Scott K. Robinson

Structure and Organization of an Amazonian Forest Bird Community

To help fill the gap in detailed knowledge of avian community structure in tropical forests, we undertook a census of a 97—ha plot of floodplain forest in Amazonian Peru. The plot was censused over a 3—mo period spanning the 1982 breeding season. The cooperative venture entailed ≈12 person—months of effort. Conventional spot—mapping was the principal method used, but several additional methods were required to estimate the numbers of non—territorial and group—living species: direct counts of the members of mixed flocks, saturation mist—netting of the entire plot, opportunistic visual registrations at fruiting trees, determination of the average size of parrot flocks, color banding of colonial icterids, etc. Two hundred forty—five resident species were found to hold territories on the plot, or to occupy all or part of it. Seventy—four additional species were detected as occasional—to—frequent visitors, wanderers from other habitats, or as migrants from both hemispheres. By superimposing territory maps or t...

Interspecific aggression and habitat selection by Amazonian birds

We report that interspecific territoriality is a common spacing mechanism among closely related bird species distributed along a primary successional gradient in the meander belt of an Amazonian whitewater river. In the first phase of the research, we mapped the territories of more than 330 bird species in large census plots encompassing the complete successional gradient. We found species pairs in over 20 genera that showed contiguous but non-overlapping territories, such that early stages of the successional gradient were occupied by one member of the pair, and later stages by the other. Other species pairs showed additional types of spatial relationships, including partially overlapping and completely overlapping territories. Using reciprocal heterospecific playback experiments, we tested for the existence of aggressive interactions between the members of species pairs (usually congeners) showing all three types of spatial relationship. Among 12 species pairs showing contiguous, non-overlapping territories, 10 showed evidence of interspecific aggression (approach to the playback speaker). In eight of these 10 cases, the responses were markedly asymmetric. Target individuals of one species approached the speaker, whereas individuals of the other species remained in place or moved away. The heavier species was consistently the aggressor. In six species pairs showing partial territorial overlap along the successional gradient, only one species displayed statistically significant interspecific aggressiveness, although some individuals in all six pairs approached or avoided the speaker. In species pairs distributed with completely overlapping territories, we found interspecific aggression in only one of five genera tested. We suggest that spatial segregation of congeneric bird species on habitat gradients (presumably including elevational gradients), is commonly underpinned by interspecific territoriality mediated through directed, asymmetrical interspecific aggression. Aggressive asymmetry suggests a despotic model of habitat occupancy, in which the larger species occupies the more productive end of habitat gradients, and the smaller species occupies less productive habitats. The pronounced successional gradients characteristic of Amazonia may explain much of the increased species richness, especially within genera, of Amazonian bird communities.

FOREST BIRD COMMUNITY STRUCTURE IN CENTRAL PANAMA: INFLUENCE OF SPATIAL SCALE AND BIOGEOGRAPHY

Historical and biogeographic contexts can play important, yet sometimes overlooked, roles in determining structure of local communities. In particular, few examinations of historical influences on patterns of species richness and relative abundances in tropical communities have been conducted. In part, that gap in our knowledge has been caused by a paucity of data on tropical communities, even for relatively well-studied taxa such as birds. In the Neotropics, only two sites, a 97-ha plot in lowland Peru and a 100-ha plot in French Guiana, have been inventoried on a spatial scale sufficient to estimate population densities for a majority of resident bird species. Results from those studies revealed extremely similar species richness, community biomass, and patterns of relative abundance. A third site in lowland Panama was originally censused in 1968–1969 and has often been compared with many other tropical and temperate sites. Results from Panama suggested an exceptionally different community structure fro...

Population dynamics of avian brood parasitism.

