Haff & Magrath. to Call Or Not to Call? Biology Letters

Electronic Supplementary Material

Haff & Magrath. To call or not to call? Biology Letters.

Methods

Study species

Scrubwrens breed in pairs or in cooperative groups with a single female and alpha male, together with one or sometimes more subordinate males [1, 2]. Scrubwrens usually lay 3 eggs in a clutch, which they incubate for 18 days. Fledglings are in the nest for an average of 15 days [3]. Individual scrubwrens were banded either in previous years, or in the year of the study but before the breeding season.Daily nest predation rates jump from about 1% during incubation to 5% after hatching [4], a magnitude of increase that is high compared to most other passerines [5]. These data suggest that nestling vocalizations, parental activity, or a combination of the two betray the location of otherwise cryptic nests.

Experimental design and playback methods

Playbacks and model presentations were designed to mimic natural circumstances. Peeps were played back when adults were not at the nest because of the potential for these calls to convey information to adults while the adults are in the vicinity of the nest[6]. We chose to place models 2 m from the nest because at this distance potential predators might not notice a nest if nestlings were silent, but would be able to overhear young if they were calling loudly. We chose 2 minutes as the length of model exposure to mimic a realistic situation while minimizing the chance of more than one adult responding to the model at a time, and to avoid habituation to the model.

In order to control for potential carryover effects of playbacks on adult behaviour, we presented parents with both model types in a single day, but kept playback type the same. Each nest received the second playback type with model presentations the subsequent day. We judged this design to be more realistic than manipulating brood noisiness within a day, as very hungry broods are likely to be consistent in their behaviour over the course of several hours, while the presence of other species near the nest can fluctuate over short periods. We assessed the response of a single focal parent instead of both members of the pair because we could only control the length of model exposure to a single individual. We focused on the first parent (female or alpha male) to approach the nest after the first 30 min playback period and then maintained that adult as the focal individual for all subsequent trials. When two treatments at a nest were conducted on the same day we left nests undisturbed for at least 60 minutes between the end of the first treatment and the beginning of the second treatment.

We recorded sounds for playbacks at each nest using two Audio-Technica ATM15a miniature cardioid condenser lapel microphones connected by 15 m audio cables to a Marantz PMD670 solid state digital recorder sampling wave files at 44.1 kHz and 16 bits. We recorded the nestlings when they were 4-5 days old, allowing us time to prepare playbacks for experiments. We examined all recordings on spectrograms in Raven Pro 1.3 [7], and selected sections for peep, whine, and background playbacks. We filtered out sounds below 1 kHz in recordings of peep calls and background sounds, but did not filter whine calls because of their broad frequency range. For all playbacks we removed loud calls or other abrupt noises in the background. Playbacks were broadcast from 16-bit wave files using a Sony MDR-A106 headphone speaker placed directly above and facing the same direction as the nest entrance, connected by a 15 m cable connected to an Edirol R-09HR solid-state digital player.In order to minimize our impact on predation rates, we used wire predator exclosures that allowed adults free access to nests, but kept out mid-sized predators such as currawongs [8]. Exclosures were installed before recording or playback at nests. Parents generally ignored the exclosures, and no nest was abandoned after caging.

At least one day before experiments we placed dummy microphones and speakers at nests to habituate adults to their presence. At this time we also secured a garden mesh dome covered with camouflage cloth 2 m from the nest, and placed models under these domes at least 1.5 h before presentation. Adults quickly resumed normal behaviour after we placed equipment. We replaced dummy equipment with recording and playback equipment on the day of experiments, which we ran from a hide 10-15 m from the nest. We presented models to focal parents during stage 2 of the experiment by using a fishing line to pull the camouflage cloth off the model dome at a constant rate of approximately 30 cm/s when parents were 10 m from nests. We used three different currawong and two rosella models, and within model type there were no effects of different models on adult behaviour (results). We recorded adult vocalizations using a Sennheiser ME62 omni-directional microphone covered in camouflage cloth and placed next to the model. We also recorded a commentary on adult behaviour during model presentation using an audio recorder.

