the first nestling that appears in the nest. Such a "first to appear" learning mechanism underlies rejection of cowbird eggs in the Gray Catbird (Rothstein 1974).

Perhaps the most familiar behavioral adaptation of brood parasites is ejection behavior of the cuckoo chick. Within a few hours of hatching, day-old Common Cuckoo chicks eject host eggs or nestlings by pushing themselves under the egg or chick and pushing the luckless victim from the nest (Chance 1922). There is a recent report of ejection behavior by a Brown-headed Cowbird chick (Dearborn 1996), and it remains to be determined whether this is an isolated case of an accidental ejection or an example of a rare but purposeful behavior. Newly hatched honeyguides are equipped with piercing mandibles and use them to kill their foster siblings (Friedmann 1955).

Adult brood parasites also may use destruction of host eggs or young to persuade hosts to accept their eggs. Soler et al. (1995) presented experimental evidence that Great-spotted Cuckoos (Clamator glandarius) punish their magpie hosts by destroying the magpie eggs if the magpies eject parasitic cuckoo eggs. Additional work on this system is needed to determine whether such punishment is a regular feature of the cuckoo-magpie interaction, because the same research group had previously shown that the magpies recently increased the rate at which they reject cuckoo eggs despite the possible punishment (Soler et al. 1994). It is unlikely that comparable punishment occurs with cowbird parasitism, because numerous researchers have removed cowbird eggs from parasitized nests without reporting such behavior (Rothstein 1982). However, a related claim has been made for cowbirds by Arcese et al. (1996). They proposed that female cowbirds prey on host eggs and nestlings to force hosts whose nests cannot be parasitized successfully to relay and thus increase the supply of future nests that can be parasitized. Experimental work by Smith and Taitt (unpubl. data) supports a prediction of this idea, that the rate of host nest failure should decline sharply when cowbirds are removed, a result also reported in this volume by Whitfield. Other studies, however, have not found this result (Braden et al. 1997, Stutchbury 1997). If predation by brood parasites occurs commonly, it could depress host breeding success to a greater degree than is evident from the normal costs of parasitism, egg removal, and losses of host young while rearing parasitic young.

Brood parasites face several other interesting behavioral problems. They have to locate appropriate host nests at the best time for laying, during the laying period of the host. They also have to lay in the nests of potentially aggressive hosts, who usually recognize them as enemies (Robertson and Norman 1976, Neudorf and Sealy 1992) and may be capable of injuring them. Brood parasites solve these problems by laying very rapidly (Davies and Brooke 1988, Sealy et al. 1995) and by doing so at dawn, when hosts are least likely to detect them (Scott 1991, Burhans Chapter 18, Sealy et al. Chapter 19).

Finding host nests must require strong skills in nest searching behavior, a measure of caution when approaching nests of large host species, and a specialized memory. Sherry et al. (1993) have shown that eastern female Brown-headed Cowbirds (M. a. ater) develop a larger hippocampus, the site of spatial memory in birds and mammals, during the nesting season, presumably to help them keep track of the locations of potential host nests. There may, however, be regional variation in this trait, as Uyehara and Narins (in review) failed to find a hippocampal difference in western cowbirds (M. a. artemisiae).

Another problem faced by brood parasites is the development of species recognition and of species-specific behavioral patterns such as songs. just how cowbirds come to know that they are cowbirds is unclear, but the process appears to be virtually foolproof. There are no reports of cowbirds in nature attempting to court heterospecifics or singing the songs of other species. The lack of such reports led some workers to speculate that the cowbird's development of species recognition and of song is innate and resistant to experience (e.g., Mayr 1979). Cowbirds would thus differ from most other songbirds, whose song development depends heavily on learning.

In fact, we now know that although some cowbird behaviors develop nearly normally in complete isolation from conspecifics (King and West 1977), much of the behavior of cowbirds is learned, and even innate cowbird behaviors can be modified in response to experience (O'Loghlen and Rothstein 1993, West and King 1996). Indeed, some aspects of the cowbird's vocal repertoire, such as the development of flight whistles, show a greater input from learning than do the songs of most nonparasitic songbirds, as described by Rothstein et al. (Chapter 7). In recent years, cowbird studies have become prominent in the vast literature on vocal development and are widely cited in basic animal behavior texts (e.g., Alcock 1993) for two reasons: first, because of their intricate developmental systems, and second, because of the large geographic variation in their vocalizations on both local and continental scales (Rothstein and Fleischer 1987, King and West 1990). Last, cowbirds have been recognized as attractive subjects for controlled learning experiments because they adapt so readily to captivity.

