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Winter Movements of Sitta canadensis L. (Red-breasted Nuthatch)
in New England and Beyond:
A Multiple-scale Analysis
W. Herbert Wilson Jr.1,* and Bets Brown1
Abstract - We analyzed 55 years of abundance data (1960–2014) for Sitta canadensis (Redbreasted
Nuthatch) to seek patterns of winter irruptions on temporal and spatial scales. This
species shows an erratic pattern of irruption into southerly areas from its northern breeding
areas. Irruptions show a broad geographic synchrony. At the narrower level of the state or
province and even more so at the level of individual counts, correlations of abundance in
adjacent areas become weaker. The abundance of irruptive birds is best considered a mosaic.
At the regional scale, correlations of Red-breasted Nuthatch abundance with irruptive
northern finches that also depend on conifer seeds are weak to absent. The data suggest
that birds irrupt because of failure of conifer seed production on the breeding grounds, not
because the birds are seeking masting conifer stands to the south.
Introduction
Seasonal migrations between breeding and wintering grounds characterize a
large number of bird species in the Northern Hemisphere. A smaller group of species
migrates south in movements called irruptions in some but not all years. In this
paper, we define an irruption as a significant fall to early winter invasion of birds
with distributions centered in the high-latitudes to more southerly areas where the
species do not regularly winter (Bock and Lepthien 1972, 1976; Lack 1954).
The best known of these irruptive species in North America are a group of
fringillids called the northern finches, including Acanthis flammea (L.) (Common
Redpoll), A. hornemanni (Holbøll) (Hoary Redpoll), Spinus spinus (L.) (Pine Siskin),
Haemorhous purpureus (Gmelin) (Purple Finch), Pinicola enucleator (L.)
(Pine Grosbeak), Coccothraustes vespertinus (W. Cooper) (Evening Grosbeak),
Loxia leucoptera Gmelin (White-winged Crossbill), and L. curvirostra L. (Red
Crossbill) (Bock and Lepthien 1976, Erickson 1970, Evans 1966, Reinikainen 1937).
All of these species rely on conifer seeds for much of their food (Bock and Lepthien
1976) and their irruptions are often synchronous (Bock and Lepthien 1972, 1976).
Sitta canadensis L. (Red-breasted Nuthatch) is also an irruptive species. Redbreasted
Nuthatches frequent coniferous forests (Adams and Morrison 1993, Ball
1947, Wydemerer 1933) and breed across most of southern Canada and the northern
tier of the US, with populations extending southward along the Appalachians to
North Carolina and the Rocky Mountains to Colorado. The diet of Red-breasted
1Department of Biology, Colby College, Waterville, ME 04901. *Corresponding author -
whwilson@colby.edu.
Manuscript Editor: Heather York
Winter Ecology: Insights from Biology and History
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Nuthatches consists mostly of arthropods during the breeding season and conifer
seeds during the winter (Ghambalor and Martin 1999). Red-breasted Nuthatches
cache seeds for later use during the winter.
The conventional explanation for irruptions is variability in seed production by
conifers or other trees whose seeds provide a major food source for the irruptive
birds (Formozov 1960, Lack 1954, Newton 1970, Smith 1986). During mast years,
birds are predicted to remain on their breeding grounds through the winter, but are
forced southward when seed production is low. We currently lack continent-wide
data on conifer cone production to test this hypothesis directly. However, we can
employ an indirect test by looking for synchrony between Red-breasted Nuthatches
and northern finches. Though not closely related, Red-breasted Nuthatches and
finches depend on conifer seeds for winter sustenance. If levels of conifer seed production
drive irruptive movements, then the finches and the nuthatch should show
similar patterns. We also consider 2 alternative hypotheses: (1) birds periodically
move south in search of masting conifers, irrespective of local cone abundance, and
(2) birds are competitively forced south by population increases following particularly
successful nesting seasons.
In this paper, we use data from the National Audubon Society Christmas Bird
Counts (CBCs) to determine patterns of irruption in Red-breasted Nuthatches.
We examine variability broadly from a continental perspective as well as smaller
geographic units (state- and province-level down to the level of individual
count-circles). Finally, we examine the strength of correlations of abundance of
Red-breasted Nuthatches with several species of irruptive finche s.
Materials and Methods
The primary source for the data used in this paper was the National Audubon
Christmas Bird database. A CBC is conducted annually on a single day sometime
between mid-December and early January by thousands of observation teams who
each count birds within a fixed 460-km2 (15 miles in diameter) circle. We downloaded
the state and province data from the National Audubon Society website
(https://www.audubon.org/conservation/science/christmas-bird-count). These data
included the mean abundance (measured as number of birds per party-hour) for
each state or province of all the CBCs conducted in a particular year. We requested
and received the count-level data for all counts on which Red-breasted Nuthatches
appeared since the beginning of the program in 1900.
