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# Abiotic conditions shape spatial and temporal morphological variation in North American birds

## Abstract

Quantifying environment–morphology relationships is important not only for understanding the fundamental processes driving phenotypic diversity within and among species but also for predicting how species will respond to ongoing global change. Despite a clear set of expectations motivated by ecological theory, broad evidence in support of generalizable effects of abiotic conditions on spatial and temporal intraspecific morphological variation has been limited. Using standardized data from >250,000 captures of 105 landbird species, we assessed intraspecific shifts in the morphology of adult male birds since 1989 while simultaneously measuring spatial morphological gradients across the North American continent. We found strong spatial and temporal trends in average body size, with warmer temperatures associated with smaller body sizes both at more equatorial latitudes and in more recent years. The magnitude of these thermal effects varied both across and within species, with results suggesting it is the warmest, rather than the coldest, temperatures that drive both spatial and temporal trends. Stronger responses to spatial—rather than temporal—variation in temperature suggest that morphological change may not be keeping up with the pace of climate change. Additionally, as elevation increases, we found that body size declines as relative wing length increases, probably due to the benefits that longer wings confer for flight in thin air environments. Our results provide support for both existing and new large-scale ecomorphological ‘rules’ and highlight how the response of functional trade-offs to abiotic variation drives morphological change.

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## Data availability

Data from the MAPS programme are curated and managed by The Institute for Bird Populations and were queried from the MAPS database on 16 October 2019. MAPS data used here are available on Dryad (https://doi.org/10.5068/D1DT2T).

## Code availability

All code used to produce analyses are freely available on Github (https://github.com/caseyyoungflesh/MAPS_morph_changes) and archived on Zenodo (https://doi.org/10.5281/zenodo.6977666).

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## Acknowledgements

We thank MAPS station operators for collecting and sharing their data. D. Kaschube provided critical data access, assisted by R. Guralnick and R. LaFrance. We thank C. Che-Castaldo for helpful discussions regarding the statistical modelling. Illustrations were provided by L. Helton. Funding was provided by National Science Foundation grants EF 1703048 (M.W.T.) and EF 2033263 (M.W.T.).

## Author information

Authors

### Contributions

C.Y. led formal analysis. C.Y. and M.W.T. shared conceptualization and writing of the original draft. R.B.S. and J.F.S. facilitated data access. All authors contributed to review and editing of drafts.

### Corresponding author

Correspondence to Casey Youngflesh.

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### Competing interests

The authors declare no competing interests.

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Nature Ecology & Evolution thanks Nir Sapir and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Extended data

### Extended Data Fig. 1 Morphological data availability over time.

Each horizontal line represents one of 1124 MAPS stations. Stations are ordered by latitude, from north (top) to south (bottom).

### Extended Data Fig. 2 Derivation of morphological indices.

(A) Logged wing length as a function of logged mass for the 105 bird species considered in this study. Points represent mean values for each species. (B) The relationship between wing length (W) and mass (M) can be described by a power law, where c represents the scaling exponent. Logging both sides of the equation linearizes this model. Using a phylogenetic regression, c was estimated to be approximately 1/3 across species, as predicted by scaling theory. The negative arc tangent of this estimate (to convert the slope to radians) was used to create a rotation matrix. (C) For each species, the rotation matrix was used to reproject logged wing length and logged mass onto a new coordinate plane (top panel). Values for both the x and y axes were standardized to have a standard deviation of 1, to create a size index and wing index, representing the overall size of each individual bird and the degree to which wing length deviates from its expected value given the body mass of the individual, respectively (bottom panel).

### Extended Data Fig. 3 Posterior estimates for (A) SI βIDX (Eq. 6), (B) SI γIDX (Eq. 11), and (C) SI θIDX (Eq. 11), denoting the change in size index for each species per 10 years, 10 degrees latitude, and 1000 m elevation, respectively.

Points represent the posterior medians, while thick and thin lines represent the 50% and 89% credible intervals, respectively. The dashed grey line represents zero in all cases.

