Alexander Cruz

Noise Pollution Changes Avian Communities and Species Interactions

Humans have drastically changed much of the worlds acoustic background with anthropogenic sounds that are markedly different in pitch and amplitude than sounds in most natural habitats. This novel acoustic background may be detrimental for many species, particularly birds. We evaluated conservation concerns that noise limits bird distributions and reduces nesting success via a natural experiment to isolate the effects of noise from confounding stimuli and to control for the effect of noise on observer detection biases. We show that noise alone reduces nesting species richness and leads to different avian communities. Contrary to expectations, noise indirectly facilitates reproductive success of individuals nesting in noisy areas as a result of the disruption of predator-prey interactions. The higher reproductive success for birds within noisy habitats may be a previously unrecognized factor contributing to the success of urban-adapted species and the loss of birds less tolerant of noise. Additionally, our findings suggest that noise can have cascading consequences for communities through altered species interactions. Given that noise pollution is becoming ubiquitous throughout much of the world, knowledge of species-specific responses to noise and the cumulative effects of these novel acoustics may be crucial to understanding and managing human-altered landscapes.

Noise pollution alters ecological services: enhanced pollination and disrupted seed dispersal

Noise pollution is a novel, widespread environmental force that has recently been shown to alter the behaviour and distribution of birds and other vertebrates, yet whether noise has cumulative, community-level consequences by changing critical ecological services is unknown. Herein, we examined the effects of noise pollution on pollination and seed dispersal and seedling establishment within a study system that isolated the effects of noise from confounding stimuli common to human-altered landscapes. Using observations, vegetation surveys and pollen transfer and seed removal experiments, we found that effects of noise pollution can reverberate through communities by disrupting or enhancing these ecological services. Specifically, noise pollution indirectly increased artificial flower pollination by hummingbirds, but altered the community of animals that prey upon and disperse Pinus edulis seeds, potentially explaining reduced P. edulis seedling recruitment in noisy areas. Despite evidence that some ecological services, such as pollination, may benefit indirectly owing to noise, declines in seedling recruitment for key-dominant species such as P. edulis may have dramatic long-term effects on ecosystem structure and diversity. Because the extent of noise pollution is growing, this study emphasizes that investigators should evaluate the ecological consequences of noise alongside other human-induced environmental changes that are reshaping human-altered landscapes worldwide.

Noise pollution filters bird communities based on vocal frequency.

Background Human-generated noise pollution now permeates natural habitats worldwide, presenting evolutionarily novel acoustic conditions unprecedented to most landscapes. These acoustics not only harm humans, but threaten wildlife, and especially birds, via changes to species densities, foraging behavior, reproductive success, and predator-prey interactions. Explanations for negative effects of noise on birds include disruption of acoustic communication through energetic masking, potentially forcing species that rely upon acoustic communication to abandon otherwise suitable areas. However, this hypothesis has not been adequately tested because confounding stimuli often co-vary with noise and are difficult to separate from noise exposure. Methodology/Principal Findings Using a natural experiment that controls for confounding stimuli, we evaluate whether species vocal features or urban-tolerance classifications explain their responses to noise measured through habitat use. Two data sets representing nesting and abundance responses reveal that noise filters bird communities nonrandomly. Signal duration and urban tolerance failed to explain species-specific responses, but birds with low-frequency signals that are more susceptible to masking from noise avoided noisy areas and birds with higher frequency vocalizations remained. Signal frequency was also negatively correlated with body mass, suggesting that larger birds may be more sensitive to noise due to the link between body size and vocal frequency. Conclusions/Significance Our findings suggest that acoustic masking by noise may be a strong selective force shaping the ecology of birds worldwide. Larger birds with lower frequency signals may be excluded from noisy areas, whereas smaller species persist via transmission of higher frequency signals. We discuss our findings as they relate to interspecific relationships among body size, vocal amplitude and frequency and suggest that they are immediately relevant to the global problem of increases in noise by providing critical insight as to which species traits influence tolerance of these novel acoustics.

The decline of an adaptation in the absence of a presumed selection pressure

The colonial nesting Village Weaver (Ploceus cucullatus) lays eggs that vary in ground color and pattern, but individual females lay similar eggs each time. Tests on captive African stocks have shown that females reject eggs of other cohorts if such eggs are sufficiently different. The Village Weaver may have evolved rejection behavior and variable eggs in response to cuckoo parasitism in Africa. The Village Weaver was introduced into Hispaniola from Africa as early as the 18th century. Before the arrival of the Shiny Cowbird (Molothrus bonariensis) in the early 1970s, there were no brood parasites on Hispaniola. Furthermore, in an experimental parasitism study, Hispaniolan Village Weavers accepted both dummy eggs and dissimilar Village Weaver eggs. The Village Weaver may have decreased the egg‐rejection behavior in the absence of the selective pressure of brood parasitism. Now Hispaniolan populations of the Village Weaver are parasitized by the Shiny Cowbird, which lays eggs dissimilar to those of the weaver. Brood parasitism by the Shiny Cowbird exerts a detrimental impact on the Village Weaver by reducing nest success and productivity.

Different behavioural responses to anthropogenic noise by two closely related passerine birds

Anthropogenic noise, now common to many landscapes, can impair acoustic communication for many species, yet some birds compensate for masking by noise by altering their songs. The phylogenetic distribution of these noise-dependent signal adjustments is uncertain, and it is not known whether closely related species respond similarly to noise. Here, we investigated the influence of noise on habitat occupancy rates and vocal frequency in two congeneric vireos with similar song features. Noise exposure did not influence occupancy rates for either species, yet song features of both changed, albeit in different ways. With increases in noise levels, plumbeous vireos (Vireo plumbeus) sang shorter songs with higher minimum frequencies. By contrast, grey vireos (Vireo vicinior) sang longer songs with higher maximum frequencies. These findings support the notion that vocal plasticity may help some species occupy noisy areas, but because there were no commonalities among the signal changes exhibited by these closely related birds, it may be difficult to predict how diverse species may modify their signals in an increasingly noisy world.

Mechanisms of egg acceptance by marsh-dwelling blackbirds

The parameters of egg acceptance in Red-winged Blackbirds (Agelaius phoeniceus) and Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) were investigated by introducing into their nests various eggs and objects. With the exception of miniature eggs, both species accepted all eggs, real and artificial, and rejected all nonegg-shaped objects. Egg shape and size were necessary components of egg acceptance whereas color and markings were not. The rejection of nonegg-shaped objects as large as Brown-headed Cowbird (Molothrus ater) eggs demonstrated that inability to remove cowbird eggs was not responsible for blackbird acceptance responses. Nests parasitized before the onset of egg laying did not increase the probability of either the egg being rejected or the nest being inactive.

Spatial and temporal activity patterns of the brood parasitic brown-headed cowbird at an urban/wildland interface

We examined the impact of the urban environment on the spatial and temporal activity of brood parasitic Brown-headed Cowbirds (Molothrus ater) in Boulder County, CO, USA. We found that cowbirds used the urban areas for foraging and roosting and traveled into the 3240 ha wildland preserve of ponderosa pine in the mornings to parasitize songbird hosts. Cowbird abundance decreased with distance from the urban/wildland boundary, and Plumbeous Vireo (Vireo plumbeus) nests closer to the urban/wildland boundary were more likely to be parasitized by cowbirds than those farther away. A linear regression accurately predicted the relative abundance of cowbirds based on parameters of distance from residential areas, and distance from roads and trails within the wildland preserve. For species of concern that are known cowbird hosts, creating larger preserves, reducing residential encroachment, and reducing preserve perforation by roads and trails might alleviate high frequencies of parasitism for a portion of the host population. However, even large preserves, such as found in Boulder, CO, USA cannot insulate all focal nesting species from the urban effect of increased brood parasitism. Efforts to reduce food resources and cover for cowbirds in the urban areas might prove to ameliorate host reproductive success close to the urban/wildland boundary through decreases in cowbird abundance.

