The ecology of behavioral plasticity and behavioral syndromes
In changing environments, behavioral plasticity should be favored by selection when the cost of plasticity is low and individuals can match their behavior to current environmental conditions. However, some behaviors may be fixed and covary in consistent and predictable ways, known as behavioral syndromes or animal “personalities.” Some personalities are sensitive to environmental changes with shyer, less aggressive individuals having better fitness in rapidly changing habitats than bolder, more aggressive individuals. However, personality traits have been found to be plastic in some cases, with variation depending on context or experience. This raises the question: do the selective pressures of frequently disturbed environments favor plasticity in personality traits?
My work has tested this question in the Bachman’s sparrow, a songbird that resides in habitats that experience frequent fire every 1-3 years. In recent work, I found that aggression and boldness, two personality traits that are often related in a behavioral syndrome, were not related in Bachman’s sparrows. Birds were more aggressive soon after a fire and in greater sparrow densities (Niederhauser et al. 2021), but boldness was not related to any environmental features. Furthermore, I found that aggressiveness changes over time, whereas boldness may not. |
The development and evolution of song repertoires and song learning
One of the most striking qualities of bird song is the remarkable variation that is found among and within species, from the size and complexity of vocal repertoires to the ways in which repertoires are learned and organized, and ultimately, how they are used. Why has such variation evolved in songbirds? Variation in song learning depends on when and how it occurs, what is learned versus improvised, who is providing the adult song models, and how well they are copied. When and how it occurs and how well songs are copied depends on genetic and neurodevelopmental factors. What is learned and who is providing the song models depend on social factors like whether the tutor songs were heard in direct or eavesdropping interactions. Song repertoire size is thought to be shaped by intra- or intersexual selection. With intrasexual selection, individuals of the same sex need a large repertoire in order to communicate signals of strength and aggressive intentions. In intersexual selection, large repertoires are favored by the opposite sex as an honest signal of a mate’s quality.
My work has explored song use and song learning in Bachman’s sparrow, a species with a large vocal repertoire that has gone unstudied. I found that Bachman’s sparrows at my study site in South Florida sing 33-55 song types, which is a larger repertoire size than previously reported for this species. Males from this population share on average 47 percent of their repertoires, which may indicate that songs might be used for both courtship and competition as well as how repertoires are learned. High levels of sharing at close distances has been seen in songbirds that either learn their songs throughout their lives or delay the crystallization of their repertoires until a later age. I found that Bachman’s sparrows do not change their repertoires over time, so they are more likely to delay crystallization. |
Ecology and transmission of vocal communication signals
Physical ecology has an important role in shaping vocal signals. Ecological conditions such as temperature, humidity, air turbulence, and physical structures affect how sounds transmit, so properties of vocal signals should be selected to maximize transmission in those environments. As some environments are changing rapidly due to anthropogenic factors, how will animal vocal signals evolve to deal with those changes?
A major aim of my research has been to determine how the environment affects how signals are produced and transmit. During my Master’s research, I found that Florida scrub-jay nestlings did not change vocal begging between natural and suburban habitats. However, breeding females may change their brooding behavior to reduce the signaling of her offspring to reduce predation risk (Niederhauser and Bowman 2014). During my doctoral research, I tested the hypothesis that the low amplitude songs of song sparrows transmit less effectively through the environment than high amplitude “broadcast” songs. I found that both low and high amplitude songs transmitted similarly in field and hedgerow habitats, despite other acoustic differences that should make them transmit differently (Niederhauser et al. 2018). Song sparrows may have evolved songs with a wide frequency range to allow effective song transmission across a variety of environments. |