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Reducing the availability of food to control feral pigeons:
changes on population size and composition
Journal:
Pest Management Science
Manuscript ID
PM-15-0663.R1
Wiley - Manuscript type:
Research Article
Date Submitted by the Author:
n/a
Complete List of Authors:
Senar, Juan Carlos; Natural History Museum of Barcelona, Evolutionary and
Behavioural Ecology Unit
Montalvo, Tomas; Agència de Salut Pública de Barcelona, Servei de
Vigilància i Control de Plagues Urbanes
Pascual, Jordi; Natural History Museum of Barcelona, Evolutionary and
Behavioural Ecology Unit
Peracho, Victor; Agència de Salut Pública de Barcelona, Servei de Vigilància
i Control de Plagues Urbanes
Key Words:
Integrated management, Feral pigeon, public information, food reduction,
control, limiting factors
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Reducing the availability of food to control
feral pigeons: changes in population size
and composition
Running title: Integrated management of feral pigeons
Juan Carlos Senar
1
, Tomás Montalvo
2
, Jordi Pascual
1
& Victor
Peracho
2
1
Natural History Museum of Barcelona, Pº Picasso s/n, 08003 Barcelona, Spain
2
Servei de Vigilància i Control de Plagues Urbanes, Agència de Salut Pública de Barcelona,
Av.Príncep d'Astúries 63, 3r.2a, 08012 Barcelona, Spain
Correspondence autor: Juan Carlos Senar, Natural History Museum of
Barcelona, Pº Picasso s/n, 08003 Barcelona, Spain. E-mail: jcsenar@bcn.cat
Word count for the abstract: 196
Word count: 4,085
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BACKGROUND: Since feeding by humans is one of the main food resources to
1
pigeons (Columba livia), there is general agreement that public education that
2
aims to reduce the food base may be the most feasible way to reduce pigeon
3
abundance. However, except for the classic example of Basel, the method has
4
rarely been tested or implemented. We provide results from a one-year study in
5
the city of Barcelona where we tested the effect of public education on pigeon
6
population abundance and composition.
7
RESULTS: The quantity of food provided by people to pigeons was significantly
8
reduced during the study. Feral pigeon density was reduced by 40% in the two
9
experimental districts, but no variation was detected in the control district.
10
Detailed analyses in one of the districts showed that the reduction was mainly
11
related to the reduction in food availability but not to culling. Pigeons captured at
12
the end of the experiment were larger than at the start of the study but body
13
condition was reduced.
14
CONCLUSION: Results show the effectiveness of public information to manage
15
feral pigeon populations in a large city and that control operations can exert
16
important selection pressure on the population leading to changes in population
17
composition.
18
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Key words: Feral pigeon, population size, public information, food reduction,
20
culling.
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1. INTRODUCTION
22
The size of the populations of feral pigeons Columba livia has increased
23
dramatically in many cities during the second half of the 20th century both in
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Europe and in North America
1
. This increase has been associated with many
25
problems related to the damage to urban architecture and the transmission of
26
infectious diseases
1-4
, hence giving rise to an increasing concern by city
27
authorities and managers. Since damage is related to the number of pigeons
5
,
28
we have to reduce their numbers if we want to reduce pigeon damage in cities.
29
Many methods have been suggested to reduce urban feral pigeon populations
30
4,6-10
. Nevertheless, population models suggest that restricting the availability of
31
food and nesting resources in the city should be the most effective and long-
32
lasting method
4,9
. Since feeding by humans is one of the main food resources
33
to pigeons, public education that aims to reduce the food base may be the most
34
feasible way to reduce pigeon abundance
1,5
. The method was successfully
35
implemented in Basel
5
in the 1980s and more recently in Venice
4
. However,
36
implementation in a large city with high pigeon density
11
and where dispersal
37
movements between close areas can be important may entail more difficulties
38
than in other locations. For instance, Basel had a density of 840 pigeons/km
2
5
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before public information programs were undertaken while Barcelona has a
40
density of 4,242 pigeons/km
2
12
. Movements within the city between close areas
41
are also important in Barcelona
13
and they could limit the success of a public
42
information program about pigeon control. Additionally, and for a proper
43
validation of the method, control populations should also be used to ascertain
44
whether the reduction in feral population is the result of management operations
45
or natural fluctuations in the population.
46
A topic of great interest from an evolutionary perspective is that reducing food
47
availability and distribution could exert selection pressure that could change
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population composition and hence select the population towards a different
49
optimum
14,15
. In feral pigeons it has been shown that in urban populations fed
50
by people birds adopt a sit-and-wait foraging strategy which selects for longer
51
tarsi, while short tarsi are selected for in populations with less feeding by
52
humans, which promotes active searching for food
16
.
