Canadian Journal of Zoology
Demography of sooty fox sparrows following a shift from a migratory to resident life history
Journal: Canadian Journal of Zoology
Manuscript ID cjz-2017-0102.R2
Manuscript Type: Article
Date Submitted by the Author: 08-Oct-2017
Complete List of Authors: Visty, Hannah; University of British Columbia, Forest and Conservation Sciences Wilson, Scott; Environment and Climate Change Canada, National Wildlife Research CentreDraft Germain, Ryan; University of British Columbia, Forest and Conservation Science; University of Aberdeen, Institute of Biological and Environmental Sciences Krippel, Jessica; University of British Columbia, Forest and Conservation Science Arcese, Peter; Univ of British Columba,
sooty fox sparrow, demography, population growth, MIGRATION < Keyword: Discipline, colonization, Passerella unalaschcensis
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Demography of sooty fox sparrows following a shift from a migratory to resident life history
Hannah Visty1, Scott Wilson2, Ryan Germain1,3, Jessica Krippel1, and Peter Arcese1
1Department of Forest and Conservation Sciences, 2424 Main Mall, Vancouver, BC V6T 1Z4;
[email protected]; [email protected]; [email protected]
2Environment and Climate Change Canada, National Wildlife Research Centre, 1125 Colonel by
Drive, Ottawa, ON K1A 0H3; [email protected]
3Institute of Biological and Environmental Sciences, Zoology Building, University of Aberdeen,
Tillydrone Avenue, Aberdeen, AB24 2TZ, United Kingdom; [email protected] Draft Contact author: Hannah Visty, Department of Forest and Conservation Sciences, 2424 Main
Mall, Vancouver, BC V6T 1Z4; [email protected]; Phone: 778 985 6200; Fax: 8229103
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Demography of sooty fox sparrows following a shift from a migratory to resident life history
Hannah Visty (H. Visty), Scott Wilson (S. Wilson), Ryan Germain (R. Germain), Jessica Krippel
(J. Krippel), and Peter Arcese (P. Arcese)
Abstract
Identifying causes and consequences of variation in species life history has the potential to
improve predictions about how climate and land use change may affect the demography and
distribution of species in future. Sooty fox sparrows (Passerella unalaschcensis J.F. Gmelin,
1789; or commonly grouped within Passerella iliaca B. Merrem, 1786) were migrants that rarely bred in the Georgia Basin of British Columbia prior to ~1950 but have since established resident populations. Data on 270 color banded birdsDraft and 54 nests on Mandarte Is., BC, allowed us to estimate demographic vital rates and population growth in one recently established population.
Annual fecundity (F), estimated as the product of the number of broods initiated (1.5 ± 0.01; mean ± sd), clutch size (2.82 ± 0.44), and probability of survival to fledging (0.68 ± 0.02), exceeded values reported for migrants, supporting the hypothesis that residents invest more in reproduction on average than migrants within species. Estimating juvenile and adult overwinter survival (Sj = 0.32 ± 0.06, and Sa = 0.69 ± 0.05) next allowed us to simulate an expected
distribution of population growth rates as: λexp = Sa + (Sj × F), given parameter error. Our
estimate of λexp (1.61 ± 0.57) implies expeditious population growth, consistent with the species’
recent colonization of the region.
Keywords: sooty fox sparrow, Passerella unalaschcensis, demography, population growth, migration, colonization
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Introduction
Climate, land cover and species life histories often co vary (e.g., Hudson and Keatley 2010;
Gimona et al. 2015), which suggests that by identifying causal mechanisms we can improve
predictions about how environmental change may affect species life histories in future (e.g.,
Winkler et al. 2002; Visser 2008; Pacifici et al. 2015; Beever et al. 2017). Fox sparrows
(Passerella iliaca B. Merrem, 1786) offer an interesting case in point, given the species’ current
status as a polytypic, single brooded migrant (Bendire 1889; Swarth 1920; Threlfall and
Blacquiere 1982; Garrett et al. 2000; Weckstein et al. 2002). In contrast, Zink (1994) provided
genetic evidence in support of a phylogenetically distinct sooty fox sparrow (Passerella
unalaschcensis J.F. Gmelin, 1789; sometimes recognized as a subspecies group within P. iliaca,
e.g., Chesser et al. 2016) in coastal regionsDraft of the Pacific Northwest, and Wahl et al. (2005)
reported that sooty fox sparrows have become residents of the coastal lowlands and Gulf and San
Juan Islands of British Columbia (BC) and Washington State (WA), possibly producing multiple
broods annually. We ask in this paper whether these differences in historical and modern
accounts of sooty fox sparrows represent a life history shift from a migratory to residential
lifestyle, similar to shifts reported in a variety of species as an example of acclimatization to
environmental change (e.g., Winkler et al. 2002; Visser 2008; Beever et al. 2017).
