Female Choice Assay:
Prior to the mate choice assay, each female’s cage was moved into a
sound attenuation chamber equipped with one audio
speaker. Females were given 24
hours to habituate to the chambers, at which point operant perches were
installed on either side of the front cage door, so that total perches
in the cage included two operant perches and two normal perches placed
diagonally in the back of the cage. Operant perches were placed
approximately 10 cm above the cage floor and 25 cm apart. Perches were
approximately 9 cm long, ½ cm in diameter, and made of wood. Perches
were attached to Honeywell lever arm depression micro-switches. The
perch on the left when facing the cage was designated perch A, and the
perch on the right was designated perch B. Hopping on a perch caused the
perch to lower slightly and trigger a pressure-sensitive button on the
micro-switch, which in turn triggered playback of a song through the
speaker. In this type of operant conditioning, the song itself serves as
the reward and encourages females to continue eliciting song through
hops (Stevenson 1967; Riebel 2000; Anderson 2009). All songs were
volume-adjusted to 65 dB SPL at the chamber center. All operant data
were collected through Sound Analysis Pro (see Tchernichovski et al.
2000).
Females learned to trigger song during an initial training period
(Anderson et al. 2014). For training, females were presented with
conspecific/ heterospecific song pairs (heterospecific song was from
rufous-collared sparrows, Zonotrichia capensis ). Whether perch A
played conspecific and perch B played heterospecific, or vice versa, was
alternated between birds, and switched every day for the same bird.
Switching songs between perches helped control for side bias. In order
to pass training, females had to hop on both perches for two days in a
row. All females passed training within 4 days.
After training, females were
presented with Solver/Non-Solver stimuli.
Females were given a new pair of
Solver and Non-Solver songs every day, in which a single song from a
Solver male was triggered by a hop on one perch, and a single song from
a Non-Solver male was triggered by a hop on another perch. The side that
played Solver song was switched each day to control for side bias. Each
bird heard 4 of 6 Solver/Non-Solver pairs, as female response tends to
drop off after prolonged periods in preference testing. Testing multiple
stimulus sets on each female also greatly reduces the risk that a chosen
stimulus set is non-representative of its category and yet repeatedly
measured (i.e. pseudoreplication in acoustic experiments; Kroodsma
1989). Trials ran from 11am to 5pm each day, during which the number of
hops on each perch was recorded. Outside of trial times, hops on a perch
did not trigger playback.