Analysis of the number of CBC count circles included each year indicated a
dramatic rise beginning around 1960 and continuing to the present. We therefore
excluded data from earlier than 1960 because of limited sample size for some states
and provinces.
CBCs have the advantage of sampling a standardized area. However, the number
of observers varies greatly among counts. Therefore, we adopted the metric used
by National Audubon Society biologists—number per party-hour. This method
standardizes the abundance to account for differences in effort. CBC data do not
take into account detectability of different species, but this shortcoming is obviated
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by our consideration of a single species. The early-winter timing of CBCs may not
capture the full irruption in a given year.
To examine the claim that Red-breasted Nuthatches show a biennial pattern
of irruption, we graphed numbers of birds per party-hour against year. For our
analyses, we included 3 states within the breeding range and 3 states in which Redbreasted
Nuthatches only occur as winter visitors.
We employed extensive exploratory graphical analysis in this research. We
used the choroplethr package in R (R Core Team 2016) to make choropleths (maps
where areas are shaded in proportion of a measured variable) of various data for 59
administrative regions (the lower 48 states, the District of Columbia, the 10 provinces
along the southern tier of Canada). We use the term “regions” in this paper to
denote US states and Canadian provinces. We created choropleth maps to show the
region-level mean abundance from the period 1960 until 2014. The coefficient of
variation (CV) for each region was also mapped with a choropleth.
To examine the effect of distance at the regional level, we performed correlation
analyses on all possible combinations of the 59 regions. We then chose 8 regions
for graphical analysis. Using a choropleth, we mapped the correlation coefficient
(Pearson’s r) for a focal region with all other regions.
At a finer scale, we tested for similarities of abundance at the count-circle level.
We chose 8 count circles in 8 different regions as focal areas. We chose these circles a
priori based on the length of the count history and the number of other counts within
a 250-km radius. We employed a geosphere in R (R Core Team 2016) to determine
the distance between the focal circle and all other count circles. We filtered the data
to only include those circles within 300 km of the focal circle. For those circles, we
used the rcorr package in R (R Core Team 2016) to determine the Pearson-r and the
P-values for each correlation between the focal circle and the other circles in the 300-
km radius. We prepared histograms of distance and indicated significance (P < 0.05)
or lack of significance by color for each count-circle considered.
To test for synchrony of irruptions at the regional level, we performed correlation
analyses using the rcorr package in R (R Core Team 2016) for Red-breasted
Nuthatches with 3 fringillids: Common Redpoll, Pine Siskin, and Purple Finch.
We report data for the 10 northern regions where Red-breasted Nuthatches may be
found year-round.
Results
Figure 1 presents annual mean abundance (number per party-hour) for 6 regions.
The graphs do not show a consistent saw-tooth pattern one would expect for a true
biennial pattern. The expected consecutive abundance alternating above and below
the grand mean does not consistently appear. Indiana and Ohio, areas south of the
breeding range, show a bit more evidence of biennial alternation of highs and lows.
However, some peaks occur in odd-numbered years and others in even-numbered
years, forcing us to reject the hypothesis of biennial irruptions.
The highest number per party-hour values occur in the western provinces,
Washington, Montana, and the 3 three northern New England states (Fig. 2A). As
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expected, the lowest abundances occur in the areas most distant from breeding areas
along the southern tier of states.
Figure 1. Yearly abundance of Red-breasted Nuthatches, measured as number per partyhour
for 6 states. The mean number per party-hour for each state is represented by a
horizontal red line. Montana (MT), Minnesota (MN), Maine (ME), and Colorado (CO) host
breeding populations of Red-breasted Nuthatches. Indiana (IN) and Ohio (OH) are south of
the breeding range.
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As a measure of variation within a region among years, we present the CV for
each of the 59 regions (Fig. 2B). We expected the relatively high CV values in the
southern states where irruptions are rare. More surprising is the high CV for Newfoundland,
which has abundant coniferous forests. Most of the regions with the
highest densities showed relatively low CVs (Montana, British Columbia, Maine,
New Hampshire, Vermont).
When we treated each region as a separate focal area, correlations of abundance
with all other areas generally indicated a strong regional pattern. A given region was
more likely to be correlated with adjacent regions compared to more distant regions.