### Extended Data Fig. 4 Posterior estimates for (A) WI βIDX (Eq. 6), (B) WI γIDX (Eq. 11), and (C) WI θIDX (Eq. 11), denoting the change in wing index for each species per 10 years, 10 degrees latitude, and 1000 m elevation, respectively.

Points represent the posterior medians, while thick and thin lines represent the 50% and 89% credible intervals, respectively. The dashed grey line represents zero in all cases.

### Extended Data Fig. 5 Posterior estimates for (A) Lag 0 γTVT (Eq. 15), (B) Lag 1 γTVT (Eq. 15), and (C) Lag 2 γTVT (Eq. 15), denoting the change in Size Index for each species per 1C change in temperature at lag 0, lag 1, lag 2, respectively (i.e., the effect of change in temperature over time).

Points represent the posterior medians, while thick and thin lines represent the 50% and 89% credible intervals, respectively. The dashed grey line represents zero in all cases.

### Extended Data Fig. 6 Posterior estimates for βSVT (Eq. 19) denoting the change in size index for each species per 10 °C change in mean station temperature (i.e., the effect of change in temperature over space).

Points represent the posterior medians, while thick and thin lines represent the 50% and 89% credible intervals, respectively. The dashed grey line represents zero in all cases.

### Extended Data Fig. 7 Posterior estimates for (A) $${\bf{\omega}}_{{\boldsymbol{ M}}_{{\mathbf{ TIME}}}}$$ (Eq. 25), (B) $${\bf{\omega}}_{{\boldsymbol{M}}_{{\mathbf{LAT}}}}$$ (Eq. 25), and (C) $${\bf{\omega}}_{{\boldsymbol{M}}_{{\mathbf{ELEV}}}}$$ (Eq. 25), denoting the percent change in mass for each species over the 30-year study period, the latitudinal range across which each species was sampled, and the elevational range across which each species was sampled, respectively.

Points represent the posterior medians, while thick and thin lines represent the 50% and 89% credible intervals, respectively. The dashed grey line represents zero in all cases.

### Extended Data Fig. 8 Posterior estimates for (A) $${\bf{\omega}}_{{\boldsymbol{ W}}_{{\mathbf{ TIME}}}}$$ (Eq. 25), (B) $${\bf{\omega}}_{{\boldsymbol{W}}_{{\mathbf{LAT}}}}$$ (Eq. 25), and (C) $${\bf{\omega}}_{{\boldsymbol{W}}_{{\mathbf{ELEV}}}}$$ (Eq. 25), denoting the percent change in wing length for each species over the 30-year study period, the latitudinal range across which each species was sampled, and the elevational range across which each species was sampled, respectively.

Points represent the posterior medians, while thick and thin lines represent the 50% and 89% credible intervals, respectively. The dashed grey line represents zero in all cases.

### Extended Data Fig. 9 Absolute value of the estimate rate of change (represented in units of haldanes [standard deviations per generation]) for body mass (|h|) for focal species in this study (red) and for species and traits presented in (79) (blue).

Traits considered by (79) varied by species and only species undergoing anthropogenic disturbance [as defined by (79)] were considered. The x-axis of the plot is truncated at 0.2 to facilitate visualization.

### Extended Data Fig. 10 General trends (decrease in SI over time, increase in SI over latitude, and increase in WI over elevation) observed in this study as exhibited by Turdus migratorius (American robin).

(a) Observed size index measures at one MAPS banding station (located at 39.3°N, 84.8°W) plotted over time. (b) Observed size index measures plotted across latitude. (c) Observed wing index measures plotted across elevation. In all cases, each black point represents one individual, the posterior mean of the linear predictor is plotted in red, while the red ribbon represents the 89% CI.

## Supplementary information

### Supplementary Information

Supplementary Fig. 1 and Supplementary Tables 1–4.

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Youngflesh, C., Saracco, J.F., Siegel, R.B. et al. Abiotic conditions shape spatial and temporal morphological variation in North American birds. Nat Ecol Evol (2022). https://doi.org/10.1038/s41559-022-01893-x

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• DOI: https://doi.org/10.1038/s41559-022-01893-x