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ARTICLE
Local weather and regional climate influence breeding
dynamics of Mountain Bluebirds (Sialia currucoides) and Tree
Swallows (Tachycineta bicolor): a 35-year study
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S.L. McArthur, A.E. McKellar, N.J. Flood, and M.W. Reudink

Abstract: Many songbirds are under increasing pressure owing to habitat loss, land-use changes, and rapidly changing climatic
conditions. Using citizen science data collected from 1980 to 2014, we asked how local weather and regional climate influenced
the breeding dynamics of Mountain Bluebirds (Sialia currucoides (Bechstein, 1798)) and Tree Swallows (Tachycineta bicolor (Vieillot,
1808)). Mountain Bluebird reproduction was strongly associated with local weather: number of nestlings and fledglings both
decreased in years of high rainfall. Clutch size and number of fledglings also declined over the study period. Abundance of
Mountain Bluebirds was higher in years of lower early-season snowfall and warmer local temperatures, as well as more negative
Southern Oscillation Index (SOI) values, indicating a positive influence of El Niño conditions. Tree Swallow reproduction (clutch
size, number of nestlings, and number of fledglings) was negatively associated with SOI values, and the number of Tree Swallow
nestlings decreased in years of higher rainfall and warmer temperatures. Tree Swallows also showed a marked decline in
abundance over the period of the study, consistent with recent range-wide declines. Together, our results demonstrate that local
weather and regional climate differentially affect the reproductive dynamics of Mountain Bluebirds and Tree Swallows and
highlight the importance of long-term citizen science data sets.
Key words: Sialia currucoides, Mountain Bluebird, Tachycineta bicolor, Tree Swallow, climate, reproductive success.
Résumé : De nombreux oiseaux chanteurs subissent des pressions croissantes découlant de la perte d’habitat, des changements
d’utilisation des sols et de l’évolution rapide des conditions climatiques. En utilisant des données issues de la science citoyenne
recueillies de 1980 à 2014, nous nous sommes demandé comment la météo locale et le climat régional influencent la dynamique
de reproduction des merlebleus azurés (Sialia currucoides (Bechstein, 1798)) et des hirondelles bicolores (Tachycineta bicolor (Vieillot,
1808)). La reproduction des merlebleus était fortement associée à la météo locale, le nombre d’oisillons et de jeunes à l’envol
diminuant durant les années très pluvieuses. La taille des nichées et le nombre de jeunes à l’envol ont également diminué durant
la période d’étude. L’abondance des merlebleus était plus grande durant les années caractérisées par un plus faible enneigement
au début de l’hiver et des températures locales plus chaudes, ainsi que par des valeurs plus négatives de l’indice d’oscillation
australe (IOA), indiquant une influence positive des conditions associées aux épisodes El Niño. La reproduction des hirondelles
bicolores (taille des nichées, nombre d’oisillons et nombre de jeunes à l’envol) était négativement associée aux valeurs de l’IOA,
et le nombre d’oisillons d’hirondelle diminuait durant les années plus pluvieuses et chaudes. Les hirondelles bicolores présentaient également une baisse d’abondance durant la période d’étude, cette observation concordant avec des baisses récentes à
l’échelle de leur aire de répartition. Collectivement, ces résultats démontrent que la météo locale et le climat régional ont des
incidences différentes sur la dynamique de reproduction des merlebleus azurés et des hirondelles bicolores et soulignent
l’importance d’ensembles de données de longue durée issus de la science citoyenne. [Traduit par la Rédaction]
Mots-clés : Sialia currucoides, merlebleu azuré, Tachycineta bicolor, hirondelle bicolore, climat, succès de reproduction.

Introduction
In North America, songbirds face a multitude of threats, including land-use change, habitat loss, introduced predators, and a
rapidly changing climate. The impact of climatic conditions can
manifest at both local and regional levels. Local weather can have
significant effects on the reproductive success and abundance of
birds. Higher spring temperatures are often associated with earlier laying dates, which may result in larger clutch sizes (Winkel
and Hudde 1997). Warmer temperatures have also been associated
with greater hatching success, fledging success, and recruitment
(Martin 1987; Reid et al. 2000; Greño et al. 2008). Higher rainfall

may increase reproductive success by increasing primary productivity and thus the availability of food items such as seeds and
insects (Noy-Meir 1973; Cody 1981; Boag and Grant 1984). However,
for cavity-nesting species, too much rain can result in reduced
reproductive success owing to nest soaking and abandonment
(Wesołowski et al. 2002; Bordjan and Tome 2014) or starvation of
young (Fisher et al. 2015). In addition to effects on reproductive
success, weather extremes such as early-season storms and cold
spells can negatively influence breeding bird survival and abundance (Whitmore et al. 1977; Newton 2007; Hess et al. 2008).
Large-scale weather patterns can also influence avian reproductive success. The El Niño South Oscillation (ENSO) cycle is a major