Historical records prior to ~1950 indicate that sooty fox sparrows were short distance
migrants in coastal British Columbia, with only three breeding records from western Vancouver
Island (Swarth 1920; Munro and Cowan 1947). By 1983, Guiguet (1983) listed sooty fox
sparrows as resident on Vancouver Island, and other sources recognize them as residents of the
Southern Gulf and San Juan Islands (Baron and Acorn 1997; Campbell et al. 2001; Wahl et al.
2005). Multiple lines of evidence also indicate that sooty fox sparrows are increasing in this
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region (National Audubon Society 2010; Hearne 2015), as might be expected following
successful colonization and the adoption of a residential lifestyle. This expectation arises from
well known trade offs in migratory and reproductive tactics, which influence species
demography, ecology and evolution (Rolshausen et al. 2009; Tökölyi and Barta 2011). Shifts
from migratory to residential lifestyles have been linked to increases in reproductive effort (Gillis et al. 2008; Bruderer and Salewski 2009). Comparative studies of birds also showed that short distant migrant birds bred longer and initiated more broods with smaller clutches than long distance migrants of the same species (Sandercock and Jaramillo 2002).
In this paper we estimate demographic vital rates and population growth in a resident, individually marked population of sooty fox sparrows that colonized Mandarte Island, BC, in
1975. Based on the studies above, we expectedDraft to observe that the sooty fox sparrows we studied would have longer breeding seasons, initiate more nests annually, and lay smaller clutches than observed in migratory fox sparrows. In the absence of commensurate reductions in survival (e.g.,
Sandercock and Jaramillo 2002), we also expected to observe evidence of positive population growth.
Methods
We studied sooty fox sparrows on Mandarte Is. (c. 6 ha), located ~11km south of Victoria
International Airport in southwestern BC, Canada, also the site of a long term study of song sparrows (Melospiza melodia; 1960 63 and 1975 2017, Tompa 1963; Arcese et al. 1992; Smith et al. 2006). Sooty fox sparrows colonized Mandarte Is. in 1975 and became the most common passerine on the island ~2010 (Tompa 1963; Drent et al. 1964; Johnson 2015). We began collecting demographic data on sooty fox sparrows opportunistically in 2010 while also monitoring song sparrows. Observations typically spanned March – August annually, including
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5 20 days per year of netting and census during overwinter trips in 2012 – 14, and up to 60 days
per winter from 1982 – 1987, indicating that most or all sooty fox sparrows observed on the
island were resident year round. From 2010 – 2016, 123 nestling, 82 juvenile, and 65 adult sooty
fox sparrows were fitted with a numbered metal and 1 3 coloured plastic bands to facilitate re
sighting. A smaller number of distinctly marked fox sparrows (1989; or ‘naturally’ identifiable by
a unique distribution of white feathers from 1982 – 1987) originally confirmed year round
residence in this population, but were not observed in sufficient detail to use in analyses
presented here.
We thus used birds marked from 2010 – 2016 to estimate juvenile annual survival (Sj)
and adult annual survival (Sa) using recapture and re sighting data and program MARK (version
8.x). When doing so, individuals bandedDraft outside the formal re sighting period (May 1 –June 30)
in year t to t+1 were entered into the survival encounter history as if they were observed in the
re sighting period in year t+1. ‘Juveniles’ include birds banded as nestlings (~4 8 days old),
fledglings (~12 24 days old), or independent young (<76 days old). Our initial analyses
indicated no difference in overwinter survival estimates whether nestlings and older young were
treated separately or pooled; we therefore pooled these groups to increase sample size and
simplify our population model by estimating survival for single period (nestling to recruit:
‘juvenile survival’). Candidate models were developed using combinations of the most common
determinants of survival (age in two classes: juvenile or adult, variation by year of observation,
or simply held constant); the ‘logit’ link function was used to test all models. Model goodness
of fit (GOF) was examined using a bootstrap GOF test in MARK on the most parameterized
model.