Host Choice by Brood Parasites

There are several parallels between the reproductive decisions of brood parasites and the choices made by foraging animals. Like foragers (Stephens and Krebs 1986), brood parasites exhibit functional responses to host density, i.e., they lay more eggs in denser host populations (Smith and Arcese 1994). Also, like foragers choosing more nutritious foods, generalist parasites ought to select the host species that are most successful at rearing their young. Host species of cowbirds vary in their suitability for several reasons: some are egg ejectors, some desert their nests when parasitized, and others feed their nestlings a mainly vegetable diet, on which young cowbirds cannot survive. We might therefore expect cowbirds to avoid such unsuitable species and to select out the same set of best host species across regions.

In apparent agreement with this general expectation, certain host species do seem to be consistently favored by cowbirds across large areas. The Red-eyed Vireo (Vireo olivaceus) stands out as perhaps the most heavily used of all hosts (see, e.g., Thompson et al. Chapter 32, Winslow et al. Chapter 34, Hahn and Hatfield Chapter 13). Other larger hosts, like Song Sparrows, may be used less frequently than Red-eyed Vireos (e.g., Peck and James 1987) but may contribute more cowbirds to the continental population. There are, however, host species like the Red-winged Blackbird (Agelaius phoeniceus) that are sometimes used heavily but their frequency of use varies greatly among regions and sites (see Carello and Snyder Chapter 11). Other similar examples are described in this book.

Cowbirds may avoid parasitizing ejector host species (Sealy and Bazin 1995), but it is hard to estimate how often ejector host species are used by brood parasites, because parasitic eggs disappear rapidly from the host nests if they are laid there (Scott 1977). Some studies have shown frequent use of ejectors, even when unparasitized nests of acceptors were available (Rothstein 1976b, Scott 1977, Friedmann et al. 1977:36-37).

While there have been few explicit studies of the fitness consequences of host selection for cowbirds, the evidence to date is far from compelling that they generally select the best hosts available. Weatherhead (1989) found similar frequencies of parasitism in Red-winged Blackbirds and Yellow Warblers (Dendroica petechia), even though the blackbirds were far more effective hosts. At the same study site, Briskie et al. (1990) found that Least Flycatchers (Empidonax minimus), another good host, were used much less often than the less suitable Yellow Warbler. Although such comparisons of species pairs are a useful start in examining host choice, a major current gap in the cowbird-host literature is communitywide studies that compare the use of hosts by cowbirds with their relative nest abundances (e.g., Hahn and Hatfield Chapter 13). In summary, Brown-headed Cowbirds clearly exhibit host selectivity, but they apparently do not always make the best choices among the local spectrum of potential hosts. Shiny Cowbirds may be even less selective (Kaftan 1997).

Another reproductive decision facing a brood parasite is, when it finds a suitable nest, how many eggs to lay in that nest. Since songbird nests often fail completely, and sibling parasites are likely to compete strongly with each other in the nests of smaller hosts, a simple rule would seem to be to lay one egg only per nest. However, if suitable nests are in short supply, and if hosts can rear more than one parasite chick, it should then be adaptive for the parasite to lay more than one egg in the same nest. Multiple parasitism occurs in about one third of all reported cases of parasitism (Friedmann 1963, Lowther 1993), and most students of brood parasitism (e.g., Orians et al. 1989) have assumed that eggs are generally laid at random by cowbird females without regard to whether nests are already parasitized. When several parasitic eggs are laid in a single host nest, it is thus possible that this results from scramble competition between several female parasites for limited hosts and not from adaptive laying behavior (but see Smith and Arcese 1994). In opposition to this argument, laying more than one egg per host nest is commoner, and likely advantageous, in the larger hosts that can often rear more than one cowbird per nest (see Robinson et al. Chapter 33, Trine Chapter 15).

Population Dynamics of Avian Brood Parasites

Cowbirds are abundant in many parts of North America. One conservative estimate of continental numbers is 20 million-40 million (Lowther 1993), but Ortego (Chapter 5) reports 38 million cowbirds at a single roost. It is therefore likely that Brown-headed Cowbirds number 100 million or more across the continent. There have been no detailed studies of populations of cowbirds or other brood parasites because of their high mobility and the large areas over which breeding individuals move (Rothstein et al. 1984, Thompson and Dijak Chapter 10). Thus far, only models of host-parasite interactions have provided much useful insight into the dynamics of brood parasite populations (e.g., May and Robinson 1985, Pease and Grzybowski 1995).