Received 4 August 2016. Accepted 7 January 2017.
S.L. McArthur,* N.J. Flood, and M.W. Reudink. Department of Biological Sciences, Thompson Rivers University, Kamloops, BC V2C 0C8, Canada.
A.E. McKellar. Environment and Climate Change Canada, 115 Perimeter Road, Saskatoon, SK S7N 0X4, Canada.
Corresponding author: M.W. Reudink (email: mreudink@tru.ca).
*Present address: Ministry of Forests, Lands and Natural Resource Operations, BC Public Service Agency, 3401 Reservoir Road, Vernon, BC V1B 2C7, Canada.
© Her Majesty the Queen in right of Canada 2017. Permission for reuse (free in most cases) can be obtained from RightsLink.
Can. J. Zool. 95: 271–277 (2017) dx.doi.org/10.1139/cjz-2016-0184

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272

driver of large-scale regional climatic patterns in the north Pacific
region (Schonher and Nicholson 1989; Chase et al. 2005) and has
been associated with variation in the reproductive success of terrestrial birds in western North America (Sillett et al. 2000; Chase
et al. 2005; Weatherhead 2005; Wilson et al. 2011). El Niño periods
occur, on average, every 4 years and are characterized by increased rainfall in western North America throughout the year
(Trenberth 1997). El Niño events are often associated with increased reproductive success of songbirds, which may be due to
enhanced plant productivity and food abundance during the
breeding season (DeSante and Geupel 1987; Morrison and Bolger
2002; Nott et al. 2002; Chase et al. 2005). ENSO effects during the
nonbreeding season have also been found to influence survival
and reproductive success because of the carry-over effects of overwintering food availability, body mass, and spring departure of
migrants (Sillett et al. 2000; Studds and Marra 2007). Because both
are important drivers of reproductive success in birds, the direct
effects of local weather and the indirect effects of regional climate
on reproductive success should be considered together (Weatherhead
2005).
Here, we use a citizen science data set spanning 35 years to
investigate the relative influence of environmental factors on the
reproductive success of two species of songbirds with different yet
overlapping ecological niches breeding in nest boxes in the semiarid grasslands of British Columbia, Canada. Specifically, we
examine the influence of large-scale climate drivers (i.e., ENSO
cycles) and local weather (temperature and precipitation) on the
reproductive success and breeding abundance of Mountain Bluebirds (Sialias currucoides (Bechstein, 1798)) and Tree Swallows
(Tachycinta bicolor (Vieillot, 1808)). We predicted that (i) conditions
during El Niño years, which are characterized by warmer, wetter
conditions across the year, would positively influence reproductive success (likely due to an increase in food availability during
the breeding season), (ii) birds would experience higher reproductive success when local weather conditions were warmer and
drier, and (iii) breeding bird abundance would be higher in El Niño
years and years with milder early-season local weather conditions.

Materials and methods
Study species
Mountain Bluebirds, small thrushes that are obligate cavity
nesters, readily accept artificial nest boxes in open grassland
areas. Conservation groups throughout North America have established “bluebird trails”, each consisting of a number of nest
boxes that are generally located 100 m or more apart and are often
placed along fence lines in grasslands, to support populations of
Mountain Bluebirds, Western Bluebirds (Sialia mexicana Swainson,
1832), and Eastern Bluebirds (Sialia sialis (L., 1758)) (see North American
Bluebird Society; available from http://www.nabluebirdsociety.org/).
In North America, Mountain Bluebirds are listed as a species of
least concern. However, in Canada and within the Great Basin
Bird Conservation Region (BCR 9) where our study took place,
populations have shown moderate declines since the 1970s
(−1.37% per year and −1.48% per year, respectively; Environment
and Climate Change Canada 2014). This downward trend prompted
an increase in the number of bluebird trails throughout the region (Pardieck et al. 2015).
Like Mountain Bluebirds, Tree Swallows are obligate secondary
cavity nesters that also readily accept artificial nest boxes erected
near open fields and meadows. Tree Swallows will often breed in
boxes on bluebird trails. These medium-sized swallows are aerial
insectivores that feed over areas of open ground or water where
flying insects gather. Tree Swallows are distributed across most of
the continental U.S. and in the southern portions of most Canadian
provinces (Winkler et al. 2011). Although they are also considered a
species of least concern, their Canadian and BCR 9 populations
have shown moderate declines since the 1970s (−1.44% per year