We broadcast peeps and whines at 65 dB and 80 dB at 20 cm, respectively, which are at the upper ranges of natural amplitude of these calls for 7-8 day old nestlings [9]. Control playbacks were broadcast at the same level as the background between nestling call playbacks (mean 32 ± 2 s.e. dB).

We used Raven Pro 1.3 spectrograms to count all calls, with settings as in [10]. In total, we tallied 8,532 buzz and 1,408 chip calls. To further ensure that we manipulated parental perception of risk near nests we also compared (1) the closest distance adults approached models, to the nearest 0.25 m; (2) the closest distance adults approached nests, to the nearest 0.25 m; and (3) whether or not adults dropped or ate the food they were carrying to the nest between the two model types.

Statistical analysis

We used Friedman ANOVA and sign tests to test for differences in adult approach to different model types, Kruskal-Wallis one-way ANOVA for independent samples to test for differences in adult response to model replicates.

Results

Adults treated model currawongs but not rosella controls as predators. Parents gave more buzz alarms and fewer chip calls during predator than control model presentations (fig. S1; playback treatments combined, number of buzz alarms: Wilcoxon Z = -3.85, P < 0.0001; number of chips: Z = -2.28, P = 0.02). Parents also stayed farther away from both nests and models during model predator than control presentations (playback treatments combined, minimum approach to models: Friedman ANOVAZ = -3.74, P< 0.0001; minimum approach to nest: Z = -2.8, P= 0.005). Also consistent with perception of threat, parents dropped or swallowed food only in response to model predators (20 drops or swallows during one or both predator presentations, 0 during control presentations, two-tailed sign test P< 0.0001).

There were no differences in how adults treated different taxidermic mounts of the same type, either for predator models (number of buzz alarms: Kruskal Wallis χ22= 0.74, P = 0.69; minimum approach to models: χ 22 = 0.81, P= 0.67; minimum approach to nests: χ 22= 0.31, P = 0.86), or control models (number of buzz alarms: χ21 = 1.58, P= 0.21; minimum approach to models: χ 21= 0.11, P= 0.75; minimum approach to nests: χ 21= 0.06, P= 0.81).

References

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2.Magrath R.D., Wittingham L.A. 1997 Subordinate males are more likely to help if unrelated to the breeding female in cooperatively breeding white-browed scrubwrens. Behav. Ecol. Sociobiol.41 185-92.

3.Higgins P.J., Peter J.M. 2002. Handbook of Australian, New Zealand and Antarctic Birds, Volume 6: Pardalotes to Shrike-thrushes. Melbourne, Australia: Oxford University Press.

4.Platzen D., Magrath R.D. 2004 Parental alarm calls suppress nestling vocalization. Proc. R. Soc. B271 1271-6. (doi: 10.1098/rspb.2004.2716)

5.Clark A.B., Wilson D.S. 1981 Avian breeding adaptations - hatching asynchrony, brood reduction, and nest failure. Q. Rev. Biol.56 253-77. (doi:10.1016/j.anbehav.2007.01.025 )

6.Magrath R.D., Haff T.M., Horn A.G., Leonard M.L. 2010 Calling in the face of danger: how predation risk affects acoustic communication by parent birds and their offspring. Adv. Stud. Behav.41 187-253. (doi: 10.1016/S0065-3454(10)41005-0)

7.Charif R.A., Waack A.M., Strickman L.M. Raven Pro 1.3 User's Manual. 1.3 ed. Ithaca, NY: Cornell Laboratory of Ornithology; 2008.

8.Haff T.M., Magrath R.D. 2010 Vulnerable but not helpless: nestlings are fine-tuned to cues of approaching danger. Anim. Behav.79 487-96. (doi: 10.1016/j.anbehav.2009.11.036 )

9.Haff T.M., Magrath R.D. 2011 Calling at a cost: elevated nestling calling attracts predators to active nests. Biol. Lett.7 493-5. (doi: 10.1098/rsbl.2010.1125)

10.Magrath R.D., Pitcher B.J., Dalziell A.H. 2007 How to be fed but not eaten: nestling resonses to parental food calls and the sound of a predator's footsteps. Anim. Behav.74 1117-29.

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