Faculty of Science
THE NEIGHBOURHOOD WATCH: DOES THE PROXIMITY TO
NEIGHBOURS INFLUENCE NEST SUCCESS IN TREE SWALLOWS
(TACHYCINETA BICOLOR)?
2021 | RAVEN SANTANA MOLLER

B.Sc. Honours thesis – Biology

THE NEIGHBOURHOOD WATCH: DOES THE PROXIMITY TO NEIGHBOURS
INFLUENCE NEST SUCCESS IN TREE SWALLOWS (TACHYCINETA BICOLOR)?
by

RAVEN SANTANA MOLLER

A THESIS SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF SCIENCE (HONS.)
in the
DEPARTMENT OF BIOLOGICAL SCIENCES
(Animal Biology)

This thesis has been accepted as conforming to the required standards by:
Matthew Reudink (Ph.D.), Thesis Supervisor, Dept. Biological Sciences
Stephen Joly, Co-supervisor, Dept. Biological Sciences
Sean Mahoney (Ph.D.), Co-supervisor, Dept. Biological Sciences
Tom Dickinson (Ph.D.), External examiner, Dept. Biological Sciences

Dated this 23th day of April, 2021, in Kamloops, British Columbia, Canada
© Raven Santana Moller, 2021

ABSTRACT
Due to the similarities in habitat requirements, tree swallows (Tachycineta bicolor) live in
close proximity to other secondary cavity nesters, such as the mountain bluebird (Sialia
currucoides. While these birds do not compete directly after territory establishment, relationships
between neighbours can impact the success of swallows. Theoretical and empirical work suggests
that neighbours can increase success through processes of reciprocal altruism in nest defence or
decrease success through density-dependent competition. The goal of this study is to understand
the impact that conspecific and heterospecific neighbours have on tree swallow reproductive
success. To do this, we analyzed 659 tree swallow nest success over an 8-year period. We found
that the number of nestlings per nest increased as the distance to the nearest swallow neighbour
increased and the number of swallow neighbours within 1000m decreased. Also, we found that the
number of fledglings per nest increases as the number of swallow neighbours within 500m
increased but decreased when there were more swallows within 250m. Lastly, we found that
mountain bluebird neighbours had a marginal negative effect on reproductive success. These
results indicate that the relationship between neighbours is scale-dependent, but consistent with
reciprocity, density-dependent competition, and the dear enemy phenomenon.

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ACKNOWLEDGEMENTS
Throughout the process of completing my thesis, I have experienced much guidance and
support from those around me. I would like to extend my sincere thanks to my supervisor, Matthew
Reudink for guiding me down this path, being available for many edits, and sharing his expertise
with me. I would also like to thank my co-supervisors, Stephen Joly and Sean Mahoney. Stephen
Joly for showing me the ropes of compiling data and lending his bird-watching expertise. Sean
Mahoney for the work he put into the data analysis to prevent me from losing my mind trying to
write R code. I’d like to acknowledge the moral support from the other members BEAC Lab team.
I’d like to thank my fiancé, Riley Mager, for being a sounding board for ideas and helping me
work through any challenges I faced. Lastly, I am extremely grateful for the support of my family
throughout my entire academic career.

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TABLE OF CONTENTS
List of Tables ..………………………………………………………………………..... v
List of Figures ………………………………………………………………………….. vi
1. Introduction

……………………………………………………….………..….. 1

2. Materials and Methods
3. Results

…………………………………………………………………………. 8

4. Discussion
Literature Cited

………………………………….…….….…………… 5

……………………………………………………………………. 13

………………………………………………………………………… 18

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LIST OF TABLES:
Table 1. Kamloops Naturalist Club monitored artificial nest boxes for each observed. ................ 6
Table 2. A complete summary of the negative binomial generalized mixed effects models analysis
results of each distance factor in comparison to reproductive success involving conspecific
neighbour. ....................................................................................................................................... 9
Table 3. A complete summary of the negative binomial generalized mixed effects models analysis
results of each distance factor in comparison to reproductive success involving heterospecific
neighbours. .................................................................................................................................... 12
Table 4. A summary of the theories involving neighbouring relationships being tested in
comparison to our predictions and findings from analysis. .......................................................... 13

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LIST OF FIGURES:
Figure 1. The number of tree swallow nestlings in relation to the distance to the nearest conspecific
neighbour scaled by subtracting each value from the mean and dividing by the standard deviation..
....................................................................................................................................................... 10
Figure 2. The number

of tree swallow nestlings in relation to the number of conspecific

neighbouring nests within 1000m of each nest . ........................................................................... 10
Figure 3. The number of tree swallow that fledged the nest in relation to the number of conspecific
neighbouring nests within 250m. .................................................................................................. 11
Figure 4. The number of tree swallow that fledged the nest in relation to the number of conspecific
neighbouring nests within 500m ................................................................................................... 11
Figure 5. The number of tree swallow nestlings in relation to the distance to the nearest
heterospecific neighbour . ............................................................................................................. 13

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INTRODUCTION
Inter- and intraspecific interactions impact the evolution of animal behaviour and ecology.
These interactions can be particularly intense during the breeding season when establishing
territories, as individuals compete for breeding sites.

Due to this high intensity of competition,

the presence of conspecific or heterospecific competitors makes it more difficult to gain an optimal
nesting site (Wiebe 2016). That said, once territorial conflicts have been resolved and nests are
established, dynamics between neighbouring birds (i.e., conspecific or heterospecific pairs of birds
that have already established a territory in an adjacent area) change as the nature of their
interactions shift. In some cases, neighbouring birds can act cooperatively, positively influencing
reproductive success (Ligon 1983; Lombardo 1985; Wiebe 2016), while in other cases, the
proximity and abundance of neighbours can negatively impact the reproductive success (Sillett et
al. 2004).
One way neighbours can increase nest success is through reciprocity in nest or territory
defence (Ligon 1983; Lombardo 1985). Reciprocity, or reciprocal altruism, is commonly observed
among closely related individuals; however, reciprocity can also occur among individuals without
kinship ties (Trivers 1971; Ligon 1983). Ligon (1983) suggested that reciprocity can occur
between individuals if one individual helps another with the expectation that the other will repay
the favour in the future (Ligon 1983). Interactions of this nature would assume a certain extent of
cognitive ability as the individual must be able to recognize previously established and future
patterns of assistance. This cooperation creates a mutualistic relationship between neighbours
through reciprocal-altruistic selection (Ligon 1983; Krams et al. 2008). Krams et al. (2008)
observed that reciprocal altruism does occur naturally in some species. For example, in pied
flycatchers (Ficedula hypoleuca), conspecific neighbours assist in mobbing behaviour initiated by
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cooperative neighbours but will not assist neighbours that defected from past assistance (Krams et
al. 2008).
The evolution of reciprocal behaviour may be explained by the TIT for TAT model, which
was based off the Prisoner’s Dilemma game (Axelrod and Hamilton 1981; Ligon 1983; Lombardo
1985; Krams et al. 2008). According to this model, during an interaction, individuals have the
option to exhibit restraint by acting cooperatively through reciprocating acts or incite conflict
against the other party (Lombardo 1985). By experimentally simulating intraspecific competition,
Lombardo (1985) observed interactions between breeding tree swallows and non-breeding tree
swallows. The highest fitness for an individual occurs if they incite conflict and the other remains
cooperative by exhibiting restraint. However, this puts the cooperator at a fitness disadvantage,
making the relationship unstable. If both parties defect from cooperation, both suffer reduced
fitness. If both interacting birds show restraint, their average fitness increases to intermediate
between the prior two options, leading to the most evolutionarily stable strategy (Lombardo 1985).
Neighbours can also be beneficial by enhancing predator detection both directly (e.g., through
mobbing) or indirectly through eavesdropping on neighbour alarm calls (Templeton and Greene
2007; Magrath et al. 2009; Fallow et al. 2013). Eavesdropping, in combination with defensive
responses by neighbours toward mutual threats (i.e., reciprocity), can result in mutual defence of
a nest site or territory from predators by stimulating a mobbing response (Templeton and Greene
2007; Magrath et al. 2009; Fallow et al. 2013).
In a theory first proposed by Fisher in 1954, the “dear enemy” phenomenon describes both a
positive and negative effect of neighbours on nest success (Briefer et al. 2008). In Fisher’s model,
aggression towards neighbours depends on the degree of familiarity with the neighbours (Briefer
et al. 2008). Neighbours that are familiar and nearby tend to have interactions that are less