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The aim of this work was to test the success of feral pigeon management based
54
on public education combined with culling operations in Barcelona where
55
pigeon density is high. We used a design with experimental and control areas.
56
A successful public education should entail a reduction in the quantity of food
57
available to pigeons along the study. As a consequence, we should expect a
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concomitant reduction in pigeon density in experimental areas compared to the
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control one. We predicted that if public education was the main reason for the
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reduction in pigeon density, population size reduction should better correlate
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with the quantity of public informed than with culling effort. Additionally, because
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of the fact that larger individuals may enjoy a priority of access to the reduced
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food supplies
17,18
, we predicted an increase in the size of the pigeons along the
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study.
65
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2. MATERIAL AND METHODS
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The study was carried out in Barcelona city in 2009. Barcelona has an area of
68
102 km
2
, 72% of which is built up. The city is divided into ten districts and
69
73 neighbourhoods, which allows decentralized local administration. The
70
experimental study was carried out in two districts: Sant Andreu [SA] and Horta-
71
Guinardó [HG]. In SA we sampled four neighbourhoods: 1. Navas, 2. Congres i
72
els Indians, 3. La Sagrera and 4. Sant Andreu. In HG we sampled two
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neighbourhoods: 5. Guinardó and 6. Baix Guinardó (Figure 1). An additional
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neighbourhood (7. Vilapicina-Torre Llobeta), within the district of Nou Barris,
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was used as a control area where no experimental action was carried out. We
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choose this neighbourhood as a control area because it was adjacent to the two
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experimental districts, so that habitat structure and socioeconomic variables
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were quite similar, and because it was someway in between the two
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experimental districts. We used squares of 250x250m (6.25 ha) as the sample
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unit (Barcelona contains 1,568 of these units). The size of this unit was
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determined on the basis of the home range area of pigeons in Barcelona, which
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is about 3.5 ha
13
. The study was based on a total of 44 experimental (32 in SA
83
and 12 in HG) and 12 control squares. The size of the control area was smaller
84
than the SA experimental area, but similar to the size of the HG experimental
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area. In any case, we thought that 12 sample control units should be enough to
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ascertain whether the reduction in feral population was the result of
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management operations or natural fluctuations in the population. As a
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consequence, we preferred to concentrate efforts in increasing the number of
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sample units in SA to allow for a powerful multiple regression, within the same
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district, to test for the differential effect of public information and culling efforts
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on population size reduction (see below).
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In the six experimental neighbourhoods we carried out a campaign of public
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education, aimed to reduce the food base for pigeons. The campaign started on
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1st February 2009 and finished on 22th February 2010. It consisted in
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distributing a pamphlet explaining the negative effects of feeding pigeons both
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for pigeons and for the public in a similar way as in Haag-Wackernagel (1995).
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We used seven city council information agents to contact people in city parks,
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gardens and streets for every working day from 0800 to 1200 hours (or from
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0900 to 1300 hours, depending on the week). The agents explained the content
100
of the pamphlet making an especial effort to inform people which were observed
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feeding pigeons. They also informed the local shopkeepers about the project. In
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total, we contacted 2,190 citizens. The information agents collected also data
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on the number of individuals engaged in feeding pigeons (N= 74) and the
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availability of food for pigeons disposed in the streets (see below). We also
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established three capture sessions with the elimination of individuals (pigeons
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culled: 06/03/09: 5,935; 03/07/09: 4,083; 13/11/09: 2,252). As in Haag-
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Wackernagel (1995) culling was done to adapt pigeon population size to the
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reduced food supply initiated by the public restriction of feeding. Pigeons were
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captured using pneumatic cannon nets. Capture areas were baited at the point
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of capture during 4-5 days prior to capture to increase success.
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The experimental and control squares were surveyed by walking along all the
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roads in each sample unit (circuitous path) where we counted all visible pigeons
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(quadrate counts)
19,20
. Because a part of the population can be hidden and
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remain undetected bird detection probability must be considered
20-24
. In
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previous work we derived a correction factor of 3.5 to account for detectability of
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pigeons based on a double sampling procedure
19
using visual surveys and
117
capture-recapture approaches
20
. This value was consistent across different
118
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cities
20,25,26
, and so we assumed that although the index for Barcelona was
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derived many years ago, it could also be used now to estimate feral pigeon
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population size. We are uncertain whether the index can change thorough the
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year
9
; however, since we are comparing population size values between
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experimental and control areas, yearly changes in detection probability should
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affect the different areas in a similar way, so that comparisons are still valid.