The population dynamics of avian brood parasitism have received little attention in spite of mounting evidence that many host species are adversely affected by brood parasites. We develop models for the dynamics of brood parasitism and apply these models to host-parasite associations for which demographic data are available. For generalized brood parasites, which use many host species, we assume that the population dynamics of the parasite are unaffected by any one host species and that the hosts accept the eggs of parasites. The demographic consequences of parasitism for a population of a given host species can be estimated if one knows the probability of parasitism, juvenile and adult mortality rates, and the average number of offspring successfully fledged by parasitized and unparasitized females (eqs. 3, 4). Because these demographic data are difficult to obtain in the field, they are available for few species. Applications of this model using available data suggest that females of many parasitized species must renest several times within a season in order to replace themselves. Furthermore, several species are in danger of extirpation as a result of brood parasitism by cowbirds (Molothrus spp.), which are increasing in population and have expanded their ranges and come into contact with many species that have not evolved a resistance to brood parasitism. One-host-one-brood-parasite associations represent a generalization of insect host-parasitoid associations, which have been extensively studied. Using reasonable estimates of the demographic parameters for both avian parasite and host, we show that these associations have a stable equilibrium point (eqs. 6, 7), in contrast with the oscillatory behavior exhibited by the simplest (Nicholson-Bailey) host-parasitoid models. In general, the longer the host lives, the more likely the system is to be stable rather than oscillatory. A review of the scant demographic data for one-host-one-parasite associations suggests that most specialized brood parasites are too rare to have an impact on the overall host population, although some local populations may be affected. These models can be modified to include the effects of host resistance (individuals of some host species reject the eggs of parasites), the effects of spatial heterogeneity (whereby some host populations are more vulnerable to brood parasites than others), and renesting following parasitism. More complete demographic data are needed if we are to understand the population dynamics and conservation requirements of host species subject to brood parasitism.

Edge effects on nest predation in the Shawnee National Forest, southern Illinois

Abstract Edge habitats may be considered ‘ecological traps’ for breeding birds if they attract many birds because of apparently favorable nesting conditions but have higher nest predation levels than interior habitats. Four alternative, nonexclusive hypotheses have been suggested to explain why edges might have higher predation levels than interior habitats: (1) predator activity is higher in areas with higher prey density (density-dependent predation); (2) predators are more abundant on edges than in forest interior; (3) the predator community is richer in species on edges than in forest interior; and (4) predators forage along travel lanes (linear geographical features) such as edges. Here we evaluated whether forest-farm edges in southern Illinois are ecological traps, and examined the relevance of these four hypotheses at our study site with several different experiments during May–July 1992 using artificial nests ( n = 605) baited with quail eggs and placed on the ground, in shrubs, or in saplings. Our results showed that, in general, the forest-farm edges of southwestern Illinois did not attract significantly more individuals or species of nesting songbirds, but they did have higher nest predation levels than forest interior sites, primarily as a result of higher predation levels on sapling nests. We did not find evidence strongly supporting any of the four hypotheses suggested as explanations for higher nest predation levels near edges. Two data sets showed that predation levels on artificial nests were density-independent. Forest-farm edges had neither more total species of potential nest predators nor more individual predators. However, there were more species of avian predators on edges than in interior sites. Correlations between predator abundance and nest predation levels on individual transects were weak. The travel lane hypothesis was not supported because nest predation levels were either not affected by distance from linear geographical features (roads and ravines) or were significantly less when close to than when far from presumptive travel lanes. High spatial heterogeneity in predation levels, numbers of singing birds, and potential nest predators may have obscured general patterns and suggest a need for larger sample sizes. Edges may be detrimental to some species of singing birds but not to others, and for different reasons.

Forest fragmentation in the temperate zone and its effects on migratory songbirds

