Monahan & Hijmans, Mechanisms of migratory range expansion
1
Ecophysiological constraints shape fall migratory response to climate change
in the North American field sparrow
William B. Monahan* & Robert J. Hijmans†
Museum of Vertebrate Zoology, 3101 Valley Life Sciences Building, University of California,
Berkeley, CA 94720-3160, USA
Department of Integrative Biology, monahan@berkeley.edu; †Present address: International Rice Research
Institute, Los Baños, Laguna, Philippines.
*
ESM. ADDITIONAL MATERIAL AND METHODS
(a) Diagnostic analyses to determine the suitability of the Audubon Christmas Bird Count
(CBC) for quantifying changes in the distribution and abundance of the field sparrow
The CBC is a unique historical dataset has been used extensively and highly successfully to
study the winter distribution and abundance of North American birds (Bock & Root 1981;
Koenig 1998), including the field sparrow (Root 1988). However, some known limitations exist
with its methodology (Dunn et al. 2005). Chief among these in the context of the present study
are false absences (Fielding & Bell 1997) and inadequate sampling of the species’ range
(Hurlbert & White 2005). We considered these potential issues as follows:
First, as indicated in the main text, we restricted our analysis to the 72 spatially unique
CBC sites (at 0.5 × 0.5° spatial resolution, the native resolution of the weather data, see below)
in suitable habitats with ≥ 95% temporal coverage. This was done to eliminate a spatiotemporal
bias in the data. Specifically, the number of sites surveyed in eastern North America has
increased dramatically since the year 1940 (ESM figure 1a). Inclusion of the extra sites would
have made it impossible to determine whether an apparent range expansion was real or artefact.
Second, we asked whether the mean observed northern range limit of the field sparrow
was constrained geographically by the mean potential limit as determined by the northernmost
CBC sites included in our analysis. From 1940 to 1997, the observed northern range boundary
was separated by a minimum distance of 67 km from the stable potential limit of 41.2° N latitude
(ESM figure 1b). Hence, our estimates of the observed northern range limit were not constrained
by the spatial distribution of survey sites. Incidentally, we did not consider the southern range
limit of the species because it was bounded by the Gulf of Mexico.
Third, we considered whether the observed range expansion was influenced by a
spatiotemporal bias in the probability of species detection based on effort. While mean survey
effort per site increased from 1940 through 1963, there was no consistent tendency for absence to
be associated with low effort and presence to be associated with high effort (ESM figure 1c).
Temporal changes in the observed northern range boundary thus could not have been caused by
spatiotemporal differences in survey effort.
Fourth, we asked whether general spatiotemporal changes in effort might have influenced
the broad estimated trends in relative abundance. We were particularly concerned with the
possibility of an asymptotic relationship between the number of birds counted and effort since
this would tend to falsely overemphasize declines. However, bird counts failed to show an
asymptotic relationship with effort (ESM figure 1d). Additionally, both the CBC and the North
American Breeding Bird Survey suggest that the species has been declining since the mid-1960s
(Sauer et al. 2007).
Monahan & Hijmans, Mechanisms of migratory range expansion
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In summary, CBC data provide reasonable approximations of historic changes in the
winter distribution and abundance of the field sparrow, at least as considered in the context of the
present study. We acknowledge that our methods of analysis are by no means exhaustive. In
particular, new methods are being developed to better account for uncertainties in survey data
when estimating population sizes and population trends (Royle 2004; Link et al. 2006). We also
note that many aspects of the CBC are for historical reasons well suited to studying northern
distributional limits of eastern species like the field sparrow. Hence, our methods are not
intended to generally provide a detailed analysis framework for other taxa and systems; the
adequacy of our approach must be carefully evaluated on a case-by-case basis and, where
appropriate, modified using better data and methods.
(b) Estimates of physiological parameters
We based our calculations of individual energy demand on existence metabolism, which is
typically determined by measuring the amount of food consumed by captive birds over a period
in which they maintain constant weight (Kendeigh 1969), and is formally calculated as gross
energy intake minus total excretory energy. Using physiological parameters obtained from a
published experimental study conducted using large umbers of winter-acclimatized birds (Olson
& Kendeigh 1980), existence metabolism in kJ/bird per day was given by 63.34 − 1.14Tm, where
Tm represents minimum temperature of the coldest month in °C, and lower lethal temperature
was reached at –13°C.
(c) Weather data
We used monthly weather grids for each year at 0.5 × 0.5° spatial resolution (Mitchell & Jones
2005). We conservatively used minimum temperature of the coldest month to capture peak
metabolic energy demand during the same month of the year that the species would also be most
susceptible to cold lethal conditions. Environmental energy supply was approximated by net
primary productivity (NPP) calculated with the Miami model (Adams et al. 2004), which uses
mean annual temperature (T) and total precipitation (W) to estimate the annual amount of Carbon
per m2: NPP(T,W) = 1/1000 min{1350/(1 + e1.315−0.119T), 1350(1 − e−2.656W) } .
(d) Mapping ecophysiological and geographic limits
Selected CBC sites were mapped as 0.5 × 0.5° presence/absence arrays that could be directly
associated with the weather surfaces. For each year, estimates of metabolic energy demand and
NPP were extracted from the selected sites; annual means were used to assess temporal changes
in each variable within the potential range of the species. The lower lethal temperature threshold
of –13°C was used in conjunction with annual measures of minimum temperature of the coldest
month to generate potential presence/absence surfaces. We estimated the mean observed and
potential northern latitudinal range margins by averaging across latitudes associated with the
northernmost grid cells (for each 0.5° longitude) where the field sparrow was either observed or
capable of surviving. Calculations of potential margins were restricted to the portions of the grids
bounded by the species’ observed longitudinal extremes. Due to the fact that the potential range
limit represented a mean approximation, lower lethal temperature isoclines as plotted in figure 1
were not intended to fully specify distributional dynamics on a per year basis, but rather patterns
of change (or stasis) across multiple years.
Monahan & Hijmans, Mechanisms of migratory range expansion
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Monahan & Hijmans, Mechanisms of migratory range expansion
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ESM Figure 1. Diagnostic analyses to determine the suitability of the Audubon Christmas Bird
Count (CBC) for quantifying changes in the distribution and abundance of the field sparrow. (a)
Total number of CBC sites surveyed over time (black) relative to the 72 selected sites (grey line).
(b) Observed (black) and potential (grey line) mean northern range boundary estimated using the
72 selected sites. (c) Temporal changes in survey effort for sites where the field sparrow was
either present (black) or absent (grey). (d) Log-transformed number of birds counted as a
function of survey effort.