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All counts were carried out between 9-14h, which is the period with maximum
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detectability
11
. We carried out a minimum of 3 counts per square within each
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sample period and used the mean of the three values. Whenever one of the
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censuses was clearly different from the mean value (>50%) we carried out two
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additional censuses and used the mean value. Population size surveys were
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carried out 9-25 February, 8-24 June, 19 October-4 November and 28
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December-15 January. These periods are denominated as the February, June,
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October and January census.
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Data were analyzed with a repeated measures ANOVA, where census data
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from each district were paired. Feral pigeon density (number of pigeons by 6.25
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ha), at each of the sampled squares, was the dependent variable. Independent
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variable Time included the four paired census periods previously detailed
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(February, June, October and January). Independent variable District included
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the two experimental districts (SA and HG) and the control district.
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For the district of SA, where most squares were monitored, we tested several
139
variables for correlation to population size. i). Food availability provided by
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people. We ranked food deposited in streets 1: <200g, 2: 200-500g, 3: 500-
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1,000g, 4: 1,000-3,000g of food per square and day. Food availability was
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estimated by information agents meanwhile visiting each square to inform
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people. The quantity of food (according to the previous scale) found at different
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points within each square were summed to obtain a daily estimation of food
145
available per square. Values estimated from different days were averaged. ii).
146
Reduction in food supply. The main aim of the information agents was to make
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citizens aware of the problems of feeding pigeons and that people do not
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continue to provide food to the birds. We computed an index of variation in food
149
availability as the quantity of food available in the period between the first two
150
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censuses minus the quantity of food available in the period between the last two
151
censuses. The reduction in food supply in SA district was tested comparing the
152
quantity of food available to pigeons (semi quantitative scale, see i) between the
153
two periods (see ii), within each square, which were paired, using a non-
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parametric Wilcoxon Matched Pairs test. iii). Culling effort. We used the total
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number of pigeons captured per square. This data set was analyzed with a
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multiple regression, using ranked data to avoid problems related to the lack of
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normality in the variables used
27,28
. The dependent variable included the
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reduction in the number of pigeons at the squares of the SA district (census 4 -
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census 1), and independent variables included absolute quantity of food
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provided by people (i), reduction in the quantity of food available per square
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since the start of the experiment (ii), and number of pigeons culled at each
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square (iii) (N=32 squares).
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Body measures were recorded for a sample of individuals (N=483) from the
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different sampling units at the start (1 February) and at the end of the
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experiment (13 November). For each individual we measured body mass, skull
166
length and wing length with a ruler to the nearest mm. Additionally, an index of
167
body condition was computed by regression using standardized residuals of
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body mass and skull length
29
.
169
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3. RESULTS
171
At the start of the experiment, the density of pigeons at the SA district was
172
higher than at the HG and control districts (figure 2)(Post hoc Planned
173
Comparison tests; SA vs. HG: F
1,53
= 9.26, p<0.01; SA vs. CTL: F
1,53
= 9.92,
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p<0.01; HG vs. CTL: F
1,53
= 0.01, p=0.93). The number of pigeons at the two
175
experimental districts was reduced by a 40% between February and June
176
(figure 2)(Post hoc Planned Comparison tests; SA: F
1,53
= 75.90, p<0.001; HG:
177
F
1,53
= 9.84, p<0.01). Number of pigeons at the control district did not vary during
178
the study (Feb vs. June: F
1,53
= 0.44, p=0.51; whole period: F
1,53
= 0.51,
179
p=0.48)(figure 2, note significant interaction between Districts and Time).
180
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The quantity of food available to pigeons from the start to the end of the study
181
was significantly reduced in the study area (Median 1.5 vs. 1.0 units of food by
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square; Wilcoxon Matched Pairs Test: Z= 2.58; p<0.01; N=32;). The reduction
183
in the number of pigeons at the squares of the SA district (census 4 - census 1)
184
was correlated to the reduction in the quantity of food available per square since
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the start of the experiment (r partial= 0.37, p<0.05), so that squares with a
186
higher reduction in food availability reduced population size to a higher degree.
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The number of pigeons culled at each square (mean value= 159; 95% CI: 94-
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224) and the absolute quantity of food provided by people had no effect on the
189
reduction in pigeon density (culled individuals r partial= 0.07, p=0.72; food
190
availability r partial= -0.18, p=0.33; N=32 squares).
191
Pigeons captured at the end of the experiment were larger by 1-3% than at the
192
start of the study, before culling and public information were implemented
193
(figure 3). The body condition of pigeons, however, was reduced during the
194
study by a 6% (figure 3).