Can. J. Zool. Vol. 95, 2017

and −0.91% per year, respectively; Environment and Climate
Change Canada 2014), and they are part of the larger guild of aerial
insectivores that is generally showing population declines across
the continent (Nebel et al. 2010).
Field methods
Fieldwork for this project was conducted along bluebird trails,
hereafter called routes, throughout the area around the confluence of the Thompson and North Thompson rivers near Kamloops, British Columbia, Canada (50.68°N, 120.34°W), from 1980 to
2014. Volunteers from the Kamloops Naturalist Club monitored
and recorded reproductive data for active nest boxes along each
route during the breeding season. Bluebird routes vary in the
number of nest boxes erected along the route (2–50 boxes; Appendix A, Table A1) and the length of the route (300 m to 14 km). Boxes
are typically placed on fenceposts at chest height, separated by
approximately 200 m. The vast majority of boxes border ranchlands that have been in operation for decades, with minimal
change at the landscape level. Physical copies of bluebird route
records for each year of the monitoring program were obtained
from the Kamloops Naturalist Club and digitized in 2014. The
protocol for volunteers was to check all nest boxes along their
routes every 7–10 days throughout the breeding season (late April
through early August). Clutch size and number of nestlings were
determined by repeated trips to the nest; number of offspring
fledged was based on the number of offspring present at the last
trip to the nest and a subsequent check after fledging to determine the presence of any dead offspring. Nesting attempts in
which all offspring perished (number fledged = 0) were included.
Summary data were available for all years and included the following
for each species and each route: number of clutches, number of first
nests and second nests, clutch size, number of nestlings, number
of fledglings, total number of nest boxes, and number of active
nest boxes (i.e., those that were used for nesting). We analyzed
data from 26 routes from 1980 to 2014, which included a total of
3892 clutches from Mountain Bluebirds and 2837 clutches from
Tree Swallows. Only first nesting attempts were included in all
analyses.
Nest monitoring was conducted by volunteers with the Kamloops Naturalists Club; the authors assisted in collecting data as
part of ongoing research on several routes starting in 2011 under
federal banding and collection permits granted by the Canadian
Wildlife Service. All work was done in accordance with Canadian
Council on Animal Care guidelines. The Thompson Rivers University Animal Care Committee approved this research (file number
10603) and no animals were harmed during this study.
Local weather
We obtained local daily weather information by accessing the
weather station online archives of Environment and Climate
Change Canada for the Kamloops A station, located at the Kamloops airport (50.70°N, 120.44°W), for the years 1980–2012. Data
from this station were not available for 2013 or 2014, so temperature, rainfall, and snowfall data from the nearby (approximately
20 km SE) Pratt Road station (50.60°N, 120.20°W) were used instead. Because weather data from Kamloops A and Pratt Road
stations were highly correlated for the years 1988 to 2013 for temperature (r2 = 0.98, p < 0.01) and rainfall (r2 = 0.70, p < 0.01), we felt
justified in using data from the latter location for 2013 and 2014.
The mean distance from the start of each monitoring route to the
nearest weather station was 11.7 km, with a maximum distance of
30.4 km and a minimum distance of 4.9 km.
To examine the effects of weather on reproduction, we averaged daily temperature and rainfall values for three different
periods during the breeding season: laying period, incubation
period, and nestling period. Date ranges were calculated based on
detailed daily records of Mountain Bluebird reproduction from
2011 to 2014 (for details see Morrison et al. 2014). To define the
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McArthur et al.