We present choropleths for 8 states to document this pattern. Maine shows a strong
Figure 2. Choropleth maps showing (A) the abundance of Red-breasted Nuthatches
measured as number per party-hour in the lower 48 states and the 9 southern Canadian
provinces, and (B) the variability in abundance (measured as the coefficient of variation)
in the same regions.
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correlation of yearly density with New Hampshire and Nova Scotia, but not Vermont
or Quebec (Fig. 3A). Quebec shows a more consistent pattern with high correlations
with New Brunswick, Nova Scotia, and Ontario (Fig 3B). Tennessee, well south of the
Red-breasted Nuthatch’s breeding range, shows a regional signature with high correlations
in adjoining Alabama, North Carolina, Virginia, and nearby West Virginia
(Fig. 3C). Similarly, Minnesota shows a regional signature with abundances strongly
correlated with Wisconsin, Ontario, and Michigan (Fig. 3D).
Four western regions show the same evidence of regional variation (Fig. 4).
Alberta shows strong correlations with the 3 other western provinces of British Columbia,
Saskatchewan, and Manitoba (Fig. 4A). North Dakota and Montana show
strong positive correlations with the Alberta abundances. The annual abundances
in Montana are strongly correlated along the northern tier of regions, surprisingly
strongly with Wisconsin, Michigan, Ontario, Quebec, and New Brunswick (Fig.
4B). A strong northwestern pattern of similar abundances is manifested in the Oregon-
centered choropleth (Fig. 4C) where Washington, British Columbia, and Idaho
are strongly correlated, and Alberta, Saskatchewan, and Montana are only slightly
less strongly correlated. Colorado shows a more restrictive pattern, with a strong
correlation with Utah and a fairly strong correlation with New Mexico (Fig. 4D).
Other adjoining states show weaker correlations.
Examination of correlations of number per party-hour among individual CBC
circles allows for a more granular comparison of variability (Fig. 5). For distances
among counts separated by no more than 300 km, we found surprisingly poor
significant correlations with a central focal circle. The number of significant correlations
did not fall off with increasing distance from the focal circle. For each of
the 4 focal circles in Saskatchewan (Fig. 5A), Maine (Fig. 5B), Oregon (Fig. 5C),
and Montana (Fig. 5D), most of the correlations were not signifi cant.
Table 1 presents correlation analyses testing for synchrony in the abundance of
Red-breasted Nuthatches with 3 species of irruptive finches that also rely on small
cones from conifer trees. Common Redpoll abundance was significantly correlated
with Red-breasted Nuthatch abundance in Washington and in New York. None
of the other regions showed a significant correlation between these 2 species. We
Table 1. Regional-level correlation analysis of Red-breasted Nuthatch abundance (number per partyhour)
with the abundance of Common Redpoll, Pine Siskin, and Purple Finch from the period 1960 to
2014. * denotes r-values that are significant at the P < 0.01 level.
Region Common Redpoll Pine Siskin Purple Finch
ME 0.031 0.154 -0.086
NH 0.151 0.001 0.052
VT 0.081 0.249 0.619*
NY 0.542* 0.134 0.060
MI 0.023 0.275 -0.322
MN -0.157 0.200 0.039
MT 0.023 0.275 -0.323
WA 0.490* 0.124 0.010
OR 0.177 0.120 -0.078
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Figure 3. Choropleth
maps showing
correlations in
abundance with 4
representative focal
centers from
eastern and central
North America: (A)
Maine, (B) Quebec,
(C) Tennessee,
and (D) Minnesota.
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Figure 4. Choropleth
maps showing
correlations in
abundance with 4
representative focal
centers from
western North
America: (A) Alberta,
(B) Montana,
(C) Oregon,
and (D) Colorado.
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found no significant correlations for nuthatch abundance with that of Pine Siskin,
and only Vermont showed a significant positive correlation for nuthatch and Purple
Finch abundance.
Discussion
Irruptive migrations are common, though unpredictable, behaviors of a number
of northerly species in North America and Eurasia (Ball 1947, Lack 1954). Despite
extensive documentation of these movement patterns, our understanding of the
drivers of the phenomenon is limited. Attempts to understand a phenomenon that
occurs on the level of continents or hemispheres is a daunting task. Nevertheless, 3
hypotheses have emerged to explain irruptive migrations.