We estimated life history traits and fecundity based on 54 nests observed from nest
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6 building through fledging. For nests found with nestlings (n = 36), we estimated hatch date and
the date the first egg of the clutch was laid (DFE) by back dating nestling age in days using traits
well established for song sparrows (e.g., feather development and body size; Smith et al. 2006;
Germain et al. 2016) and assuming a 14 day incubation period (Ryan 1974). Detailed
observations of two sooty fox sparrow nests found prior to laying and followed closely thereafter
confirmed that nesting phenology and development in sooty fox sparrows is nearly identical to
that observed in song sparrows on Mandarte Island (Smith et al. 2006). We used t tests to
quantify differences in mean clutch size between the sooty fox sparrows we observed versus
values reported for migratory fox sparrows (e.g., Threlfall and Blacquiere 1982; Rogers 1994). A
lack of demographic data from other populations prevented more detailed comparisons.
To compare the timing of nest initiationDraft and number of broods produced by sooty fox
sparrows in different years, we standardized all sooty fox sparrow DFEs by the median date of
first nests initiated by song sparrows in the same year (e.g., Wilson and Arcese 2006). This
standardization helped to account for annual variation in climate, given that sooty fox sparrows
were monitored opportunistically, and more often early in the breeding season when their nests
were easier to find and effort required to monitor song sparrows slightly less. We quantitatively
assessed the similarity of breeding timing in fox and song sparrows by running an Anderson
Darling test in program R (version 3.3.3, package kSamples) to test whether the shapes of the
continuous distributions of sooty fox sparrow and song sparrow breeding times differed
significantly (using 10,000 resampling permutations).
Annual fecundity (F) was estimated by multiplying the mean observed clutch size (CS;
Nnests = 49), the estimated number of broods initiated annually (NB), and the fraction of young
surviving to independence from parental care (Sn) as: F = CS × NB × Sn. Sn was estimated
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following Mayfield (1961) as a probability of nest success (Nnests = 23), and following Shaffer
(2004) using a small sample of nests observed over the entire nesting cycle. Because these
methods yielded very similar estimates of daily survival, but very few nests were observed over
the entire nesting cycle, we report Mayfield estimates here. These estimates are robust to the
assumption of constant mortality based on the simple predator community present on Mandarte
Is. and detailed results obtained in the sympatric song sparrow population (Johnson and Shaffer
1990, Arcese et al. 1992, Smith et al. 2006).
We next estimated expected population growth rate as: λexp = Sa + (Sj × F) by creating a
stochastic distribution of λexp, simulating over the observed standard deviations of component
estimates of Sa, Sj, CS, and Sn (Walters 1986). The stochastic simulation was necessary to
incorporate error and account for the opportunisticDraft collection of data, which varied in detail and
amount between years (Bartlett 1960). We adopted a deterministic estimate of NB equal to 1.5,
based on a comparison of song sparrow and fox sparrow DFEs (see Results). To accurately
reflect the potential for environmental variation in survival due to climate—as opposed to
sampling error—we used standard deviations for Sa and Sj estimated over 42 years for the
sympatric song sparrow population (cf Arcese et al. 1992; Smith et al. 2006).
Results
Clutch size in 49 nests found prior to fledging was 2.82 ± 0.44 (mean ± sd) and nearly identical
to our estimate based on all 13 nests found prior to hatching (2.85 ± 0.38). Eight nests were
found with eggs that did not hatch due to abandonment or infertility (4 of 9 eggs opened after
broods fledged showed no development, 5 contained embryos in arrested development; cf Taylor
et al. 2010). Of all 49 nests, 10 had a CS of 2 eggs, 38 had 3 eggs, and 1 nest had 4 eggs. Mean
clutch size in our resident fox sparrow population is significantly smaller than clutches in
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migratory populations; sooty fox sparrows in Alaska averaged 4.11 ± 0.33 (n = 9, Rogers 1994;
t= 8.35, p < 0.0001), while red fox sparrows in Newfoundland had a mean clutch size 3.24 ±
0.60 (n = 34, Threlfall and Blacquiere 1982; t = 3.68, p < 0.0004).