laying period, we calculated the 95% confidence interval for firstegg dates, which ranged from 29 April to 2 June, and then added
an additional 5 days to the latest first-egg date account for days of
sequential laying (laying period: 29 April – 7 June). To define the
incubation period, we used the start of the laying period as our
start date (because events during egg laying of early clutches
may influence hatching success) and then added an additional
14 days, which was the mean incubation time in our population
(M.W. Reudink, unpublished data) (incubation period: 29 April –
21 June). Finally, the nestling period was defined as the earliest
incubation start date (3 May) plus 14 days (17 May, the earliest
hatch date) until 20 days (mean time from hatch to fledge;
M.W. Reudink, unpublished data) after the latest hatch date
(21 June + 20 days = 10 July) (nestling period: 17 May – 10 July). We
lacked the same high-resolution data on nestling phenology for Tree
Swallows, but Tree Swallows and Mountain Bluebirds in our population reproduce largely concurrently (M.W. Reudink, unpublished data)
and have similar incubation and nestling durations (Winkler et al.
2011), so we used the same date ranges for both species.
To examine the effects of early-season weather conditions on
box occupancy rates (a proxy for abundance), we calculated mean
temperature, rainfall, and snowfall from when the birds begin to
arrive on migration in our region until the latest first-egg dates
(dates as above). Mountain Bluebirds begin arriving in the Kamloops region in early February, whereas Tree Swallows begin arriving in early March (Campbell et al. 1997). Thus, early-season
weather variables for Mountain Bluebirds were averaged over the
period from 1 February to 2 June, whereas early-season weather
variables for Tree Swallows were averaged over the period from
1 March to 2 June.
Regional climate
We used the standardized Southern Oscillation Index (SOI) to
measure variation in regional climate, and we obtained monthly
SOI values for all months from January 1980 to December 2014
from the U.S. National Oceanic and Atmospheric Administration
(NOAA) online climate database (available from http://www.ncdc.
noaa.gov/cdo-web/). The SOI, which is based on the difference in
air pressure at sea level between Tahiti and Darwin, Australia, is
one of several measures of the fluctuations in air pressure that
occur between tropical regions of the eastern and western Pacific
Ocean. Particularly during El Niño and La Niña events, these fluctuations affect the jet stream over western North America, and
thus influence regional climatic patterns (Melack et al. 1997). Negative SOI values are associated with warm, wet El Niño events in
western North America, whereas positive values are associated
with cooler, dry La Niña events (Rasmusson and Wallace 1983;
Melack et al. 1997).
Statistical analyses
To examine the effects of local weather and regional climate on
reproductive success, we constructed a series of linear mixedeffects models (LMM). Models were constructed separately for
Mountain Bluebirds and Tree Swallows. We examined five different response variables (clutch size, number of nestlings, proportion of eggs hatched (number of nestlings/clutch size), number of
fledglings, and proportion fledged (number of fledglings/clutch
size). We included continuous fixed effects of SOI and year in all
models; year was included to account for potential changes in
reproductive success over time. For the clutch-size model, we included fixed effects of laying-period temperature and rainfall; for
the nestling models (number of nestlings and proportion of eggs
hatched), we included fixed effects of incubation-period temperature and rainfall; and for the fledgling models (number of fledglings and proportion fledged), we included fixed effects of
nestling-period temperature and rainfall. Because all reproductive variables were collected at the route level and routes were
repeated in multiple years, we included route as a random effect

273

in all models. We tested for issues of multicollinearity, and there
were no strong correlations among weather, climate, and year
(i.e., no correlation coefficients were greater than 0.5). After construction of the full model, we used a backward stepwise procedure to eliminate nonsignificant (p > 0.05) effects to arrive at a
best-fit final model.
To examine if local weather and regional climate influenced
nest-box occupancy rates, we constructed similar LMMs for Mountain Bluebirds and Tree Swallows in which we examined the
effects of early-season temperature, early-season rainfall, earlyseason snowfall, SOI, and year on occupancy rates (defined as
number of boxes occupied by breeding birds of each species/total
boxes on the route). Early-season weather variables were specific
to each species (see above, Materials and methods). We included
route as a random effect and weighted the model by the number
of boxes on each route. Year was included in all models to account
for potential changes in occupancy over time. As above, we used a
backward stepwise procedure to eliminate nonsignificant (p > 0.05)
effects to arrive at a best-fit final model. All statistical analyses were
conducted in JMP version 12 (SAS Institute, Inc. 2015).