195
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4. DISCUSSION
197
The sustainable reduction of the number of pigeons in urban habitats is one of
198
the main aims of urban wildlife managers
4
. As in the case of other urban
199
nuisance wildlife, reducing the food provided by humans should be the target of
200
managers
4,9,30
. However, this is rarely attempted, especially in large cities (see
201
an exception in Haag-Wackernagel
5
and Giunchi et al.
4
). Results from our
202
experimental study in Barcelona city showed that public education aimed to
203
reduce the food base, succeeded in reducing both food available and feral
204
pigeon abundance.
205
Pigeon abundance was reduced by 40% between February and June and did
206
not increase until the following January. The effect was not apparent in the
207
control areas, where no action was carried out. The reduction in the number of
208
pigeons was mainly affected by the reduction in the quantity of food available to
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pigeons rather than by the culling actions. In fact, culling reduces pigeon density
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at the capture sites but if food abundance is not reduced simultaneously the
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pigeons from the surroundings quickly refill the emptied area so that in a few
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days the density recovers
13
. Instead, the reduction of food availability has
213
permanent effects, since the area cannot hold the same number of pigeons as
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before. Hence, data strongly support the view that the reduction of the carrying
215
capacity of the environment through food reduction is the best way to attain an
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efficient feral pigeon population size control
4,5
.
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The main reduction in pigeon abundance was attained in just four months. This
218
period probably may be enough to cause a reduction in pigeon survival and
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breeding success when linked to a reduction in food availability. However, given
220
the fast dispersal responses of feral pigeons to variations in food availability and
221
pigeon density
13
, it is also possible that a part of the population emigrated from
222
the experimental squares to other areas of the city, so that the reduction found
223
in pigeon numbers could be the combined effect of both processes.
224
Nevertheless, and from the perspective of a city manager, pigeon numbers
225
were successfully reduced permanently whatever the main reason for the
226
reduction.
227
The lack of variation in pigeon density in the control area along the year is
228
surprising. Population size should increase for instance during and after the
229
breeding season, and should decrease after the late summer population crisis.
230
We think that stability found in the control area may be a by-product of using a
231
constant detectability along the year when in fact, this detectability most
232
probably changes according to period
9
. During the breeding season,
233
detectability should be reduced and the correction factor should increase, since
234
many females may be incubating and hence, are not available during census.
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Detectability of juvenile birds may also be different from that of adult birds. All of
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this can mask census values. Nevertheless, we have to emphasize that this
237
does not affect to the main results of the paper, since we are comparing
238
experimental and control sample units and detectability should probably covary
239
between units in a similar way.
240
It has been shown earlier that in urban pigeon populations where people
241
provide abundant food, pigeons are selected for longer tarsi, while short tarsi
242
are selected for in populations with less feeding
16
. Population size
243
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management also had an effect on the size and body condition of the pigeons.
244
Skull and wing length increased and body mass and body condition decreased.
245
This could be a consequence of a trapping bias, if the first captures were a
246
biased part of the population. However, this is improbable since the birds to be
247
trapped first, and being removed from the population, would have been
248
dominant and large individuals that monopolize abundant food sources
17
.
249
Alternatively, our results could be interpreted as a consequence of dominant
250
and hence larger individuals, being favoured because of their priority of access
251
to the reduced food supplies
17,18
. It could also be that the smaller birds (as
252
young individuals or females) emigrate first from the experimental squares. In
253
both scenarios, the presence of high competence to access reduced food
254
resources may have caused the reduction in the body condition of the birds.
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Whatever the case, results show how reducing food availability and distribution
256
because of control operations can exert important selection pressure that can
257
change population composition.
258
Summarising, reducing feral pigeon abundance in cities is clearly better
259
achieved by reducing the food provided by humans, and public education aimed
260
to reduce the food base should be the target of managers.
261
262
ACKNOWLEDGEMENTS
263
We thank two anonymous referees for their comments and suggestions. This
264
work was supported by the Barcelona Public Health Agency, and by research
265
project CGL2012-38262, Ministry of Economy and Competitivity, Spanish
266
Research Council. We thank Lluïsa Arroyo, Jordi Faus, Daniel Riba and
267
Margarida Barceló for field support.
