Research Methods:

• Adult male Abert’s Towhees, Melozone aberti, were caught during March and April 2013 using a mist net and playback of conspecific song. We determined sex and age by behavior (singing and territorial aggression only in males), skull pneumatization, and morphology (developed CP only in adult males, incubation patch only in adult females).
• Within 3 minutes of capture, we bled all birds (i.e., birds from both studies, see below) from the right jugular vein using a heparinized syringe with a 29-gauge needle. Blood samples (~100 μl) were immediately placed on ice until centrifuged later the same day (within 6 h) and the plasma harvested using a Hamilton glass syringe. Plasma samples were then frozen at -80°C until they were assayed. We also measured body mass (± 0.5 g), wing chord (± 0.5 mm), CP width (± 0.1 mm, using digital calipers), and fat stores of birds in both studies.
• For birds in sheet 1, within 2 minutes of the initial bleed described above, we randomly assigned birds (n = 8 per treatment per habitat; i.e., total sample size = 32) to receive an intravenous injection via the jugular vein of either synthetic chicken GnRH-I (Sigma Chemical Co., MO, USA; at a dose equal to ≃25 μg/kg) in 0.1 ml saline solution (0.9% NaCl) or 0.1 ml saline (control). We collected a second blood sample (~100 μl; as described above) from each bird 20 min after the injection to determine post-injection plasma T. Between the injection and the post-injection bleeds, birds were held in individual cloth bags.