Results
Mountain Bluebird
When we examined the effects of weather and climate on the
reproductive success of Mountain Bluebirds, we found that clutch
size and number of fledglings declined over the study period
(Table 1). Number of nestlings, proportion of eggs hatched, number of fledglings, and proportion of eggs that fledged young were
all negatively associated with rainfall (Table 1, Fig. 1). Nest-box
occupancy rate was positively associated with warmer early-season
temperature and was negatively associated with early-season snowfall; box occupancy was higher in years with lower SOI values, which
are associated with warm, wet El Niño events (Table 1).
Tree Swallow
Clutch size, number of nestlings, proportion of eggs hatched,
and number of fledglings were all negatively associated with SOI
values, and there were also negative effects of incubation-period
temperature and rainfall on number of nestlings (Table 2, Fig. 2A);
proportion of eggs hatched was higher in years with lower SOI
values, but cooler, drier weather during incubation. The proportion
of eggs that produced fledged young was negatively associated with
rainfall (Table 2). Nest-box occupancy rate of Tree Swallows declined
over time and was also positively associated with early-season snowfall (Table 2, Fig. 2B). There was no relationship between occupancy
rates of Tree Swallows and Mountain Bluebirds over the course of
the study (n = 35 years, r = 0.13, p = 0.44).

Discussion
Based on citizen science data collected over a 35-year period in
the semiarid grasslands of British Columbia, Canada, Mountain
Bluebirds and Tree Swallows exhibited marked yearly variation
in reproductive success and abundance, which was linked to both
local weather and regional climate conditions. However, the effects of these variables differed between the species. Mountain
Bluebirds were significantly affected by rainfall, but neither temperature nor SOI influenced any measure of reproductive success,
and although the clutch size of this species declined over the
duration of the study, it did not seem to be affected by either local
weather or regional climate conditions. In contrast, regional climate had a major influence on Tree Swallows, with lower SOI
values being associated with a reduction in several measures of
reproductive success. Local weather conditions, including both
rainfall and temperature, were also important for Tree Swallows.
Nest-box occupancy rates (a proxy for abundance) of Mountain
Bluebirds and Tree Swallows were also affected by climate and
local weather, but interestingly, the pattern was opposite of that
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Table 1. Final best-fit linear mixed-effects models examining the effects of year, local weather, and regional climate
(Southern Oscillation Index (SOI)) on clutch size, number of nestlings, proportion of eggs hatched, number of
fledglings, proportion fledged, and nest-box occupancy in Mountain Bluebirds (Sialia currucoides).
Factor

Estimate

SE

F

df

p

Clutch size
Year

−0.01

−0.003

4.23

384.9

0.04

Number of nestlings
Incubation-period rainfall

−0.18

0.08

5.55

365.6

0.02

Proportion of eggs hatched (number of nestlings/clutch size)
Incubation-period rainfall

−0.03

0.01

4.78

370.2

0.03

Number of fledglings
Year
Nestling-period rainfall

−0.01
−0.28

0.01
0.09

6.28
10.50

396.0
348.5

0.01
0.001

Proportion fledged (number of fledglings/clutch size)
Nestling-period rainfall

−0.05

0.02

10.29

352.3

0.002

Rate of nest-box occupancy
SOI
Early-season temperature
Early-season snowfall

−0.04
0.02
−0.51

0.02
0.01
0.14

5.70
4.07
14.21

380.7
382.2
379.4

0.02
0.04
0.0002

Fig. 1. Number of Mountain Bluebird (Sialia currcuoides) fledglings was negatively associated with rainfall during the nestling period. Each
point represents a year, with the size of the circle corresponding to the number of clutches analyzed during that year. Error bars represent SE.

observed for reproductive success: in this case, Mountain Bluebirds were significantly influenced by variables in both categories,
whereas Tree Swallow occupancy varied only with year and local
weather.
For Mountain Bluebirds, the negative association between rainfall and number of fledglings, in particular, is consistent with
previous studies of this species, which suggest that stormy conditions and high rainfall during the nestling period can lead to
increased chick mortality (Power and Lombardo 1996). However, it
is unclear, in our study, whether rainfall directly or indirectly
contributes to reduced reproductive success. During the nestling
stage, cooler temperatures, especially in days following rainfall
events, can reduce the thermoregulatory capabilities of nestlings
(Gullett et al. 2015), leading to direct mortality; early-season
storms have also been observed to kill Mountain Bluebirds of all
ages (Houston 1982). In addition, rainfall might reduce the provisioning rates of nestlings by Mountain Bluebird parents, causing
indirect mortality of nestlings owing to starvation. Mountain