Summary Although much attention has been paid to the impacts of tropical deforestation on populations of Neotropical migrants, fragmentation of breeding habitat may be an equally serious problem for many of these birds. Populations of many migrant songbirds have been declining in recent decades, especially within small woodlots. Censuses from woodlots of different sizes also consistently show that many migrant songbirds are area-sensitive, i.e. they are absent from all but the largest woodlots in a region. In contrast, long-term censuses from large, unfragmented forests show few consistent patterns of decline in Neotropical migrants. Population declines are therefore linked to forest fragmentation because they are most pronounced in small, isolated woodlots. Fragmentation leads to significant increases in nest predation and cowbird Molothrus spp. parasitism, the two most important causes of population declines and area-sensitivity. Predation and nest-parasitism rates are higher in small woodlots and along the edges of larger tracts than in the interior of large tracts. Data from fragmented forests in the American Midwest show that reproductive rates of several forest species are probably well below levels necessary to compensate for adult mortality. Among Wood Thrushes Hylocichla mustelina nesting in central and southern Illinois, for example, 89-100% of nests contain cowbird eggs (average of 2.2-4.6 cowbird eggs/nest) and nest-predation rates range from 50 to 96%. For the Wood Thrush and other forest songbirds, fragmented landscapes may be population sinks with populations sustained by immigration from larger, unfragmented forest tracts. These data emphasize the importance of protecting large, unfragmented forests for breeding habitat. We need far better data on dispersal rates and distances, fecundity and survival rates before we can determine what levels of predation and parasitism migratory birds can tolerate. The effects of silvicultural practices such as clear-cutting and selective logging on migratory songbirds may depend upon the landscape context. Preliminary evidence from a fragmented national forest in the Midwest suggests that selective logging can have relatively little impact on forest songbirds. We tentatively propose that low-volume selective logging be used as an alternative to clear-cutting. Logging roads should be closed and revegetated soon after harvest, and rotation times should be lengthened to permit regeneration of large, old trees.

Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs

Why do many hosts accept costly avian brood parasitism even when parasitic eggs and nestlings differ dramatically in appearance from their own? Scientists argue that evolutionary lag or equilibrium can explain this evolutionary enigma. Few, however, consider the potential of parasitic birds to enforce acceptance by destroying eggs or nestlings of hosts that eject parasitic eggs and thereby reject parasitism. This retaliatory “mafia” behavior has been reported in one species of parasitic cuckoo but never in parasitic cowbirds. Here we present experimental evidence of mafia behavior in the brown-headed cowbird (Molothrus ater), a widely distributed North American brood parasite. We manipulated ejection of cowbird eggs and cowbird access to predator-proof nests in a common host to test experimentally for mafia behavior. When cowbird access was allowed, 56% of “ejector” nests were depredated compared with only 6% of “accepter” nests. No nests were destroyed when cowbird access was always denied or when access was denied after we removed cowbird eggs, indicating that cowbirds were responsible. Nonparasitized nests were depredated at an intermediate rate (20%) when cowbirds were allowed access, suggesting that cowbirds may occasionally “farm” hosts to create additional opportunities for parasitism. Cowbirds parasitized most (85%) renests of the hosts whose nests were depredated. Ejector nests produced 60% fewer host offspring than accepter nests because of the predatory behavior attributed to cowbirds. Widespread predatory behaviors in cowbirds could slow the evolution of rejection behaviors and further threaten populations of some of the >100 species of regular cowbird hosts.

Urban mockingbirds quickly learn to identify individual humans.

Practically all animals are affected by humans, especially in urban areas. Although most species respond negatively to urbanization, some thrive in human-dominated settings. A central question in urban ecology is why some species adapt well to the presence of humans and others do not. We show that Northern Mockingbirds (Mimus polyglottos) nesting on the campus of a large university rapidly learn to assess the level of threat posed by different humans, and to respond accordingly. In a controlled experiment, we found that as the same human approached and threatened a nest on 4 successive days, mockingbirds flushed from their nest at increasingly greater distances from that human. A different human approaching and threatening the nest identically on the fifth day elicited the same response as the first human on the first day. Likewise, alarm calls and attack flights increased from days 1–4 with the first human, and decreased on day 5 with the second human. These results demonstrate a remarkable ability of a passerine bird to distinguish one human from thousands of others. Also, mockingbirds learned to identify individual humans extraordinarily quickly: after only 2 30-s exposures of the human at the nest. More generally, the varying responses of mockingbirds to intruders suggests behavioral flexibility and a keen awareness of different levels of threat posed by individuals of another species: traits that may predispose mockingbirds and other species of urban wildlife to successful exploitation of human-dominated environments.

Size‐Abundance Relationships in an Amazonian Bird Community: Implications for the Energetic Equivalence Rule

We studied size‐abundance relationships in a species‐rich Amazonian bird community and found that the slope of the logarithmic relationship between population density and body mass ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape