Catherine P. Ortega

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.

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.

Validating the use of artificial nests in predation experiments

Artificial nests with Japanese quail (Coturnix coturnix) eggs have often been used in experiments to compare patterns of predation with different ecological and nest-site attributes. Many investigators assume that predators respond to artificial nests as they would to natural nests; however, this assumption has rarely been tested. In 1996, in La Plata County, Colorado, we investigated differences in predator response to quail eggs in natural American robin (Turdus migratorius) nests in their original location, and artificial wicker-basket nests placed both in a 6 × 8 grid pattern at 30-m intervals and in a natural pattern 30 m in a random direction from each natural nest. Over the 15-day trial, predators responded differently to quail eggs in natural and artificial nests. By the first (5-day) check, predators had depredated 44.4% (n = 27) of natural nests, 11.1% (n = 27) of artificial nests 30 m from natural nests, and 14.6% of artificial nests in the grid (n = 48; P 0.10). However, by the final (15-day) check, more artificial nests in the grid had been depredated (91.7%) than either artificial nests located 30 m from natural nests (66.7%) or natural nests (70.4%; P 2 m aboveground. We suggest that because predators respond differently to natural and artificial nests, caution should be exercised in making management decisions based upon results of artificial nest experiments.

Effect of research activity on the success of American robin nests

We investigated researcher effects on the success of American robin (Turdus migratorius) nests from 1993 through 1995 in La Plata County, southwest Colorado. We handled the contents of 56 nests in 1993, 33 nests in 1994, and 20 nests in 1995. In these nests, which we refer to as touched nests, each egg was handled one time, and each chick was handled for about 3 minutes every 1-3 days. We made no attempt to mask our scent, such as with use of rubber gloves. With the use of a mirror on an extension pole, we monitored an additional 14 nests in 1993, 12 nests in 1994, and 21 nests in 1995 that we did not touch. Although we monitored nests that we could not reach, in the analyses we included only nests that we could have touched, ≤3.7 m high. In 1993, a significantly greater proportion of touched nests was successful in fledging at least one robin (57.1%) than untouched nests (21.4%, P < 0.05). Similarly, in 1994, 60.6% of touched nests were successful whereas 8.3% of untouched nests were successful (P < 0.01). In 1995, when we controlled for nest height by touching every other nest that was ≤3.7 m, success between touched and untouched nests was similar (35.0% for touched nests and 23.8% for untouched nests). Our results strongly suggest that monitoring nests and handling eggs and chicks did not affect negatively the success of American robin nests at our study site.

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

Use of artificial brown-headed cowbird eggs as a potential management tool in deterring parasitism

We added artificial and real brown-headed cowbird (Molothrus ater) eggs to red-winged blackbird (Agelaius phoeniceus) nests in 1985 and 1991 to determine the effect of egg addition on subsequent nest parasitism. We added large artificial eggs measuring 26.1 × 17.2 mm, similar in size to red-winged blackbird eggs, and small artificial eggs measuring 20.1 × 16.1 mm, similar in size to brown-headed cowbird eggs. Brown-headed cowbirds avoided parasitizing nests to which we added real or artificial cowbird eggs. In 1985, only 5.3% (3 of 57) of experimental nests were parasitized by cowbirds, compared with 31.5% parasitism of control nests (P = 0.001, n = 54)

EFFECTS OF BROWN-HEADED COWBIRDS ON THE NESTING SUCCESS OF CHIPPING SPARROWS IN SOUTHWEST COLORADO

Abstract We documented effects of Brown-headed Cowbird (Molothrus ater) parasitism on Chipping Sparrows (Spizella passerina) in southwest Colorado. Overall, 17 of 77 nests (22%) were parasitized. Abandonment was significantly higher among parasitized nests (29%) than unparasitized nests (5%). More Chipping Sparrows hatched and fledged per unparasitized nest than per parasitized nest. Reduction of host hatching and fledging rates in parasitized nests was attributable to smaller clutch size and higher abandonment. The major source of nest failure was predation, not parasitism; overall, 53% failed from predation, and only 6% failed from parasitism. There were no differences in weight, ulna length, tarsometatarsus length, or length of outermost primary between sparrow nestlings in parasitized and unparasitized nests. Only 18% of cowbird eggs laid resulted in a fledged cowbird. We observed no differences in nest placement or nest concealment between unparasitized and parasitized nests.