We estimated the probability of success to independence using Mayfield calculations for
nest success (Mayfield 1961). With a subset of 23 nests visited multiple times during incubation
and nestling stages, we calculated a daily survival rate of 98.5%. Given a total nesting period of
26 days (from laying to fledge; see Methods) the overall probability of nest success Sn = 0.68 ±
0.02.
We observed two successful nesting attempts in a given season by two pairs of color banded sooty fox sparrows, and a third by a pair composed of a banded male and unbanded
female. In each case the first egg of the Draftfirst nest was laid in April (14th, 19th, and 23rd), followed by 30, 36, and 48 days respectively until the first egg of the subsequent clutch was laid. To
approximate the breeding period and estimate the number of broods initiated annually, we
standardized initiation dates (DFEs) of all nests by the median DFE observed in the sympatric
song sparrow population (see Methods). Pooling data in this way suggests two apparent peaks in
laying, implying that at least some sooty fox sparrows initiated multiple nests annually (Figure
1). The second peak of sooty fox sparrow nests observable in Figure 1 is unlikely to reflect re
nesting following failures given that only 3 failures were recorded at all, and because the period between peaks suggests a ~40 day interval between the initiation of nests, also seen in the
sympatric song sparrow population. The qualitative alignment of the distributions of fox and
song sparrow nest initiation dates is consistent with the idea that each species responded
similarly to annual variation in environmental conditions, despite a quantitative comparison of
the distributions indicating that the samples are not drawn from the same distribution (Anderson
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Darling test value = 4.01, p < 0.01). This statistical difference may arise because the sooty fox
sparrow breeding season is shorter than observed in the sympatric song sparrow population, or
due to low monitoring effort in sooty fox sparrows as compared to song sparrows (see Methods).
The earliest and latest DFE recorded for sooty fox sparrows suggests a breeding season of ~86
days, spanning March 24 – June 18 (Figure 1).
To estimate juvenile and adult overwinter survival, we summarized encounter histories
(re sighting and re capture data) for 65 juveniles and 120 adults and used a variety of model
structures in program MARK to estimate survival and re sighting probabilities varying year (t),
age class (a = 1 or 2), or held constant (.) (Table 1). A fully parameterized model, Φ(a/2 – t/t)
p(t), showed no over dispersion (bootstrapped GOF test ̂ = 1.15). The best fitting model based
on AICc, Φ(a) p(t), estimated survival byDraft age class and re sighting probability by year, as
expected given that re sighting effort varied annually (see Methods; mean re sighting probability
= 0.50 ± 0.08). Overall, survival estimates from our best model were both relatively high and
precise (Sa = 0.69 ± 0.05 SE, and Sj = 0.32 ± 0.06).
We next estimated population growth as a stochastic distribution derived by Monte Carlo
simulation (see Methods). Specifically, we used juvenile and adult survival rates estimated above
(Sa = 0.69 and Sj = 0.32), paired with standard deviations for those estimates from song sparrows
(e.g., SD Sa = ± 0.16, Sj = ± 0.18; see Methods) to more accurately reflect the potential for annual
variation in the environment to affect survival. Using the above values and estimated fecundity
for sooty fox sparrows allowed us to estimate population growth rate as: λexp = Sa + (Sj × F),
replicated 10,000 times. To do so, we calculated fecundity (F) as the product of the mean number
of broods per year (NB = 1.5 ± 0.01), clutch size (CS = 2.82 ± 0.44), and the probability of
survival to independence (Sn = 0.68 ± 0.02). The resulting distribution included 86.2% of 10,000
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estimates of λexp > 1, and a mean λexp of 1.61 ± 0.57 (SD; Figure 2), suggesting a potential for rapid growth.
Discussion
Our results support earlier accounts which suggest that sooty fox sparrows established resident populations in coastal regions of the Pacific Northwest after ~1950, in contrast to the migratory habits of all other fox sparrow populations studied to date (Threlfall and Blacquiere 1982;
Rogers 1994; Weckstein et al. 2002). Sooty fox sparrows colonized Mandarte Is. in 1975 and
have since resided there year round. Females in this population laid on average 2.82 eggs in
multiple nests annually; fewer than reported for migratory fox sparrows in Newfoundland (3.24; Threlfall and Blacquiere 1982) or AlaskaDraft (4.11; Rogers 1994). Sooty fox sparrows on Mandarte Is. also initiated breeding earlier and produced more broods annually than migrants (Figure 1;
Threlfall and Blacquiere 1982; Weckstein et al. 2002), exhibiting a breeding season ~86 days
long (March July) versus ~63 days (May 1 July 2) among migrants in Alaska (Rogers 1994).
