University of Southampton - Acoustic Response of songbirds to the calls of three species of predatory bird
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Institution
University Of Southampton (UOS)
This study aims to understand the response of songbirds to differing predators, where each provide a different threat level. To do so, calls from the predators were played to songs birds on a road and in a forest and their response was recorded on an audiomoth. The magnitude was calculated to show ...
Acoustic response of
songbirds to the calls of
three species of predatory
bird
BIOL2055: Behaviour and Ecology field course
[BSc Zoology]
Word Count: [2147]
Abstract
This study aims to understand the response of songbirds to differing predators, where each
provide a different threat level. To do so, calls from the predators were played to songs birds
on a road and in a forest and their response was recorded on an audiomoth. The magnitude
was calculated to show the difference in magnitude before and after the predator call was
played. This showed that the birds had the greatest response to the novel predator (an African
fish eagle) which suggested a neophobic response. The least response was in response to the
native predator (peregrine falcon), which suggested the birds may be following the adaptive
silence hypothesis causing the birds to reduce their noise output to conceal themselves from
the predator.
Introduction
Animal communication is ‘the action of or cue given by one organism [the sender] is
perceived by and thus alters the probability pattern of behaviour in another organism [the
receiver] in a fashion adaptive to either one both participants’ [1]. Understanding animal
communication not only gives insights into the worlds of animals and their interactions, but
also the evolution and speciation of some species [2]. This is shown in animals such as
lacewings, where 3 species which all look identical in appearance are reproductively isolate
from each other due to a difference in call frequency, which allows a female to determine
which male is from her species[3].
Alarm calls are a well-studied form of animal communication, by which one individual in the
group gives a call to the other members, to alert them of danger [4]. The purpose of an alarm
call may vary between or among a species with a various purposes suggested, some of which
are: to avert a predators attention to other prey (often when an alarm call would lead to group
, mobbing, aggregation or pandemonium); the discouragement of predator pursuit by
informing a predator it has been seen so the element of surprise is lost and to alert relatives to
the presence of a predator to increase their chance of survival[5]. The latter of the three is the
most common suggested for passerines, in which it has been suggested that kin selection led
to the evolution of the altruistic behaviour of alarm calling[6].
Eavesdropping on predator calls have been shown in various prey species to gain information
on the presence of predators, which has strong selection pressures on the ability to correctly
interpret these calls due to the fitness benefits of doing so[4,7].
This study focuses on songbirds of varying species, which use easily recordable
communication. There was also a high abundance and diversity in the study location
(Bolonia, Spain), which made them an ideal subject. Multiple studies have already shown that
song birds will respond to a recording and hence would provide insight into their
communication and alarm call responses[8,9].
The three predator calls used were an African fish eagle, Haliaeetus vocifer, (a non-native,
non-predator), a black kite, Milvus migrans, (an opportunistic predator) and a peregrine
falcon, Falco peregrinus (a native predator). These were chosen as they each had a different
relationship to the songbirds and hence would elicit a different response.
The aim of this study is to test the mechanisms which underlie the ability to discriminate
between predator calls in songbirds and the subsequential response to these calls. The null
hypothesis is that there is no difference in magnitude of response to each predators’ calls. The
alternate is that there is a difference in magnitude of response. The second null hypothesis is
that there is no difference between the magnitude of response in each habitat. With that
alternate hypothesis being that there is a difference in magnitude of response.
Methods
Data Collection
A transect was made along the CA-8202, by the same individual measuring 200 paces along
the left-hand side of the road. This generated 5 site locations on the road, with each one being
photographed, and the location marked on a map for future reference. On arrival at each site,
a 3-minute silence was held to allow for the birds to acclimatise to human presence. Followed
by 2 minutes of recording before a predator call was played and then a further 2 minutes of
recording after the predator call had been played 3 times. The predator call was played
through a Bluetooth speaker which was held in the air, as the predatory birds were unlikely to
be found on the ground. An audiomoth was used to record the acoustic landscape. This was
then repeated along the footpath through the forest to generate another 5 site locations. There
were 3 recording times each day: early morning (7:00); morning (10:30) and evening (17:30).
On the first day, an African fish eagle call was played, a black kite was used on the second
day and a peregrine falcon was used on the third. The predator bird calls were taken from the
internet and edited to remove any white noise or static during and between calls.
While each recording was being conducted, one member of the team was using the ‘BirdNet’
app to identify any birds present by their call[10].
The recordings were entered into Audacity, where a 10-second snippet was selected every 30
seconds for the 2 minutes before the predator call [11]. 10 seconds after each of the 3 calls was
selected and then a further 2 10 second snippets were taken after every minute following the
predator calls. This gave 4 snippets before the predator calls and 5 post the predator calls.
Each snippet was then used to generate a spectrogram, with the total magnitude between
1KHz and 12KHz calculated for each and then a before and after call magnitude was
calculated for each recording. Only frequencies between 1KHz and 12KHz were used as this
band covers most bird calls and helps to remove background noise.
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