Bluebird nest-box occupancy rates were higher in years with
warmer temperatures and lower levels of snowfall during the
early season. Mountain Bluebirds arrive in the Kamloops region
during mid-winter when conditions are highly variable; abundant
snowfall and low temperatures are not uncommon and could
cause increased acute mortality or reduce foraging success and
contribute to increased mortality (Whitmore et al. 1977; Newton
2007; Hess et al. 2008). Alternatively, birds may disperse to other
areas when the Kamloops region experiences harsh mid-winter
conditions.
For Tree Swallows, local weather was not associated with clutch
size or number of fledglings. However, higher temperatures and
greater rainfall during the incubation period were associated with
fewer nestlings and the proportion of Tree Swallows fledged was
reduced in conditions of higher rainfall during the nestling period. The negative effects of rainfall on the number of nestlings
and the proportion fledged, as well as the lack of any association
of rainfall with clutch size, suggests that rainfall likely influenced
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Table 2. Final best-fit linear mixed-effects models examining the effects of year, local weather, and regional climate
(Southern Oscillation Index (SOI)) on clutch size, number of nestlings, proportion of eggs hatched, number of
fledglings, proportion fledged, and nest-box occupancy in Tree Swallows (Tachycinta bicolor).
Factor

Estimate

SE

F

df

p

Clutch size
Year

−0.01

−0.003

4.23

384.9

0.04

Number of nestlings
Incubation-period rainfall

−0.18

0.08

5.55

365.6

0.02

Proportion of nestlings (number of nestlings/clutch size)
Incubation-period rainfall

−0.03

0.01

4.78

370.2

0.03

Number of fledglings
Year
Nestling-period rainfall

−0.01
−0.28

0.01
0.09

6.28
10.50

396.0
348.5

0.01
0.001

Proportion fledged (number of fledglings/clutch size)
Nestling-period rainfall

−0.05

0.02

10.29

352.3

0.002

Rate of nest-box occupancy
SOI
Early-season temperature
Early-season snowfall

−0.04
0.02
−0.51

0.02
0.01
0.14

5.70
4.07
14.21

380.7
382.2
379.4

0.02
0.04
0.0002

Fig. 2. (A) Number of Tree Swallow (Tachycinta bicolor) fledglings was negatively associated with the Southern Oscillation Index. (B) Occupancy
rate of Tree Swallows declined over the 35-year study period. Each point represents a year, with the size of the circle corresponding to the
number of clutches analyzed during that year. Error bars represent SE.

