Hybrid zones can be used to examine the mechanisms affecting reproductive isolation and speciation, like song. Song has equivocal support as a driver of speciation; we did not find song to cause reproductive isolation. We examined an emerging secondary contact zone between White-crowned Sparrow subspecies pugetensis and gambelii by measuring song variation, song recognition, plumage, morphology and mtDNA. Plumage and morphological characters provided evidence of hybridization in the contact zone, with some birds possessing plumage and song characteristics intermediate between the subspecies. Playback experiments revealed asymmetric song recognition: male pugetensis displayed greater response to their own song than gambelii song, whereas gambelii did not discriminate significantly. If female choice operates similarly to male song discrimination, we predicted asymmetric gene flow, resulting in a greater number of hybrids with gambelii mitochondrial DNA (mtDNA). Contrary to our prediction, more gambelii and putative hybrids in the contact zone possessed pugetensis mtDNA haplotypes, possibly due to greater pugetensis abundance and female-biased dispersal.

White-crowned sparrow census

• We performed song censuses to provide inference into where the hybrid zone is and how large populations are of pugetensis and gambelii.
• Censuses were carried out in the breeding season (May-July) in sites spanning the secondary contact zone and zones of allopatry across the North Cascade Mountains in Washington state.
• Counts of males singing each subspecie song type were tallied for each site.

Playback experiments

• We carried out song playback experiments to characterize male song recogntion between the two subpecies
• Recordings of pugetensis, gambelii, and Harris's sparrow (hetero-specific control) were broadcast to territorial males inside and outside the secondary contact zone. The three playback trials were carried out on consecutive days for each male.
• Counts of six different territorial variables were collected across the six minute playback trial.

Capturing sparrows for plumage and genetic analysis

• We drew male sparrows into mist nets using song playback for photographs for plumage analysis and blood for genetic analysis. 
• Capture success was counted by subspecies.
• Photographs were taken from 1.1 m with a Nikon d7100 and a 70-300mm lens at 70mm in manual mode with a shutter speed of 1/250, ISO 100, and the built-in flash on.
• Photographs were processed with Fiji to determine RGB luminance of the nape, back, and bill of each bird using the color histogram function. We used mean RGB luminance to calculate a red dominance value for each region (red dominance value = mean red luminance / [mean green luminance + mean blue luminance])
• The control region of the mtDNA was sequenced and uploaded to Genbank.

White-crowned sparrow census

• Not all site were surveyed every year
• Males were only identified to subspecies by song. Thus the subspecies counts do not reflect genetic lineage.
• Censuses are still ongoing.