First, failure of food supplies on the northern breeding grounds has been claimed
to provide the impetus for irruptions from the breeding grounds into more southerly
areas (Bock and Lepthien 1972, 1976; Lack 1954). For species like finches and the
Figure 5. Correlations of Red-breasted Nuthatch abundance among proximate CBC circles
(within 300 km of the focal circle) using a focal circle in (A) Saskatchewan, (B) Maine,
(C) Oregon, and (D) Montana. For each CBC circle, the teal color denotes a significant
positive correlation with the focal circle and the orange color denotes a non-significant correlation
with the focal circle. The distance from the focal circle is represented on the x-axis
for each circle in the analysis.
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Red-breasted Nuthatch, masting in conifers on the breeding grounds could certainly
impel these birds to forgo a southerly migration during a winter of relative plenty.
Mast years are typically followed by 1 or more years of lowered cone-production,
which could force cone-dependent birds to migrate. Although this explanation
seems reasonable, we lack information on cone production in the northern forests.
Furthermore, we do not know the geographic scale at which masting occurs for
most northerly areas. Bock and Lepthien (1976) provided gross measurements of
cone crops, showing that irruptions were more prevalent when cone crops in the
north were poor. The best data come from Widrlechner and Dragula (1984), who
used foresters’ measurements of commercially important conifers in California,
Oregon, and Washington to show that Red-breasted Nuthatches appeared less frequently
in California when cone crops were good in Oregon and Washington.
Second, some researchers believe that the promise of masting conifers south
of the normal range can induce birds to move (Lack 1954). We argue that this
hypothesis is weak because it presumes omniscience on the part of the birds. To
be sure, irruptive birds may take advantage of mast during an irruption. However,
we believe the birds opportunistically take advantage of those food supplies but
are likely driven south to wander until sufficient food can be found. Bagg (1969)
describes a correlation between mast years in Ontario and irruptions of Poecile
atricapillus (L.) (Black-capped Chickadee) into central Maine. The link between
these 2 events requires that the incidence of masting be synchronous from Maine
to Ontario. Similarly, James (1967) noted few irruptive birds in Arkansas during a
mast year in Newfoundland.
The final hypothesis is that birds irrupt from their breeding grounds after particularly
successful reproductive seasons. We simply lack the data on reproductive
success at higher latitudes to test this hypothesis now.
Examination of the scale of irruptions provides some insight into the driving
forces of such movement. For the Red-breasted Nuthatch, Bombycilla garrulus (L.)
(Bohemian Waxwing), and a suite of northern finches, Bock and Lepthien (1976)
demonstrated broad-scale geographic patterns of irruption followed by years where
few irruptives appeared. These irruptions are less predictable at smaller scales
(Figs. 3, 4). Using 55 years of CBC data, we showed that Red-breasted Nuthatch
abundance is usually strongly correlated in adjacent states or provinces but is often
quite different from regions that are farther away. For some comparisons, even
adjacent regions are poorly correlated (e.g., Maine and Quebec, Colorado and Wyoming).
Red-breasted Nuthatches readily accept supplemental food at bird feeders
(Wilson 1999, 2002; Yunick 1982); thus, differences in bird feeding in proximate
count circles may contribute to the poor correlations observed. Correlations are
even weaker at even more-localized scales (Fig. 5). This analysis implies that irruptive
species are patchy at small scales; correlations between small areas, such as
the 460-km2 of a CBC circle, are weak and insignificant for most comparison s.
We regard the distribution of Red-breasted Nuthatches as a mosaic of areas
of high and low abundance when viewed at a fine scale, but as a more even
when observed at larger scales. The count-level CBC data strongly indicate that
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Red-breasted Nuthatch abundance within focal regions is not uniform in any given
year (Fig. 4).
We believe that the best explanation for irruptive migration is food limitation on
the breeding ground. Depending on the amount of food available, smaller or larger
fractions of a population may be forced to irrupt southward. Those birds continue
south until they find sufficient food, which is often associated with local masting.
We do not believe the birds are attracted to a particular area because of likely
masting; rather, the birds wander until they find food. Masting forests south of the
breeding range may therefore never be used by irrupting birds. Furthermore, heterospecific
birds may wander in different directions, explaining the lack of strong
correlations among different species at the regional level (Table 1).
Although we provide a hypothesis to explain the irruptive behavior of Redbreasted
Nuthatches, more data and efforts are needed to fully understand the
direction, scale, and impetus for these irruptions. Tracking conifer cone production
across the continent and geo-tagging or satellite-tagging birds will be necessary to
offer a robust test of our hypothesis.
Acknowledgments
We thank Kathy Dale, Director of Citizen Science at the National Audubon Society.
Liam O’Brien and Manuel Gimond provided statistical advice. Thanks also to the thousands
of participants in the Christmas Bird Count program, whose collective efforts have made
the CBC database such a powerful source of information on changing bird populations.
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