Chemical signaling has been well studied in invertebrates and mammals but less so in birds, due to the longstanding misconception that olfaction is unimportant or even non-existent in this taxon. However, recent findings suggest that olfaction plays an important role in avian mate choice and reproductive behavior, similar to other taxa. The leading candidate source for compounds involved in avian chemical communication is preen oil, a complex mixture secreted from the uropygial gland. Preen oil contains volatile compounds and their potential wax ester precursors, and may act as a reproductive chemosignal. Reproductive signals are generally sexually dimorphic, age-specific, seasonally variable, and may also vary geographically. We tested whether preen oil meets these expectations by using gas chromatography to examine the wax ester composition of preen oil in song sparrows (Melospiza melodia). We found that the wax ester composition of preen oil was significantly different between sexes, age classes, seasons, and populations. Collectively, our results suggest that song sparrow preen oil meets the criteria of a chemical cue that may influence mate choice and reproduction. Our findings in song sparrows, which are sexually monomorphic in plumage, also parallel patterns described for dark-eyed juncos (Junco hyemalis), a closely related songbird with sexually dimorphic plumage. Behavioral tests are needed to confirm that song sparrows attend to the cues present in preen oil, but our findings support the increasingly accepted idea that chemical communication is common and widespread in birds as it is in other taxa.
Song sparrows were captured using seed-baited Potter traps and mist nets at two breeding locations in Ontario, Canada: a northeastern site on land owned by the Queen’s University Biological Station near Newboro (43.008°N, 81.291°W; hereafter Newboro) and a southwestern site at the rare Charitable Research Reserve near Cambridge (43.383°N, 80.357°W; hereafter Cambridge). These two sites are separated by 390 km, well beyond the mean range of juvenile dispersal for this species, which is estimated as about 6 km (Zink and Dittmann 1993). At each site, we captured song sparrows during the early part of the breeding season (hereafter breeding), which encompasses nest building and early egg laying (Newboro: April 12 – May 5 2016 and April 8 – May 3 2017; Cambridge: April 3 – May 1 2017), and during late summer (Newboro: July 15–28 2016; Cambridge: August 8–28 2016) after most chicks have fledged, and juveniles are largely independent (hereafter post-breeding).
In the field, we determined the age class and sex of each song sparrow captured. We used wing length, plumage, and gape characteristics to distinguish juveniles (hatch-year) from adults (after-hatch-year) and used wing length, together with the presence versus absence of a cloacal protuberance (male) or brood patch (female), to distinguish males from females. From each bird, we collected a small blood sample through brachial venipuncture for genetic analysis, and later confirmed sex for all birds using the P2-P8 PCR protocol described by Griffiths et al. (1998). Preen oil was collected by gently probing the uropygial gland with an unheparinized capillary tube until ~1–5 mg was expressed into the tube. Samples were kept on ice in the field and stored at −20 °C pending analysis. We fitted each bird with a numbered aluminum leg band (Canadian Wildlife Service: 10691).