Divergent sexual selection as a barrier to reproduction

I have an ongoing collaboration with Dr. Hakan Karaardic (at right) in Bogazkent, Turkey to study sexual selection pressures in a potential barn swallow subspecies contact zone. Females of Hirundo rustica rustica in Europe select for longer tail feathers (streamers), while female H. r. transitiva in the Middle East select for elongated streamers and dark ventral plummage. Our study in Turkey aims to determine whether these divergent preferences are maintained in an area of subspecies overlap, forming a reproductive barrier, or if subspecies hybridize in this area. 

Geographic variation in barn swallow song

Since 2009 I have been sampling songs from across the barn swallow range in order to assess song divergence at both local and global scales. So far, I have sampled Hirundo rustica erythrogaster in the US, H. r. transitiva in Israel, H. r. rustica in Romania, and H. r. gutturalis in Taiwan. In 2013, along with Dr. Liz Scordato, I performed a sampling transect across Siberia. This allowed us to sample both song and morphological divergence along a contact zone between H. r. rustica, H. r. gutturalis, and H. r. tytleri. From early analyses of song differentiation, there are no significant differences in overall song syntax, song length, or song peak frequency. However, there are pronounced differences in syllable repertoire and the speed of pulse production in the terminal trill. Because trill rate is by far the fastest in Colorado, I performed a two year playback experiment within that population to determine whether this is the result of selection for this trait.

The function of trill rate in barn swallow communication

In order to test the function of North American barn swallow trill rate in aggression and its implications for reproductive success, I performed a playback experiment in our Colorado study area in summer 2011 and 2012. Preliminary results suggest that there is no across the board strategy for responding to a fast trilling intruder over a slow trilling simulated intruder, perhaps due to individual differences in male quality. However, difference in male latency to sing across treatments predicted the number of fledged young in a male's nest. This suggests that fine-scale communication dynamics mediated through trill rate have evolutionary implications. Further analyses aim to determine whether the difference between the focal and intruder male's trill rates predicts the strength of response.

Cryptic female barn swallow song?

Over the last four years I have spent a lot of time observing barn swallows while trying to record them or assign individuals to nests. In doing so, I have become aware of a vocalization that females make in a few behavioral contexts. The call (or song) is usually used as a copulation solicitation call, as shown in the video below. Thus, copulation often follows call production. However, my observations lead me to hypothesize this signal may also be used in manipulation of mate behavior, as well as female-female competition. In the video below, another female can be heard vocalizing off camera. These females may be effectively countersinging, which is consistent with other field observations. Although female song is thought to be uncommon among temperate, migratory songbirds, instead, female song may be relegated to specific contexts which are not often observed. Moreover, due to the subtlety of this vocalization, and the fact that males sometimes produce a similar syllable in their songs, may have made it more difficult to observe previously.