Differential Growth Patterns of Nestling Brown-Headed Cowbirds and Yellow-Headed Blackbirds

-As nestlings, male Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) are significantly larger than female Yellow-headed Blackbirds in several measurements, and both male and female Yellow-headed Blackbirds are significantly larger than Brown-headed Cowbirds (Molothrus ater). We investigated the mechanisms by which female Yellow-headed Blackbirds are able to survive in nests with their larger siblings and how cowbirds are able to survive in the nests of hosts much larger than themselves. Growth was monitored of nestling Yellow-headed Blackbirds and Brown-headed Cowbirds, which were experimentally cross-fostered into Yellow-headed Blackbird nests, in Boulder County, Colorado during the 1986 breeding season. Measurements were recorded for weight, tarsometatarsus length, culmen length, gape width (width of bill at loral feathering), and length of ninth (outermost) primary. Nestling weight, tarsometatarsus length, and culmen length were larger in male than female Yellow-headed Blackbirds, and larger in both male and female Yellow-headed Blackbirds than cowbirds. Gape width differed significantly between male and female Yellowheaded Blackbirds throughout much of the nestling period; gape width in both were significantly wider than that of cowbirds. However, cowbirds had a significantly larger gape relative to their weight than did male Yellow-headed Blackbirds, and female Yellow-headed Blackbirds had a significantly larger gape relative to their weight than did males of the species. The relatively larger gape may enable females to compete with larger male siblings and enable cowbirds to compete with foster siblings much larger than themselves. Additionally, feather development was faster in female Yellow-headed Blackbirds than in males and was even more accelerated in cowbird nestlings. We used museum specimens of adults to calculate proportions of adult weight attained by nestlings. Cowbirds attained a greater proportion of their adult weight and adult ninth-primary length by fledging age than did Yellow-headed Blackbirds, and female Yellow-headed Blackbirds attained a greater proportion of their adult weight and adult ninth-primary length by fledging age than did their male siblings. Received 29 October 1990, accepted 10 January 1992. GROWTH rates vary greatly among animals and, although few grow at the highest potential rate (Needham 1964), they may grow at the maximum potential rate (Ricklefs 1969). Sibling competition may be a major component in the selection for rapid growth (Werschkul and Jackson 1979) and, in some species, may result in brood reduction (Bortolotti 1986). Brood reduction may occur through siblicide (Mock 1984) or asynchronous hatching (Lack 1954, Richter 1982, Mead and Morton 1985). Among sexually dimorphic, altricial birds that also hatch asynchronously, one might expect a high degree of sibling competition resulting in a biased sex ratio. While some investigators of sexually dimorphic birds have found skewed sex ratios of nestlings or fledglings (Howe 1977, Slagsvold 1 Present address: Department of Biology, Fort Lewis College, Durango, Colorado 81301, USA. et al. 1986, Teather and Weatherhead 1989), others have reported that sex ratios were not significantly skewed (Selander 1960, Fiala 1981, Bancroft 1983, Weatherhead 1983). Interestingly, in all the above-listed studies with skewed sex ratios, the sex ratios were female-biased. Explanations for female-biased sex ratios include: depending on hatching sequence, males suffer higher mortality during food shortages (Slagsvold et al. 1986, Teather and Weatherhead 1989); and males are more costly to raise (Howe 1977, Slagsvold et al. 1986). None of the above-listed studies, however, have shown how females may compete with their larger male siblings for food. Richter (1983) reported no significant bias in sex ratios for Yellow-headed Blackbirds (Xanthocephalus xanthocephalus), a sexually dimorphic, asynchronously hatching species. He demonstrated a significant difference between nestling growth rates of males and females, but suggested that there may be little disparity in

Conservation of the yellow-shouldered blackbird Agelaius xanthomus, an endagered West Indian species