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276

reproductive success through decreased hatching success and increased offspring mortality. Our finding that colder temperature
increased the number of nestlings contradicts other studies,
which have shown that warmer temperatures can improve hatching success (Martin 1987), as well as fledging success (Reid et al.
2000), nestling survival (Ardia et al. 2010) and postfledging survival (Sankamethawee et al. 2009; Grüebler and Naef-Daenzer
2010). However, the effects of temperature were weaker than
those of rainfall, which could directly influence reproductive success through reduced hatching success and increased offspring
mortality due to the effects of exposure (Wesołowski et al. 2002;
Bordjan and Tome 2014). It is important to note, however, that we
only examined birds nesting in nest boxes; it will be important for
future studies to address whether birds nesting in natural cavities
are similarly influenced by weather and climate. In addition, it
will be important to examine whether nest-box characteristics
(e.g., placement, entrance type, surrounding habitat) influence
reproductive success.
Tree Swallow occupancy rates were higher in years with high
early-season snowfall; this may because such weather effects reduced occupancy by Mountain Bluebirds. The two species often
compete for nest boxes (Power and Lombardo 1996), and although
Tree Swallows generally seem to have higher resource holding
potential, Mountain Bluebirds benefit from prior ownership
(Wiebe 2016), which may be especially relevant in our study area
where returning breeders often occupy the same boxes from year
to year (M.W. Reudink, unpublished data).
We also found that large-scale climatic trends were associated
with abundance and reproductive success of Mountain Bluebirds
and Tree Swallows, respectively. Negative values of the SOI were
correlated with higher box occupancy rates in Mountain Bluebirds and greater numbers of Tree Swallow eggs, nestlings, and
fledglings, and a higher proportion of nestlings, which seems to
indicate higher adult survival in Mountain Bluebirds and improved reproductive success in Tree Swallows during El Niño
years (but see Sillett et al. 2000; Mazerolle et al. 2005). El Niño
events and lower SOI values have been linked to increased regional primary productivity (Barnston and Livezey 1987; Swetnam
and Betancourt 2010) and increased insect prey abundance (Kemp
et al. 1985; Swetnam and Lynch 1993) in northern latitudes of
North America, which would improve Tree Swallow foraging
throughout the breeding season, possibly contributing to improved reproductive success (Nott et al. 2002). This result is consistent with previous studies of other species: large-scale regional
climate patterns such as ENSO have been shown to account for the
majority of interannual variability in reproductive success for at
least 10 other migratory bird species (Nott et al. 2002). The climatic
effects of El Niño occurring outside the breeding season may also
be driving these patterns for both Mountain Bluebirds and Tree
Swallows. Environmental conditions experienced throughout the
annual cycle can influence both individuals (e.g., Saino et al. 2004;
Reudink et al. 2009) and populations (e.g., Wilson et al. 2011; Pillar
et al. 2015). Our results illustrate the importance of understanding
how events occurring prior to breeding influence survival and
fecundity (e.g., Webster et al. 2002; Harrison et al. 2011; McKellar
et al. 2013; Latta et al. 2016).
In conclusion, we demonstrate that, over a 35-year period, reproductive dynamics of both Mountain Bluebirds and Tree Swallows in the semiarid grasslands of interior British Columbia were
associated with local weather conditions and large-scale regional
climate patterns. In addition, clutch size and number of fledglings
declined in Mountain Bluebirds and breeding abundance of Tree
Swallows (as indicated by nest-box occupancy) declined over time.
The latter especially raises conservation concerns, as Tree Swallows have been in decline in many parts of their range over the
past several decades (Sauer et al. 2013). This study highlights the
importance of long-term data collected by citizen scientists, such
as that collected during the monitoring of bluebird trails. Future

Can. J. Zool. Vol. 95, 2017

studies that incorporate long-term bluebird trail data sets from
across a broad geographic area (ideally range-wide) will enable us
to understand if the effects of local weather and regional climate
on breeding dynamics are widespread or population-specific and
will help us to predict the effects of climate change on birds over
the coming decades.

Acknowledgements
We thank G. Dreger, P. and J. Gray, S. Weilandt, the undergraduate students who assisted on this project, and the many members of the Kamloops Naturalists Club for collecting data on
Mountain Bluebird routes over the years of this study. S. Wilson
provided helpful feedback on an earlier draft of this manuscript
and two anonymous reviewers provided thoughtful reviews that
improved the manuscript. Funding for this project was provided
by a Natural Sciences and Engineering Research Council of
Canada Discovery Grant (M.W.R.) and a North American Bluebird
Society Research Grant (S.L.M.).

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Appendix A
Table A1. Mean and SD of available nest
boxes each year on each route in the study
area near Kamloops, British Columbia.
Route name

Mean number
of nest boxes

SD

Aberdeen
Barnes Lakes
Beaton Road
Campbell Range
Cherry Creek
Dewdrop
Edith Lake
Erin Valley
Goose Lake
Jackson Road
Juniper
Lac du Bois
Long Lake
Niskonlith Lake
Pipeline Road
Pritchard East
Pruden Pass
Rosehill 1
Rosehill 2
Rosehill 3
Rosehill Ranch
Scheidam Flats
Scott Road
South Thompson
Valleyview
Whispering Pines

11.75
25
14
21.2
2.33
40.64
25.21
2.25
21.75
18.89
3
21.21
34.57
6
11
49.92
19.42
26.76
21.3
15.49
25.28
5
15.44
41.08
4.17
29.18

7.23
0
0
0.58
0.58
8.8
1.79
0.5
1.39
1.37
0
2.55
8.1
0
0
19.07
3.73
4.4
5.88
0.97
7.23
0
9.41
5.31
1.47
2.86

Note: The number of nest boxes along each
route was inconsistent across years because boxes
were added or removed.

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