Although 36 nests on Mandarte Is. were found at the nestling stage, mean clutch sizes estimated
from 13 nests observed at all stages were similar (2.82 vs. 2.85, respectively). These observations
support our expectation that clutch size would decline and brood number increase with the
adoption of a residential versus migratory lifestyle (cf Sandercock and Jaramillo 2002; Gillis et
al. 2008; Bruderer and Salewski 2009).
Our estimates of demographic vital rates for sooty fox sparrows on Mandarte Is. suggest
the potential for rapid population growth (λexp = 1.61, Figure 2). In contrast, censuses on
Mandarte indicate sooty fox sparrows grew from one to 14 pairs from 1975 – 1983, and to 30 pairs by 2010, reflecting slower realized growth (Johnson 2015). The difference in estimated
versus realized population growth rates may indicate that sooty fox sparrows often emigrate from
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Mandarte Is., making the population a source of potential colonists within the region.
Given that sooty fox sparrows were previously known only as migrants (Swarth 1920;
Weckstein et al. 2002), and that Vancouver Island represents the northern most wintering range
of sooty fox sparrows (Swarth 1920; Bell 1997; Weckstein et al. 2002), it is possible that newly
established sooty fox sparrow populations in our study area have responded to favorable changes
in the environment that enhance individual fitness. If severe winter weather sometimes limits
sooty fox sparrow populations—as shown for song sparrows on Mandarte Is. (Arcese et al. 1992;
Smith et al. 2006)—a century of climate warming in the Georgia Basin may have relaxed those
limits sufficiently to allow for growing populations of non migratory sparrows of multiple
species (Arcese, P. and Norris, R. Unpub data). The demographic vital rates we report here imply
a continued expansion of the sooty fox sparrowDraft in the Georgia Basin in future.
Acknowledgements
We thank many people that have contributed to monitoring on Mandarte Is. and the Tsawout and
Tseycum Bands who generously support our work there. We are grateful to K. Johnson, M.
Crombie, N. Morrell, P. Nietlisbach, E. Gow, K. Näpflin, M. Andrés, E. Hampshire, and D.
Gross for help with data collection and analysis. Our work was supported by the University of
British Columbia, W. and H. Hesse, the American Ornithologists’ Union, and Natural Sciences
and Engineering Research Council of Canada.
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Table 1. Model selection results examining variation in sooty fox sparrow (Passerella
unalaschcensis) annual survival by age (juvenile or adult) and time. Analyses were conducted in
program MARK.
Model AICc AICc AICc Weight Model likelihood No. Par. Deviance
Φ(a) p(t) 376.965 0.000 0.98065 1.0000 8 65.741
Φ(a/2 t/t) p(t) 384.854 7.889 0.01899 0.0194 16 55.814
Φ(.) p(t) 392.899 15.934 0.00034 0.0003 7 83.815
Φ(t) p(t) 398.288 21.323 Draft0.00002 0.0000 11 80.530
Φ(t) p(.) 422.184 45.219 0.00000 0.0000 5 117.327
Φ(a) p(.) 434.354 57.389 0.00000 0.0000 3 133.652
Φ(.) p(.) 449.510 72.545 0.00000 0.0000 2 150.859
Note: Parameters estimated are survival (Φ) and re sighting (p) probabilities, varied by age (a),
time (t), or held constant (.).
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Fig. 1. Comparison of sooty fox sparrow (Passerella unalaschcensis) and song sparrow
(Melospiza melodia) lay dates (i.e., date of first egg, DFE), standardized between years by the
song sparrow median DFE for females’ first broods in a year. Medium grey areas indicate
overlap of the two distributions. The double peak in fox sparrow lay dates indicates at least some portion of the population is double brooding, as song sparrows are known to do.
Fig. 2. The distribution of stochastically estimated population growth rates, λ, in the sooty fox
sparrow (Passerella unalaschcensis) population. Approximately 86% of estimates have a λ
greater than 1 (i.e., a growing population), and mean λ = 1.61 ± 0.57 (SD).
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