Lauryn Benedict

California towhee vocal duets are multi-functional signals for multiple receivers

Avian vocal duets provide a model system for studying the evolution and production of complex signals which require input from multiple individuals. Duets serve a variety of communication functions in diverse species. To explore the function of duetting in California towhees (Pipilo crissalis) I collected behavioural data from a marked population, conducted sound playback experiments, and removed males from established pairs to observe the behaviour of new partners. Results indicate that California towhees use duets during extra-pair communication with conspecifics and during intra-pair communication between duetting partners. During the breeding season duetting frequency peaked during the chick-rearing period and was low during periods of high female fertility. Playbacks provoked similar levels of aggressive response from male and female territory holders. Mated pairs duetted in response to simulated intrusion by conspecific males, females and pairs. California towhee duets briefly increased in frequency after pairing, and showed no evidence of change due to learning. Duets facilitate spatial and behavioural coordination by mates, which should allow them to more effectively perform behaviours related to mutual reproductive success. No single existing hypothesis adequately explains vocal duetting among California towhees. Instead, duets function in multiple contexts and provide multiple potential benefits.

Canyon wrens alter their songs in response to territorial challenges

Many animals use low frequency and harsh sounds in aggressive contexts. Low frequencies are correlated with body size in some species, and thus can provide an indication of fighting ability. Because of this, low frequencies and harsh sounds may also indicate aggressive intent and motivation to attack. We tested for facultative adjustment of vocal behaviour in canyon wrens, Catherpes mexicanus, by using playbacks to simulate territorial intrusions. Thirteen territory-holding wrens were exposed to playback of typical conspecific songs which did not include harsh elements. Territory holders increased song rate in response to simulated intrusions, altered song type usage and altered some song sound frequency parameters. Birds singing in response to playbacks did not adjust the highest frequencies or the frequencies with maximum power in their songs, but they significantly lowered the lowest song frequencies and were more likely to append harsh notes to the ends of their songs. Results match predictions of the motivation-structural hypothesis, and suggest that canyon wrens alter their songs in multiple ways when faced with conspecific territorial intrusion.

Macrogeographical variation in the song of a widely distributed African warbler

The songs of oscine passerine birds vary on many spatial scales, reflecting the actions of diverse evolutionary pressures. Here we examine the songs of Cisticola erythrops, which effectively signal species identity across a geographical area spanning 6500 km in sub-Saharan Africa. Selection for species identification should promote stability in song traits, while sexual selection and geographical segregation should promote diversity. Cisticola erythrops share syllable types across the entire range of species and structure songs similarly, but individuals sing highly variable songs through improvisational recombination of syllables. Patterns of syllable use change gradually across the range of the species and do not show distinct breaks at subspecies boundaries. The acoustic properties of the most common syllable type also change gradually with distance. The results illustrate how songs can be simultaneously species-specific and highly variable at an individual level. At a larger level, patterns of variation indicate that cultural drift has generated song diversity through an isolation by distance mechanism.

OFFSPRING DISCRIMINATION WITHOUT RECOGNITION: CALIFORNIA TOWHEE RESPONSES TO CHICK DISTRESS CALLS

Abstract ABSTRACT Accurate offspring discrimination improves parental fitness by ensuring appropriate parental investment. In colonial avian species, offspring discrimination is often mediated by recognition of individual offspring vocalizations, but spatially segregated species do not necessarily need sophisticated recognition abilities if parents can use alternative information to distinguish offspring from nonoffspring. I experimentally tested the hypothesis that territorial California Towhee (Pipilo crissalis) parents use a location-based decision rule, instead of true vocal recognition of offspring, when deciding whether to respond to chick distress calls. Accurate responses to offspring distress calls should be favored by natural selection because they can have large fitness benefits if parents succeed in chasing away potential nest predators. Responses to nonoffspring, in contrast, may be costly and should not be favored by natural selection. Towhee parents were presented with a series of three playback experiments in which I manipulated the identity of the vocalizing chick, the age of resident chicks, and the location of the distress call broadcast. Parents showed no evidence of individual vocal recognition and no pattern of differential response to distress calls when offspring age differed from that of the calling chick. Parents did, however, exhibit a significant tendency to approach distress calls originating near their offspring more often than distress calls originating elsewhere on their territory. These results provide support for the evolution of an offspring discrimination strategy based on a simple location-based decision rule instead of true vocal recognition.