Abstract The yellow-shouldered blackbird Agelaius xanthomus, endemic to Puerto Rico and Mona Island, is endangered, mainly because of brood parasitism by the shiny cowbird Molothrus bonariensis, which reached Puerto Rico at least 30 years ago. The yellow-shouldered blackbird populations have since declined, about 770–1200 remaining (470–900 on Mona Island) by 1982–1986 compared to a population of about 2400 in 1975. Nearly all nests of blackbirds in most of its habitats are parasitized by cowbirds. This significantly reduces nesting success, but blackbirds have evolved no defenses against brood parasitism. Removal of cowbirds from the yellow-shouldered blackbird nesting grounds, modeled after similar programmes for the brown-headed cowbird Molothrus ater on Kirtlands warbler Dendroica kirtlandii nesting areas, significantly increased blackbird production. Blackbirds readily accept nest boxes, and breeding populations can be established in otherwise unusable sites and can be concentrated in mangrove habitats, were they are more easily protected by cowbird removal. Furthermore, yellow-shouldered blackbird pairs using cavities, including nest boxes, fledged more young per nest than pairs using open nests.

Avian Resource Use in a Caribbean Pine Plantation

I studied resource use by birds in a Caribbean pine (Pinus caribaea) plantation in Luquillo Experimental Forest, Puerto Rico, from 1981 to 1986. I used multivariate analyses to quantify and provide a classification of foraging relationships among bird species. The analysis of food-searching behavior by birds indicated that resource use was closely associated with vegetation physiognomy. The presence of specific kinds of vegetation was important in determining the presence and abundance of some bird species. Frugivores and nectarivores foraged in understory flowering and fruiting shrubs. Among insectivores, foliage-gleaners were found mainly in the canopy and hoverers were concentrated in the understory. Dead hanging leaves and vine thickets provided important foraging substrates for some species. Forest managers can improve habitat for birds in plantations by providing habitat heterogeneity through maintenance and encouragement of understory vegetation diversity, vine thickets, and native upperstory trees. J. WILDL. MANAGE. 52(2):274-279 Information on the impacts of land management practices on neotropical birds, and particularly the changes that occur when heterogenous tropical forest ecosystems are converted to pine plantations, is limited. In Australia, exotic pine (Pinus radiata) plantations supported 50% of breeding bird species found in the original eucalyptus forests (Gepp 1976, Suckling et al. 1976, Driscoll 1977). Only Falkenberg et al. (1983) has investigated bird communities in neotropical pine plantations; they found fewer birds in Caribbean pine plantations than in native forest sites in Jamaica. Resource use by birds in neotropical pine plantations has not been quantitatively examined. Information on bird use of neotropical pine plantations is needed because 33% of all tropical plantations are in the neotropics and the most commonly planted species are pine (Pinus spp.) (Evans 1982). The objectives of my research were to describe avian resource use in a Caribbean pine plantation and analyze niche structure, food resource relationships, and other factors associated with the bird assemblage patterns. This study was conducted under the auspices of the Institute of Tropical Forestry, U.S. Forest Service, Puerto Rico, and the U.S. Man and the Biosphere Program on Tropical Forests. I am indebted to A. E. Lugo for guidance, cooperation, and helpful suggestions. I also profited from discussions with W. J. Arendt, G. P. Bauer, C. E. Bock, B. B. Cintron, C. A. Delannoy, J. C. Figueroa, G. E. Freeman, J. A. Moreno, T. K. Nakamura, T. R. Strong, F. H. Wadsworth, P. L. Weaver, and J. W. Wiley. I gratefully acknowledge the valuable field assistance of A. Arendt, W. J. Arendt, and E. Hernandez-Prieto and the forest structural information provided by A. E. Lugo. Financial support came from a National Research Council (Ford Found.) Postdoctoral Fellowship, Grant-in-Aid from the University of Colorado, and a National Geographic Society Grant.