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aggressive compared to neighbours that are unfamiliar and farther away as they are perceived to
be less of a threat at territory bounds due to previous repeated interactions and an established social
relationship (Falls and Brooks 1975; Briefer et al. 2008). Familiar neighbours redirect the energy
that would be expended on aggressive acts towards other more productive acts, such as foraging
or predator defence (Briefer et al. 2008).
Neighbour proximity can also lead to conflict through behavioural interference with
heterospecific and conspecific neighbours (Grether et al. 2017). Behavioural interference involves
sexual or aggressive interactions performed by one species that can have either direct or indirect
negative effects on individuals of another species (Grether et al. 2017). Both sexual and aggressive
interference, depending on the severity, can result in territory loss or habitat partitioning in one or
both species (Grether et al. 2017). As a result of behavioural interference, selection may favour
divergent character displacement, whereby the species that is being negatively affected will evolve
responses that reduce the cost of the negative interactions (Grether et al. 2017). These evolved
responses can include altering habitat use to avoid the conflict with hetero- and conspecifics, or a
change in the timing of breeding (Grether et al. 2017). An example of this can be seen in as western
bluebirds have been seen to be replacing mountain bluebirds in northwestern USA due to
aggressive interference (Grether et al. 2017). This is a broad evolutionary trend that can be seen in
both heterospecific and conspecific interactions and may explain the negative effects that close
neighbour proximity can cause.
Sillet et al. (2004) demonstrated that conspecific neighbours can also negatively impact nest
success through density-dependent competition. A three-year study showed that when neighbour
density was reduced in the territory of black-throated blue warblers (Dendroica caerulescens), the

3

males spent more time at the nest, and fledged more offspring than in areas with higher neighbourdensity, likely because of overcrowding and reduced resource availability (Sillett et al. 2004).
The presence of both conspecific and heterospecific neighbours may influence patterns of
reproduction; however, these relationships are complex and likely species-specific. This gap in our
understanding can be indirectly addressed by observing the effects of nest proximity on the
reproductive success of co-occurring heterospecific individuals.
Tree swallows (Tachycineta bicolor) and mountain bluebirds (Sialia currucoides) offer an
ideal opportunity to examine the influence of conspecific and heterospecific neighbours on
patterns of reproduction, as they are both migratory bird species that co-occur across interior
British Columbia, currently nesting in bird boxes mounted on fenceposts. Both species inhabit
open spaces and forest edges and are secondary cavity nesters (i.e., they occupy cavities previously
excavated by other species). Tree swallows typically arrive on the breeding grounds approximately
2 weeks later than mountain bluebirds, which gives the mountain bluebirds a head-start on territory
establishment (Wiebe 2016; Drake and Martin 2020). Although they share the same habitat and
overlap in nesting requirements, mountain bluebirds are ground foragers while tree swallows are
aerial insectivores (Johnson and Dawson 2019), allowing the two species to coexist with little
competition other than for nest sites (Meek et al. 1994; Wiebe 2016).
When available, tree swallows and mountain bluebirds will nest in artificial nest boxes. These
nest boxes, while convenient, have some drawbacks. They are typically lower to the ground than
a natural tree cavity and have thinner walls (Johnson and Dawson 2019). Bluebird boxes are often
designed with a small diameter entrance to exclude larger species, like European starlings (Sturnus
vulgaris) (Koch et al. 2012). Tree swallows often win in direct competition for access to nest boxes
due to their aggressiveness and unrelenting harassment of nest competitors (Meek et al. 1994).

4

However, once a nest by one or the other species has been built and eggs laid, the direct aggression
usually ceases and the two species no longer appear to perceive each other as threats (Meek et al.
1994; Luke Phillips et al. 2019). Tree swallows and mountain bluebirds tend to coexist with
minimal aggression for the remainder of the breeding season (S. Joly pers. comm.).
Both species defend their nests by staying nearby their nests and swoop and snap their beaks
at perceived threats (Sharman et al. 1994; Johnson and Dawson 2019; Luke Phillips et al. 2019).
Tree swallows tend to do so more aggressively than mountain bluebirds as they produce shriek
alarm calls to ward off predators (Johnson and Dawson 2019). The level of aggression and nest
defence from birds in close proximity could thus indirectly result in shared predator defence.
Therefore, this study system allows us to examine reciprocal relationships between conspecific
and heterospecific neighbours in terms of nest defence. Since these two species are commonly
found together, heterospecific reciprocal altruism could offset ant effects of competitions and
result in higher nest success when the number of neighbours increases (Ligon 1983).
Here, we investigated the effect that the proximity and abundance of conspecific and
heterospecific (mountain bluebird) neighbours have on the reproductive success of tree swallows.
We predicted that the reproductive success of tree swallows will increase as the proximity to both
conspecific and heterospecific neighbours increases since neighbours would provide added
protection and assist in nest defence (i.e., reciprocity).

