Summary
The decline of Golden-winged Warblers (Vermivora chrysoptera) has been attributed in part to hybridization with a sister species, the Blue-winged Warbler (V. pinus), which lacks the black throat patch typical of the Golden-winged Warbler. Understanding the signal function of male plumage ornaments in Golden-winged Warblers may provide insight into the mechanisms driving hybridization. If Golden-winged Warbler males use the black throat patch for interspecific signaling, Blue-winged Warblers or hybrids may be leading hybridization between the species. We examined the signal function of the melanin-based throat patch in a population of Golden-winged Warblers on the edge of the hybrid zone. Males with increased ultra violet chroma in their throat patches were older and their mates had significantly earlier first egg dates. This suggests the black throat patch of Golden-winged Warblers may be an age-related indicator of quality. Female Golden-winged Warblers should not mate preferentially with males which lack the black throat patch if it functions as an indicator of age and or male quality.
Methodology
We captured 34 male Golden-winged Warblers (18 after-second year, ASY; and 16 second-year, SY) with mist nets from 6 May to 15 July 2005 at the Queen’s University Biological Station (44º 34' N, 76º 19' W; 40 km north of Kingston, Ontario, Canada). This population of Golden-winged Warblers is on the edge of the hybrid zone (Dabrowski et al. 2005). Arriving Golden-winged Warblers were monitored at known breeding sites and first egg date was recorded for each pair’s first nest (n = 11) for those territories where we were able to find the nest.
Three throat patch feathers were collected from each male and stored in opaque envelopes until analysis. Feathers from each bird were taped to dark black paper with less than 10% reflectance (CANSON #425 Stygian black) overlapping in a way that mimicked how they would lay on the bird. Plumage coloration was measured in a darkened laboratory room with no windows (to minimize variation in ambient light) using an Ocean Optics S2000 spectrometer with a PX-2 pulsed xenon lamp set to a white spectralon standard (WS1) (Ocean Optics Inc., Dunedin, FL, USA) and dark standard (black felt in a box). A rubber sheath fitted over the spectrometer probe excluded light and ensured the probe was at a fixed distance, perpendicular to the sample. Five readings from each sample (lifting and lowering the probe between readings) were repeatable (r = 0.61, F34,140 = 8.95, P < 0.001) (Lessells and Boag 1987) and were averaged for analyses. Analysis was limited to wave lengths from 300 to 700 nm, the approximate visual range of passerines (Cuthill et al. 1999). Average reflectance for each 10-nm interval from 300 to 700 nm was calculated to give 41 reflectance measures per bird.