1. Does vocal performance indicate the level of threat to receivers?

    Mating behavior in many species involves communication displays that are vigorous or difficult to perform. Individuals that maximize such features might be of higher quality and thus favored as prospective mates. In songbirds, males produce song both for mate attraction and territory defense. Aspects of vocal performance might allow other males to assess the fighting prowess of signalers in territorial disputes, with males who are better able to sing at higher performance levels perhaps posing a greater threat. Here we tested, in swamp sparrows (Melospiza georgiana), how territorial males respond to playback of songs of various levels of performance.

    Our main findings are twofold. First, low-performance stimuli elicited significantly lower aggressive responses. Second, male response to normal and high performance songs showed significant individual variation. Closer examination revealed that male response varied with both the subject's own vocal performance, and the degree to which the performance level of the playback stimuli was elevated. Males approached less closely and spent less time near the speaker the more the high-performance playback had been increased.  However, subject males with higher-performance song approached more closely than did males with lower performance song types. These findings provide insight into if and how vocal performance may indicate threat levels posed by a signaler.

2. What factors affect the development of female preferences -

                learning, mate-choice copying, a bias for high performance?

    Female mating preferences are a crucial component of sexual selection, yet we have poor knowledge of how female preferences develop. Specifically the extent to which experience during development informs mate choice is largely unstudied. Multiple factors may shape mate choice including experiential learning, social copying, and a sexually selected bias for certain male traits such as the performance level of displays. For bird species in which males learn their songs, it is likely that early exposure to song may also influence females' preferences later in life. I address this question in the swamp sparrow (Melospiza georgiana), using a new method to elicit preferences from lab-raised birds. Adult, wild-caught females of this species are known to prefer songs of relatively high-performance, i.e. songs that are physically difficult to produce.  In 2009, I hand-raised females with tutor songs of normal-performance levels.  As further training in their first spring, I presented females with these songs again, but this time coupled with a video of an adult female responding with a copulation solicitation display (CSD, shown below). I then used the CSD assay to test female preferences for songs they had experienced during ontogeny against these same songs altered to higher and lower performance levels.

     Females gave significantly fewer CSDs to low-performance songs than to the trained (normal-performance) songs. Females responded with the most CSDs to trained songs overall, but this value was not significantly different in a pairwise comparison to the high-performance songs, which was intermediate. A greater response to trained songs supports the hypothesis of a strong influence of early experience, but a preference for high-performance songs by some females cannot be ruled out. It appears that both experiential learning and a bias for high-performance guide female preference development.

collaboration with David C. Lahti and Jeffrey Podos

1. 3. How does developmental stress affect adult male vocal performance?
   

2. 
   

3.     Current empirical research on male signaling emphasizes mechanisms that maintain male signal reliability. One way to determine how signals may indeed be reliable would be to examine if costs experienced early in life affect the development of performance features. Using the birdsong vocal communication system, we test whether developmental stress influences learning and practice, thus resulting in compromised vocal performance capacities later in life. Such a result would have implications for signal reliability, which in turn could explain why and to what extent females attend to male traits, and how their preferences develop.  We brought male swamp sparrow nestlings (4-7 days post-hatch) in from the wild and raised them with a regime of high performance training songs in order to push the birds to their performance maxima. Males were divided into two groups, one receiving ad libitum food and the other receiving 70% of this amount; and this treatment was extended through the spring when males began practicing songs. In the spring, we then recorded songs produced by both stressed and non-stressed males, and compared these in terms of copying accuracy and performance. By examining the development of traits for which males are pushed to performance maxima, the mechanisms by which these signals can act as indicator mechanisms for females may be revealed.
   

4. 

   Nutritional manipulation and male weight gain in early life:
   We measured individual male mass every four days to the nearest 0.1g. We began the nutritional stress manipulation once the sex was determined between day 7-10 (+/-1). We continued the hand feeding manipulation through day 40(+/-1). Results from the early nutritional manipulation through day 92 can be seen in Figure 2. Mass did not initially differ between groups, but by day 12, developmental stress males weighed significantly less than control males (t -4.06, p=0.0006 n=23).

4. Does male vocal performance correlate with other sexually selected traits such as plumage? and, How does plumage vary between the sexes and with condition?

In animal communication, signals operate in diverse modalities.  In fact, multimodal signals employing different sensory systems often occur within a single species. Songbirds with extravagant plumage and complex vocalizations are a prime example of species exhibiting flashy traits in multiple modalities.  Indeed bird song and color have been the focus of recent literature on this topic (e.g. Uy et al. 2009; O'Loghlen and Rothstein 2009). However, the appropriate aspects of the complex, learned song of oscine birds to measure can be difficult to pinpoint.  Ideally one should select song features which are difficult to produce, and thus might signal reliable information (Podos et al. 2010).

The swamp sparrow is described as a monomorphic species with the male having a slightly more reddish cap in the breeding season (Pyle 1987).  Much is known about the song features of this species in terms of song learning, female preference, and male competition (e.g. Podos 1996, Ballentine et al. 2004; Ballentine et al. 2008).  Indeed song features which operate under morphological constraints are well understood in the swamp sparrow (Podos 1996; Podos 1997, Ballentine et al. 2004).  Additionally some research has correlated color patches on the head region, specifically the cap, with aggression and paternal care (Olsen et al. 2010).  When capturing and banding a population of swamp sparrows in the Quabbin reserve of Massachusetts, I observed striking variation in this color patch both within and between the sexes.  This observation prompted questions regarding the degree of sexual dimorphism in this species as well as the capacity for males to be signaling in multiple modes.  I examine three main questions regarding the feather color of the cap. 

1. 1.To what extent is cap color sexually dimorphic: do males and females differ in the spectral qualities and size of their cap feather patches?
   
   

2.  Are color and song signals examples of redundant signals or multiple messages?

3.  How might cap color and area reliably indicate male quality? 

I use the same techniques to measure two key features of the cap color, either of which might be important to visual communication in the swamp sparrow: the intensity of the chestnut color and the proportional area of chestnut relative to black in the cap.  For the first feature we use a spectrophotometer to measure reflectance of the chestnut area of the cap.  For the second I use the program ImageJ to measure the absolute and relative area of the two head patches - chestnut and black – from high-quality digital images taken on-site in the field or in the lab.

Below: A social pair of Swamp Sparrows; Male (Left), Female (Right).

collaboration with Jeff Podos

Collaboration with David C. Lahti