SIZE, INSECT PARASITISM, AND ENERGETIC VALUE OF ACORNS STORED BY ACORN WOODPECKERS

Abstract We compared species composition, insect parasitism, and mass of acorns stored in 12 granaries with acorns present within the territories of Acorn Woodpeckers (Melanerpes formicivorus) at Hastings Reservation in central coastal California. All three species of oaks present in the study area produced excellent crops the year of the study, and thus woodpeckers could readily have filled their granaries with acorns of any one of the species. Nonetheless, species composition of stored acorns was highly variable among territories and at least some acorns of all three species were stored in all granaries. There was no consistent difference in insect parasitism between stored and unstored acorns. Relative to estimated availability, birds preferred to store Quercus lobata acorns and avoided Q. douglasii acorns, but showed no preference for Q. agrifolia acorns even though they are energetically more valuable than the other two species. Stored acorns of two of the three oak species also weighed less, and were thus presumably smaller, than the average unstored acorn. On average, Acorn Woodpeckers did not maximize the energetic value of their stores and could have increased the energy stored in their granaries by 14–108% had they chosen larger acorns or more energetically rich Q. agrifolia acorns. Storing may be better understood in a more traditional optimal foraging context in which the costs of harvesting and caching, together with the energetic value of stored acorns, are considered together. Tamaño, Parasitismo por Insectos y Valor Energético de Bellotas Almacenadas por el Carpintero Melanerpes formicivorus Resumen. Comparamos la composición de especies, parasitismo por insectos y masa de bellotas almacenadas en 12 graneros presentes adentro de territorios de Melanerpes formicivorus, en la Reserva Hastings en la costa central de California. Las tres especies de roble presentes en el área de estudio produjeron excelentes cosechas durante el año de estudio, por lo que los pájaros carpinteros podrían haber llenado fácilmente sus graneros con bellotas de cualquiera de las especies. A pesar de esto, la composición de especies de las bellotas almacenadas fue altamente variable entre territorios y en todos los graneros fueron almacenadas al menos algunas bellotas de cada una de las tres especies. No hubo una diferencia consistente en el parasitismo por insectos entre bellotas almacenadas y no almacenadas. En relación a la disponibilidad estimada, las aves prefirieron almacenar bellotas de Quercus lobata y evitaron las bellotas de Q. douglasii, pero no mostraron preferencia por bellotas de Q. agrifolia, aunque éstas son energéticamente más valiosas que las bellotas de las otras dos especies. Las bellotas almacenadas de dos de las tres especies de roble fueron menos pesadas, y presumiblemente fueron más pequeñas, que la bellota promedio no almacenada. En promedio, M. formicivorus no maximizó el valor energético de sus provisiones, aunque podría haber incrementado la energía almacenada en sus graneros en un 14–108% si hubiera elegido bellotas más grandes y bellotas más energéticas de Q. agrifolia. La acción de almacenar puede ser mejor entendida en un contexto de forrajeo óptimo más tradicional, en el que los costos de cosechar y ocultar, junto al valor energético de las bellotas almacenadas, son considerados juntos.

Unusually high levels of extrapair paternity in a duetting songbird with long-term pair bonds

Previous studies suggest that extrapair young are very rare or absent in socially monogamous avian species that produce vocal duets. These results are generally consistent with functional hypotheses suggesting that duets may signal commitment between partners, or aid males as a paternity guard to ensure genetic as well as social monogamy. Additionally, species that exhibit social monogamy with the same partner across multiple breeding seasons tend to exhibit low levels of extrapair paternity, so duetting species that mate for life may be particularly likely to exhibit genetic monogamy. This study examined the social and genetic mating systems of California towhees (Pipilo crissalis), a duetting species thought to have life-long pair bonds. Observation of a color-banded population confirmed that California towhees exhibit long-term social monogamy. Known social families were genotyped at four microsatellite loci with high allelic diversity. Unexpectedly, paternity exclusion indicated that at least 13 of 31 (42%) nests contained extrapair young. All chicks exhibited maternal alleles, but 21 of 81 (26%) young were not the offspring of social fathers. Thus, in contrast to previous work, this study documents high frequencies of extrapair young among socially monogamous duetting birds with long-term pair bonds.

Overlapping home ranges and microhabitat partitioning among Canyon Wrens (Catherpes mexicanus) and Rock Wrens (Salpinctes obsoletus)

ABSTRACT Patterns of animal space use may vary according to species identity, presence of conspecifics, presence of heterospecifics, and resource availability. We evaluated joint space use by Canyon Wrens (Catherpes mexicanus) and Rock Wrens (Salpinctes obsoletus) by comparing home range sizes, home range overlap, and foraging behavior. Canyon and Rock wrens are ecologically similar species which frequently co-occur along rocky cliffs where members of both species feed on terrestrial invertebrates. Interactions between Canyon and Rock wrens provide information about avian space use and foraging strategies in understudied cliff habitats. We estimated home range for each species using ArcGIS, and quantified foraging microhabitat use. Canyon Wren home ranges were widely spaced, did not overlap conspecifics, and were significantly larger than those of Rock Wrens. Rock Wrens occurred at higher densities and home ranges overlapped conspecifics in 33% of cases by an average of 19%. Canyon and Rock wren home ranges overlapped in 68% of cases by an average of 28%, but overlapping pairs rarely shared core use areas. The two species differed significantly in foraging microhabitat use. Results suggest that heterospecific territory defense between Canyon and Rock wrens is low, and that these species have adopted different methods for using shared resources in escarpment and cliff habitats.