MATERIALS AND METHODS
Study site and species
From April to August 2012-2019, we, in collaboration with the Kamloops Naturalists Club,
monitored nesting behaviour at 11 nest-box routes in the Kamloops, BC area (Table 1). Grassland

5

habitats with little urbanization and human impact encompassed the majority of the routes that we
monitored. The artificial nest boxes were set up on fence posts enclosing farmland and on tree
trunks at the approximate height of 4 to 6 feet. The boxes varied on the shape of the entry hole,
between a slot and a hole. The entry slots had a dimension of roughly 4 x 2 inches and the holes
had a diameter of approximately 1.5 inches. The main inhabitants of these artificial nest boxes are
tree swallows and mountain bluebirds, with the occasional house wren or mountain chickadee.
Table 1. Kamloops Naturalist Club monitored artificial nest boxes for each observed route.

Volunteers checked each nest box along their assigned route every 4 to 10 days from April
to August, recording the date of observation, the species inhabiting the nest, the number of eggs
and nestlings (alive or deceased), and the number of nestlings that fledged, plus any other
6

observation that could be made. We avoided observations on days with low temperatures, high
winds, or high precipitation to avoid inflicting damage to eggs or young nestlings. We did not
actively remove incubating or brooding birds from nests to count eggs or nestlings. In those
instances, we made notes about a sitting bird and the number of eggs or nestlings were recorded
on a subsequent visit. For a nest to be considered fully fledged, there could not have been any
remaining eggs or deceased nestlings, and the nest-box had to have been undisturbed and in good
condition.
For each breeding season, the box number, type of box, species, number of eggs, nestlings,
and fledglings for each tree swallow nest per route was compiled into a dataset. The closest tree
swallow and mountain bluebird neighbour to each tree swallow nest was determined based on a
distance matrix that compared the straight-line distance between every nest-box on every route.
The distance matrix was created using GPS coordinates of each box using a Garmin etrex 10 GPS
and calculated using the geosphere package in R (Karney 2013). The distance matrix was also
used to calculate the number of conspecific and heterospecific neighbours within 250 meters, 500
meters, and 1000 meters of each tree swallow nest. Specific nest box observations were excluded
if the observation intervals were too large, creating ambiguity regarding first egg date, hatch date
and fledge date. With these edits, a total of 659 tree swallow nests were used for analysis.
Reproductive success was determined by the number of eggs, nestlings, fledglings,
proportion of eggs that hatched to become nestlings, the fledge rate, which is the proportion of
nestlings that became fledglings, and the occurrence of fledging at a nest (yes/no).
Data analysis
To assess the influence of con- and heterospecific neighbors on tree swallow nest success,
we used negative binomial generalized mixed effects models which are used for zero-inflated

7

count data (Yau et al. 2003). In our analyses, we built separate models for the number of eggs,
nestlings, fledglings, the proportion of eggs that hatched, the proportion of eggs that fledged, and
whether the nest was successful overall (i.e., if the nest fledged any young). In each model, counts
of eggs, nestling, fledglings, proportion of eggs that hatched or fledged, and overall success
(yes/no) was the response variable and number of bluebirds and swallows within 250m, 500m, and
1000m, and the distance to the nearest bluebird and swallow neighbour were fixed effects. Year,
site, and nest box type (hole or slot entrance) were included as random effects. We conducted all
analyses using Program R (R Core Development Team 2020) using the lme4 package (Bates et al.
2015).
RESULTS
Effects of Conspecific Neighbours
We found that conspecific neighbours affected tree swallow success during some nesting
periods, but not during others. During incubation, we found no relationship between the number
of tree swallow eggs and the number of con- and heterospecific neighbours (Table 2). In contrast,
during the nestling stage, we found more tree swallow nestlings in the nest boxes in closer
proximity to conspecific neighbours (nearest swallow neighbour: β=-2.10, p=0.036; Figure 1), and
marginally more swallow nestlings in nest boxes with fewer swallows in the distant area (swallows
within 1000m: β =-1.94, p=0.053; Figure 2).