In addition to this major gap, there have been few studies of the effects of brood parasitism on host population dynamics. These may be small in populations where parasitism is infrequent or when hosts can escape in time (e.g., Smith and Arcese 1994). They can be large in landscapes where parasitism is frequent and breeding habitat for cowbirds is limited (Robinson et al. 1995a,b; Brawn and Robinson 1996; Rogers et al. 1997). Cowbird-host dynamics are also likely to vary geographically (e.g., Weatherhead 1989).

It is also not well known which host species do most to promote cowbird numbers. Some of the limited available knowledge on this topic was assembled recently by Scott and Lemon (1996). Scott and Lemon noted that Song Sparrows, Red-winged Blackbirds, and perhaps Chipping Sparrows seem to be particularly successful hosts. Trine (Chapter 15) also found Wood Thrushes to be very effective hosts at rearing cowbirds. In riparian communities in coastal British Columbia, which lack many other abundant and suitable hosts, Song Sparrows care for about 75% of all fledgling cowbirds being fed by foster parents and routinely rear two or more cowbird fledglings from a single nest (J. N. M. Smith, unpubl. data).

Community Effects of Cowbirds

As abundant host generalists, Brown-headed Cowbirds have the potential to generate strong population and community-level interactions among their hosts, i.e., to act as a keystone species (Power et al. 1996). If a widespread host species is used heavily and is vulnerable to the effects of parasitism, it may be driven to become rare locally (Robinson et al. 1995b). If a rare host species is used heavily, it may be extirpated (Robinson et al. 1995a). In areas where many hosts are heavily parasitized, communities may be made up of more resistant host individuals of perhaps fewer species (De Groot et al. in press). On the other hand, cowbirds might increase local host diversity by concentrating their parasitism on especially competitive host species, thus releasing weaker competitors from interspecific competition (Holt 1984). In contrast, in areas where cowbirds are uncommon, they are unlikely to generate strong population or community effects in their hosts.

There is little information available on any of these topics, other than a recent simulation model by Grzybowski and Pease (in press). Several chapters in this book, however, attempt to determine the extent of cowbirds' effects on other species.

The Impacts of Brown-headed Cowbirds on Their Hosts

The notion that cowbirds might have serious impacts on entire host populations was initially raised by the researchers who found some of the first Kirtland's Warbler (Dendroica kirtlandii) nests (Leopold 1924, Wood 1926). The first detailed accounts of the effects of cowbirds on hosts came from Nice's (1937) work on Song Sparrows and Mayfield's studies on the Kirtland's Warbler (Mayfield 1960). Nice, Mayfield, and later Walkinshaw (1983) noted that increasing frequencies of cowbird parasitism were associated with low host reproductive success and declines in host numbers. Further examples are discussed by Robinson et al. (1995b).

Echoing an earlier paper by Mayfield (1977), Brittingham and Temple (1983) published an influential article that noted the spectacular range expansion of the cowbird and its heavy use of hosts breeding at forest edges. They argued that cowbirds could depress numbers of many host species in fragmented habitats. Brittingham and Temple's paper helped to propel the cowbird to the front of the conservation stage and stimulated much applied work on cowbirds. Soon afterward, the popular book by Terborgh (1989) took Brittingham and Temple's findings and used them to fuel the notion of the cowbird as a "conservation villain," a notion that has since taken firm hold among naturalists (Holmes 1993).

Against this backdrop of rising interest in the possible impacts of cowbirds on North American songbirds, a meeting of biologists and managers interested in cowbirds and their ecological effects was held in Austin, Texas, in November 1993. This meeting, which was discussed by Rothstein and Robinson (1994) and Holmes (1993), brought together for the first time numerous researchers with an academic interest in brood parasitism and conservationists and habitat managers who regarded the cowbird as a management problem in need of urgent action. The papers presented at this meeting and the overview chapters discussing them form the backbone of this book.

James N. M. Smith teaches at the University of British Columbia in Vancouver. Terry L. Cook works for the Nature Conservancy in Seattle. Stephen I. Rothstein teaches at the University of California, Santa Barbara. Scott K. Robinson holds joint appointments at the Illinois Natural History Survey and the University of Illinois, Champaign. Spencer G. Sealy teaches at the University of Manitoba.