Summary
Within species of birds, variation in plumage may allow potential mates or competitive rivals to quickly assess the quality of an individual. Little is known about the role of white tail feather patches (“tail white”) in male Cerulean Warblers (Setophaga cerulea) and whether variation in patch size could serve as a signal. We hypothesized that the size of tail white patches in males acts as an honest signal of quality, with larger white patches indicating high quality males. We measured and compared the area of tail white to four estimates of quality (age, structural size, body mass, and blood parasite load) in 71 male Cerulean Warblers at the Queen’s University Biological Station in eastern Ontario. We found that males 2 years old or older had significantly larger tail white patches than 1 year old males, and that structurally larger males (estimated by wing length) had significantly larger tail white patches than smaller males. Our best-performing statistical model suggested that heavier individuals had larger areas of tail white, but this relationship depended on wing length: white positively covaried with body mass in smaller individuals (shorter wings), but not in larger individuals. Our findings suggest that size of tail white patches may provide information on some, but not all, aspects of quality of male Cerulean Warblers; however, we do not know if this information is perceived and used by other Cerulean Warblers in nature.
Methodology
We sampled 71 male Cerulean Warblers from the Queen's University Biological Station (44° 34' N,76° 19' W) near Elgin, Ontario, Canada from May-July 2010. We caught males in mist-nets using targeted playback paired with presentation of a male model of a Cerulean Warbler. After capture, we banded each individual with a numbered aluminum band from the Canadian Wildlife Service and gave each warbler a unique combination of colored bands. We aged each male based on plumage coloring and feather wear, where 1 year old males ("second-year" or SY) have worn primaries and tail feathers, show differential wear and pattern of wing coverts, and exhibit greener plumage than males that are 2 years old or older ("after-second-year males" or ASY; Pyle 1997).
For each captured male, we measured its unflattened wing chord (mm) using a wing ruler, tarsus length (mm) using a caliper, and mass (g) using a Pesola spring scale. We checked the accuracy of our Pesola scale using standard weights before and after the field season. Prior to each measurement, we checked that the Pesola scale read zero, and adjusted the scale to zero if needed. Our measurements of mass were accurate to +/- 0.2 g. All field measures were made by MAC to control for inter-individual variation in measurement. In order to quantify the area of tail white for each male, we photographed six individual feathers from each bird's tail from a dorsal view. Each tail is made up of six pairs of feathers ("rectrices"). We photographed the left six rectrices of the tail because they were easiest to manipulate and photograph in the hand. If a bird was missing a feather from their left side, then we photographed the right side instead. We photographed each left feather separately in the shade using a Nikon D50 camera (Nikon Canada Inc., Mississauga, Canada). Each feather was photographed against a green paper background with a visibly marked scale of 10 mm (Fig. 1).
We also obtained a small blood sample from each individual for parasite analysis by pricking the brachial vein with a sterile 23.5 gauge needle. We then collected between 15-20 µL of blood from each male using a heparinized microcapillary tube. Using this blood sample, we made a blood smear on a clean glass microscope slide. We used a Cameo Stain Pak (model 702; Cambridge Diagnostic Products Inc., Fort Lauderdale, FL, USA) that involves a three-step process to fix and stain each blood smear. We saved the remainder of the blood sample for future genetic work.