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Table 2. Results from negative binomial generalized mixed effects models testing the effect of
conspecific abundance and proximity on Tree Swallow (Tachycineta bicolor) reproductive
success.

9

Figure 1. The number of tree swallow nestlings in relation to the distance to the nearest conspecific
neighbour (β=-2.10, p=0.036, n=631). Raw values are presented here, but for analyzes, values
were scaled by subtracting each value from the mean and dividing by the standard deviation

Figure 2. The number of tree swallow nestlings was negatively related to the number of conspecific
neighbouring nests within 1000m, however, this effect was marginal (β=-1.94, p=0.053; n=631).
Swallow nests with fewer conspecifics in the immediate area contained more fledglings
(swallows within 250m: β=-2.00, p=0.046; Figure 3). In contrast, swallow nests with more
conspecifics in the surrounding distant area contained more fledglings, however, this effect was
marginal (swallows within 500m: β=1.70, p=0.089; Figure 4).

10

Figure 3. The number of tree swallow that fledged the nest in relation to the number of conspecific
neighbouring nests within 250m (β=-2.00, p=0.046; n=631).

Figure 4. The number of tree swallow fledglings was positively related to the number of
conspecific neighbouring nests within 500m, but the effect was marginal (β=1.70, p=0.089;
n=631).
Effects of Heterospecific Neighbours
Heterospecific neighbours had little influence on swallow reproductive success (Table 3).
However, there was a trend towards swallow nests in closer proximity to a heterospecific
neighbour containing a lower proportion of nestlings that fledged the nest (nearest bluebird
neighbour: β=1.71, p=0.087; Figure 5).

11

Table 3. Results from negative binomial generalized mixed effects model testing the effect of
heterospecific (Mountain Bluebird, Sialia currucoides) abundance and proximity on Tree Swallow
(Tachycineta bicolor) reproductive success.

12

Figure 5. The number of tree swallow nestlings in relation to the distance to the nearest
heterospecific neighbour (β=1.71, p=0.087, n=631).
DISCUSSION
Our study demonstrates that tree swallow reproductive success is associated with the
distance and density of local conspecific and heterospecific neighbours; however, the effect is
scale-dependent. In other words, the effect that neighbours have on tree swallow breeding success
is dependent on the number and distance of neighbours to the nest. Comparing the effects of
conspecific versus heterospecific neighbours, conspecific neighbours appear to have a much
greater effect on reproductive success than heterospecific neighbours. These results are consistent
with three out of the four of our hypotheses describing the effects of neighbours on reproductive
success (Table 4).
Table 4. Table summarizing the hypotheses tested in the present study, whether the hypothesis was
supported,
and
the
main
findings
from
the
present
study.

13

Our findings suggest that nestling success (i.e., the number of nestlings) increases as the
distance to the nearest conspecific neighbour increases but decreases as the density of conspecific
neighbours within 1000m increases. This finding suggests that nestling success is based on a
balance between reciprocity and the degree of familiarity among neighbours, as well as food
availability and predatory defense. In addition, nestling success increases as the number of
conspecific neighbours within 250m decreases but the number of more distant (≥500m) tree
swallows increases. This finding may suggest that nestling production is affected by density
dependent competition if high numbers of conspecifics within 500m reflects high quality habitat
that attracts many neighbours. Due to this finding, future studies could separate the distance effects
to determine a distinction between neighbours between 250m to 500m or 500m to 1000m to
address the density between each distance interval.
We predicted that nearby conspecific neighbours will aid in nest defense, consistent with
the reciprocity and TIT for TAT hypotheses. Our study supports these hypotheses as we found that
having a single swallow neighbour in close proximity was associated with an increase in the
number of fledglings. The repeated interactions experienced by these nearby neighbours fulfills
the requirements for reciprocity to occur (Krams et al. 2008). These frequent interactions may
encourage the neighbours to join in on mobbing behaviour to defend the nests in the area from
predators (Krams et al. 2008). In addition to this, conspecific neighbours could increase success
by facilitating eavesdropping on alarm calls throughout the area. Tree swallows, as well as other
passerines, are able to recognize the warning calls given off by both conspecific and heterospecific
neighbours, allowing them to identify the immediate danger and response that is needed
(Templeton and Greene 2007; Fallow et al. 2013). The ability to eavesdrop also provides early

