DOI: 10.5061/dryad.2280gb5n5
title: Calling in the face of danger: do nestling Red-winged Blackbirds (Agelaius phoeniceus) suppress begging in response to predator playbacks?

description: Nest predation is the most frequent cause of nest failure in birds such as
 the Red-winged Blackbird (Agelaius phoeniceus) that nest on or near the
 substrate. Nestlings should therefore exhibit adaptations to reduce the
 risk of nest predation. We tested the nestling antipredator hypothesis by
 examining the begging responses of Red-winged Blackbird nestlings to
 vocalizations of: (1) an important nest predator (American Crow, Corvus
 brachyrhynchos), (2) a predator that rarely preys on nestlings (Cooper’s
 Hawk, Accipiter cooperii), and (3) a nonpredator (Northern Flicker,
 Colaptes auratus). We performed playbacks with: (1) both parents present
 at the nest, (2) male at the nest, and (3) neither parent present.
 Following playback, we measured duration of nestling begging after the
 parent departed (begging persistence), bouts of otherwise normal begging
 when no parent was present (parent-absent begging), and calling without
 postural components of begging (nonpostural begging). When the male or
 both parents were present during playback, adults responded with alarm
 calls and nestlings significantly reduced parent-absent begging following
 American Crow and Cooper’s Hawk playbacks. Nonpostural begging was
 significantly reduced following Cooper’s Hawk playback, but there were no
 significant differences in the other begging variables. When neither
 parent was present, we found no significant differences in nonpostural
 begging in response to the three playback types, but parent-absent begging
 was significantly reduced following American Crow and Cooper’s Hawk
 playbacks when compared to Northern Flicker playbacks. These results show
 that nestlings suppress their vocal begging in response to calls of
 predators including Cooper’s Hawks even though they are not common nest
 predators.
description: We conducted our playbacks at Newark Road Prairie
 (42<sup>o</sup>32’ N, 89<sup>o</sup>08’ W), 13 km
 northwest of Beloit in south-central Rock County, Wisconsin, USA. This
 site is a wet-mesic remnant prairie and sedge meadow habitat maintained by
 regular burning. Early in the breeding season we located nests by
 systematically searching likely locations where we consistently observed
 female activity. Later in the breeding season when vegetation was too
 dense to allow effective systematic searching, we found nests by observing
 females carrying nesting material and giving nest-associated vocalizations
 when visiting specific locations. Once nests were discovered, we marked
 their locations with vinyl flagging and checked their contents daily to
 determine whether they failed or produced fledglings. Playbacks were
 performed 16 May through 6 July 2016, 24 May through 21 July 2017, and 30
 April through 13 July 2018. In 2016 and 2017 we did playbacks at active
 nests in locations that allowed us to video-record the nestlings. We
 performed playbacks between 0600 and 1400 hours CDT to nestlings 7–8 days
 after hatching. Previous studies have shown that Red-winged Blackbird
 nestlings of this age beg vigorously (Bernath-Plaisted and Yasukawa 2011).
 We obtained 10–13 samples of American Crow caws (Verbeek and Caffrey
 2002), Cooper’s Hawk cak-cak-cak calls (Curtis et al. 2006), and Northern
 Flicker long calls (Wiebe and Moore 2017) from the website xeno-canto
 (http://www.xeno-canto.org) and edited each to 6–7 seconds of calling with
 Audacity v 2.2.0 (https://audacityteam.org/). We used an Ecogear ECOXBT
 (Grace Digital Audio, Peterborough, Ontario, Canada) speaker with a
 frequency response of 20–20000 Hz to perform playbacks. The three calls
 were broadcasted in random order at the speaker’s maximum volume of about
 80–85 dB SPL at 1 m. The vocalizations were played from an iPhone (Apple,
 Inc., Cupertino, California, USA) connected to the speaker by a 30-m
 cable. Specific samples of each call were chosen using the shuffle feature
 of the iPhone. We performed playbacks under three conditions.
 In Condition 1 (2016): both
 parents present, playbacks were conducted when the female was at the nest
 and the male was nearby. In Condition 2 (2017): male present, playbacks
 were performed with the male nearby as soon as the female left the
 territory after feeding the nestlings. An observer controlled the playback
 from a position 40 m from the nest. An observer at this distance had no
 obvious effect on male or female behavior throughout the duration of the
 playback. Playback sessions comprised at least six provisioning visits by
 the female (in one case the male provisioned the nestlings). Additional
 visits were inadvertently obtained at several nests when we were unable to
 see the female feeding the nestlings. For each of the playback types we
 first observed nestling responses following the parent’s visit without
 performing a playback (Control), and then after a visit with playback
 (Playback). In sessions with extra visits, we used observations from the
 control period immediately before the relevant playback period. Control
 and playback periods for each playback call type enabled us to use a
 matched-pairs design to control for among-nest differences such as brood
 size, sex ratio, and begging intensity. Video recordings of nestling
 response to playbacks at nests less than 1.5 m above the ground were made
 with a Sony HDR XR520 camera (Sony Corp. of America, New York, New, USA)
 mounted on a tripod placed within 1 m of the nest. For nests 1.5–2 m above
 the ground, a GoPro Hero+ (GoPro, Inc., San Mateo, California, USA)
 attached to a 2.1-m pole was placed next to the nest. We used the video
 recordings to note the times at which the female left the territory,
 returned to the territory, and fed the nestlings. We used these data to
 calculate the time between each visit and rates of begging
 behavior. For
 Condition 3 (2018): no parent present, we removed a nestling that begged
 in response to light tapping on the rim of the nest and carried it to a
 vehicle parked at the study area. The nestling was placed in a nest
 collected earlier that breeding season and allowed 5 min to acclimate. We
 then played the three types of calls in random order and observed the
 response of the nestling for 5 min after each playback. This abbreviated
 schedule allowed us to return the nestling to its nest within 30 min and
 we observed no changes in provisioning or nest guarding by the parents and
 no cases of brood reduction in the days following nestling removal and
 replacement. We used
 observations of nestling begging to test the nestling antipredator
 adaptation hypothesis. In Conditions 1 (both present) and 2 (male
 present), we used the video recordings to measure: (1) begging
 persistence, (2) parent-absent begging, and (3) nonpostural begging.
 Begging persistence was the duration of begging (s) once the female
 departed. As the intervals between feeding visits varied considerably, we
 calculated the rates (number/min) for parent-absent and nonpostural
 begging. We noted the number of bouts of parent-absent begging as well as
 the duration of each control and playback period. A “bout” of
 parent-absent begging was defined as a series of begging calls in quick
 succession in the absence of a care-giving adult; a silence of at least 3
 s indicated the end of a bout of begging. We used the rate (number/min) of
 calling in the absence of any postural components of begging behavior as a
 measure of nonpostural begging. Nestlings typically remained motionless at
 the bottom of the nest during such calling. As both parents were not
 present in Condition 3, we only recorded the nonpostural begging rate and
 bout rate of parent-absent begging during the 5-min observation
 periods.