Female Song in New World Wood-Warblers (Parulidae)

Recent advances have revealed that female birdsong is widespread and multifunctional. Female song was likely ancestral among songbirds and persists in many lineages today. Nevertheless, many species lack female song, and researchers are interested in understanding the selective factors that promote and counter the persistence of this trait. Female song is associated with life-history traits including year-round territoriality, non-migratory behavior, sexual monochromatism, and monogamy. Most studies examining these relationships have looked at clades with a migratory ancestor and have found that gains of migratory behavior are strongly correlated with losses of female song (and duetting). Here we ask if the reverse pattern exists: in a large clade of songbirds with a migratory ancestor, do losses of migratory behavior correlate with gains of female song and visual signaling traits? We investigated correlations between female song, migration and dichromatism in 107 species of New World Warblers (Family Parulidae). All of these species are predominantly monogamous and territorial when breeding, 50 (47%) are migratory, 49 (46%) are monochromatic, and 25 (23%) show female song. On a robust genetic phylogeny maximum likelihood methods recover migration and monochromatism as the ancestral state in warblers. Female song is generally not reconstructed as present in any deep nodes of the phylogeny, indicating that most extant species with female song evolved this trait independently and relatively recently. Gains of female song do not correlate with losses of migration. Losses of dichromatism do correlate with losses of migration. Thus, in this clade, visual signals are associated with sedentary versus migratory lifestyles, but female acoustic signals are not. Our results show a different pattern from that seen in similar studies and support the hypothesis that losses, but not gains, of female song are driven by life history.

Paving the way: Multifunctional nest architecture of the Rock Wren

ABSTRACT Some avian species augment nest cups by building associated architectural structures that may mitigate predation, parasitism, and/or hatching failure. Because effective nest construction is integral to reproductive success, architectural structures associated with nests are predicted to provide functional benefits. Rock Wrens (Salpinctes obsoletus) stereotypically augment their soft cup nests with a pavement of stones, apparently incurring considerable energy costs. We quantified Rock Wren stone use and measured how stones occlude nest cavities. We examined whether Rock Wrens adjust individual stone-carrying effort in response to nest cavity opening size and tested 3 hypotheses about the benefits of cavity occlusion: (1) stones ameliorate temperature fluctuations and improve nest thermoregulation; (2) stones improve nest microclimates by keeping them dry; and (3) stones have the potential to reduce nest predation by alerting incubating females when predators approach. We found that individual nest pavements contained up to 1.4 kg of stones, which varied in size but were relatively uniform in thickness. Stone pavements decreased nest cavity openings by a mean of 34%, with larger openings containing significantly more stones. Presence of stones did not influence temperature in unoccupied nest cavities but did significantly decrease water infiltration into the nests during simulated rainfall. Presence of stones also changed the sound of a simulated predator approach, supporting the idea that stone patios could serve as an alarm function for vulnerable incubating females. Our data indicate that Rock Wrens adjust the amount of stones used in nests according to cavity characteristics to obtain multiple benefits. Results confirm that nest site modification can be an adaptive behavior and provide evidence that birds facultatively modify nesting environments.

A call to document female bird songs: Applications for diverse fields

ABSTRACT Research on bird song has contributed to important advances in diverse biological fields from neurobiology to conservation biology. Bird song has traditionally been studied as an elaborate male trait, but female song is also widespread in both temperate and tropical species and likely evolved in the early ancestors of modern songbirds. However, female song is underrepresented in biological collections compared to male song, and we lack documentation of female songs for most songbird species. Better documentation of female bird song is necessary for an understanding of the prevalence, regulation, function, evolution, and conservation applications of avian vocalizations. Therefore, we call on all researchers to disseminate their observations of female bird song, and to spread the word among other researchers, students, field technicians, and citizen scientists that many female songbirds sing. To this end, we provide resources for disseminating recordings and written documentation of female song, including best practices for documentation, venues for archiving and publishing, and our citizen science project, the Female Bird Song Project. We especially appeal to researchers studying marked populations who can accurately assess sex-specific singing behavior. Documenting female song across many species and geographic regions is a major endeavor. By working collectively, we can make the greatest progress toward applying the resultant knowledge to a wide variety of fields.