14

predator detection which can increase the success of the nest (Templeton and Greene 2007; Fallow
et al. 2013).
While having one conspecific neighbour close by appears to be beneficial, having too many
close conspecific neighbours can increase density-dependent competition, thereby decreasing
success. Our findings support this hypothesis as the number of fledglings per nest decreases as
more swallow neighbours were found within 250m of a given nest. Other studies have also shown
that passerine reproductive success is affected by local neighbour density as high density can
reduce clutch size, fledgling survival, and territory size (Sillett et al. 2004). As density increases,
territory size typically decreases, in turn decreasing the area that is available for foraging (Sillett
et al. 2004), putting a strain on food resources for each individual nest, ultimately decreasing nest
success. In addition to food competition, increased intraspecific density can cause males to spend
more time at their nest mate guarding than foraging to defend against extra pair paternity from
other nearby conspecific neighbours (Sillett et al. 2004). This reduction in foraging time can affect
the growth of nestlings and reduce fledging success. Further studies could investigate the rate of
extra pair paternity as intraspecific neighbour density increases and compare it to the changes in
nest success.
Our final prediction regarding conspecific neighbours involved a decrease in success when
there were high numbers of distant, unfamiliar neighbours, as they would be considered a threat to
the nest in accordance with the dear enemy phenomenon. We found that as the number of swallow
neighbours within 1000m increases, the number of nestlings decreases at a given nest, which is
consistent with the dear enemy phenomenon. These distant neighbours have fewer repeated
interactions with the parents at the nest, which may prevent them from becoming familiar with
each other and participating in reciprocity. Instead, these distant neighbours can be considered

15

strangers that are intruding on the parent’s territory (Briefer et al. 2008). Similar to density
dependent competition, the dear enemy phenomenon suggests that neighbours should redirect
parental resources towards territory defense and away from nestling care and foraging. However,
familiar neighbours would only require one act of territory defense. Strangers would require
repeated acts of defense at every interaction. Studies have shown that this effect is not consistent
throughout the entire breeding period (Sillett et al. 2004). At the beginning of the breeding season
after eggs have been laid, few interactions with close by neighbours have occurred, therefore there
may be no distinction between strangers and familiar neighbours at this point in the season. As
such, the equal level of familiarity between neighbours and strangers could explain the lack of
neighbour effects seen during the egg stage.
The presence of heterospecific neighbours appears to have a weak effect on reproductive
success. A conspecific neighbour in close proximity was marginally related to a decrease in the
proportion of nestlings that fledge the nest. This trend was contrary to our prediction that bluebird
neighbours should increase success through means of reciprocity using the TIT for TAT model.
This result is surprising, as there is no interspecific resource competition at this nesting stage, and
we hypothesized that eavesdropping on heterospecific alarm calls could increase tree swallow
success by creating an early predator warming (Templeton and Greene 2007). This result may
suggest that density-dependent factors had a stronger impact on success than nest defense through
eavesdropping and shared mobbing (Sillett et al. 2004; Templeton and Greene 2007; Krams et al.
2008).
Most of our findings fit within our predictions; however, we were limited in that our study
did not assess the varying abundance of nest boxes within every distance parameter and assumed
a uniform quality of habitat across the regions. Addressing nest box abundance and habitat quality

16

in subsequent studies may provide an explanation as to why conspecific neighbours between 250m
to 500m increase fledgling numbers and why heterospecific neighbours are associated with a
decrease the proportion that fledge the nest. Habitat quality could be assessed through proximity
to a body of water, a road, or predator, and density of neighbours. In addition, future studies could
examine behavioural interactions among individuals through observations where warning calls,
territory size, extra pair paternity, and shared mobbing through reciprocity can be measured. To
conclude, we have provided evidence that the presence of conspecific and heterospecific
neighbours can impact reproductive success in tree swallows. The dear enemy phenomenon,
reciprocity, eavesdropping, and density-dependent competition are all factors that may affect tree
swallow reproductive success in a scale-dependent manner.

17

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