268
269
Reference List 270
271
1. Johnston RF, Janiga M, Feral pigeons. Oxford University Press, New York, (1995). 272
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2. Bevan RDR. The Costs of Feral Pigeons. Feral Pigeons. Biology-Problems-Control; London: 273
British Ornithological Union; 1990 p. 10-1. 274
3. Haag-Wackernagel D, Parasites from feral pigeons as a health hazard for humans. Annals 275
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Problems, Dynamics and Control Methods. In: Integrated Pest Management and Pest 278
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popolazione urbana di Columba livia. In: Control of Synanthropic bird populations: 287
problems and prospectives,WHO/FAO; Roma, pp. 106-10, (1996). 288
9. Giunchi D, Baldaccini NE, Sbragia G, Soldatini C, On the use of pharmacological 289
sterilisation to control feral pigeon populations. Wildlife Research 34:306-318 (2007). 290
10. Ballarini G, Baldaccini NE, Pezza F, Colombi in città. Aspetti biologici, sanitari, giuridici. 291
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(Columba livia var.) de la ciudad de Barcelona. Misc Zool 8:237-244 (1984). 294
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efectividad del control por eliminación de individuos. Arxius de Miscle·lània Zoològica 4: 297
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removing individuals. Can J Zool 73:1154-1160 (1995). 300
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18. Sol D, Santos DM, Cuadrado M, Age-related feeding site selection in urban pigeons 308
(Columa livia): experimental evidence of the competition hypothesis. Can J Zool 78:144-309
149 (2000). 310
19. Williams BK, Nichols JD, Conroy MJ, Analysis and Management of Animal Populations: 311
Modeling, estimation, and decision making. Academic Press, New York, (2002). 312
20. Senar JC, Sol D, Censo de Palomas Columba livia var. de la ciudad de Barcelona: 313
Aplicación del muestreo estratificado con factor de corrección. Butll GCA 8:19-24 (1991). 314
21. Conroy MJ, Carroll JP, Quantitative conservation of vertebrates. Wiley-Blackwell, Oxford, 315
(2009). 316
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monitoring feral pigeon ( Columba livia f. domestica ) urban populations. Urban 318
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for estimating Feral Pigeons (Columba livia f. domestica) population size: a reply to 321
Amoruso et al. (2013). Urban Ecosystems 17:719-722 (2014). 322
24. Sacchi R, Razzetti E, Gentilli A, A methodological approach to feral pigeon (Columba livia) 323
census in urban areas. Rivista Italiana di Ornitologia 75:119-27 (2007). 324
25. Sacchi L, Gentilli A, Razzetti E, Barbieri F, Effect of building features on density and flock 325
distribution of feral pigeons Columba livia var. domestica in an urban environment. Can J 326
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statistics. Amer Statistician 35:124-129 (1981). 332
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30. Adams CE, Lindsey KJ, Ash SJ, Urban Wildlife Management. CRC Press, New York, (2006). 336
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Figures 339
Figure 1. Map of Barcelona showing in black and light grey the neighbourhoods where 340
we carried out the experimental work (in the Sant Andreu and Horta-Guinardó districts, 341
respectively), and in dark grey the neighbourhood used as a control. 342
343
Figure 2. Variation in population density of feral pigeons in the two experimental 344
districts (Sant Andreu- black circles and Horta Guinardó- black squares) and the 345
control district (Nou Barris: open diamonds), according to the population surveys. Error 346
bars refer to S.E. Time included four paired census periods: February (9-25 Feb), June 347
(8-24 Jun), October (19 Oct-4 Nov) and January (28 Dec-15 Jan). RMANOVA analysis: 348
District F
2,159
= 5.13, p<0.01; Time F
3,159
= 13.17, p<0.001; District x Time F
6,159
=3.60, 349
p<0.001. When comparing census 1 with 2 we found significant reductions in number 350
of pigeons for Sant Andreu (F
2,153
= 75.90, p<0.001) and Horta-Guinardó (F
2,153
= 9.84, 351
p<0.01), but not for the Control district (F
2,153
= 0.44, p=0.51). Comparisons between 352
census 3 and 4 were not significant for the three districts (all p>0.23). 353
354
Figure 3. Variation in morphometry of feral pigeons captured in the Sant Andreu district 355
prior to and after management operations. Error bars refer to S.E. ANOVA results for 356
body mass: F
1,481
= 8.70, p<0.01; body condition: F
1,481
= 32.65, p<0.001; skull length: 357
F
1,481
= 64.17, p<0.001; wing length: F
1,481
= 12.35, p<0.001. 358
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Figure 1 369
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Figure 2 371
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February June October January
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Number of pigeons / 6.25 ha
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Prior After
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Body mass (g)
Prior After
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Body condition (g)
Prior After
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32,0
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33,4
Skull length (mm)
Prior After
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Wing length (mm)
Figure 3 384
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