Observer terms change the bird community temperature trend tenfold on 1,859 North American BBS routes (1970–2024), and within-observer declines are larger for high-pitched species
Question: do North American Breeding Bird Survey (BBS) landbird communities move towards warm-associated species, and do routes that warmed more move more? Data: BBS 2025 release (1,859 routes, 322 species, 1970–2024); May–July temperature from GHCN-CAMS, CRU TS 4.09, Berkeley Earth and nClimGrid; song peak frequency for passerines (Mikula et al. 2021). Method: the community temperature index (CTI) is the log-abundance-weighted mean species temperature index. We fitted route CTI trends with observer-route fixed effects (pre-specified) and without observer terms, removed range-wide species trajectories fitted outside each region, and used a 5-degree block bootstrap. Results: with observer terms, the mean CTI trend was +0.018 °C per decade (95% CI +0.009 to +0.028), or 19% of full tracking of the +0.24 °C per decade warming. Without observer terms, on the same routes, it was +0.002 (−0.005 to +0.009). The difference (+0.013 to +0.017) was significant in all eight sensitivity variants. CTI increased between surveys by the same observer (+1.2 m°C) and decreased when a new observer started (−6.4 m°C); new observers recorded 0.82 more species. In an exploratory test with 247 passerines, within-observer trends were lower than across-observer trends, and more so for high-pitched species (Spearman rho −0.52; −0.08 log units per decade per octave within families). High-pitched passerines also had lower temperature indices (rho −0.19). This agrees with age-related hearing loss of observers. Local warming did not predict CTI change at route, 1°, 2°, 5° or regional scale. The full-tracking slope was outside the intervals except for one wide within-region interval. Route warming trends from different temperature products correlated only at r = 0.59 to 0.82. Species did not decrease more in the warm parts of their ranges. Conclusion: BBS breeding CTI trends do not identify climate tracking without an assumption about observer drift; the estimate is between about 0% and 31% of full tracking. Limitations: observer ages are unknown, so observer aging and observer-pool change cannot be separated; the song frequency tests are exploratory; counts are roadside indices.
community temperature index · North American Breeding Bird Survey · climatic debt · observer effects · observer aging · song frequency · hearing loss · climate change · birds · range shifts · gridded temperature · measurement error
References
13- usesdoi:10.5066/P14SNUV4
BBS 2025 release (1966-2024) counts, routes, weather and species list.
- usesdoi:10.1029/2007JD008470
GHCN-CAMS monthly temperature for the primary route temperature.
- usesdoi:10.1038/s41597-020-0453-3
CRU TS 4.09 temperature as a second product.
- usesdoi:10.5194/essd-12-3469-2020
Berkeley Earth gridded temperature as a third product.
- usesdoi:10.1175/JAMC-D-13-0248.1
nClimGrid monthly temperature as a fourth product for CONUS.
- usesdoi:10.17605/osf.io/fa9ky
Median song peak frequency of passerines (supporting information of Mikula et al. 2021).
- doi:10.1111/ele.13662
Source paper of the song frequency data.
- supportsdoi:10.1002/ece3.1101
We find frequency-dependent within-observer declines in BBS that agree with their observer-aging result, and show the consequence for CTI.
- doi:10.1111/1365-2656.13433
Regional BBS CTI trends and their relation to warming; our route-level analysis shows that the breeding CTI trend depends on the observer model.
- doi:10.1098/rspb.2008.0878
Definition of CTI tracking and climatic debt.
- doi:10.1371/journal.pone.0184275
CTI trends can come from drivers that are not climate.
- UK butterfly transect communities did not shift to warm-niche species at the rate of 1976–2024 site warming, while thermal niche predicted year-to-year responses sci:sha256:a37a9927e8c25069416d317953a1c1943c4017f40a065b89c7de4187728e5393
UK butterfly CTI also stayed far below full tracking of local warming.
- BBS route trends in 2007–2024 do not decrease with local abundance for 335 North American bird species, unlike eBird model trends at the same places sci:sha256:68ffdc7961f03e5fd4e9333a8cdd825fab7844e5b09e3493ef265f777869997d
BBS species trend analysis with route-observer intercepts; our results show that such within-observer trends are lower for high-pitched species.
Cited by
15 publicationsPublic catalog entries on Sciscape.
- Adult rodents at Portal, Arizona, were not lighter in warmer years and did not lose mass in 1977–2026 in 11 species; their hindfoot length decrease is mainly measurement drift
Shows a similar problem: observer changes alter long-term trends in bird survey data.
- Spatial synchrony of 183 breeding bird species did not increase on North American BBS routes in 1967–2024, while summer temperature synchrony at the same routes increased
Shows that BBS observer changes can strongly bias route trends; we use within-observer growth rates for this reason.
- supportsOff-road NEON and roadside BBS bird trends were correlated in 2015–2024 (rho 0.54 per site, 0.83 per species), but NEON site indices were noisier and the mean difference was not robust
Reported within-observer decreases of BBS counts; our BBS minus NEON trend difference halved when BBS trends also used between-observer changes, which agrees with that report.
- usesCounts from separate years showed no clear median concentration change for 152 common landbirds on 812 BBS routes, 2000–2004 to 2015–2019
Reuses its retained categorical landbird and introduced-species labels. Follows its documented run and taxon filters; counts are rebuilt from the original BBS release. Its observer results inform the limits.
- supportsNorth American breeding-bird community temperature index fell in 1970–1996 and rose in 1997–2024 in all 30 tested BBS regions, at 25% of full tracking, but not more where June warming was larger
Observer terms change the BBS CTI trend; our route-only versus route × observer results and tenure checks agree.
- Year-permutation tests show no robust increase in breeding-bird growth-rate synchrony in North American, UK, Swedish and Czech schemes, while temperature synchrony increased at NA and UK sites
Shows strong observer effects in BBS trends; we use within-observer growth rates.
- Variance and autocorrelation trends did not improve 5-year forecasts of local disappearance beyond abundance and trend in BBS and BioTIME 2.0 count series
Related SCI result on BBS observer effects on community change.
- In 2022–2024 breeding bird surveys, buteo hawks and vultures fell below trend, but waterfowl and gulls did not, and H5N1 detections did not predict which species or routes declined
Shows that observer terms change BBS trend estimates; we include route-by-observer effects.
- In North American BBS route counts (1966–2024), the variance of change grows to 55-year lags; observer changes cause most apparent trend reversal and the sub-linear growth exponent
Earlier SCI study that showed observer terms change BBS community temperature trends.
- supportsCzech breeding bird community temperature index rose at 29% of full tracking in 1982–2024, but not more than with shuffled species values and not more where sites warmed more
Our results agree that local warming does not predict site CTI trends; we also use observer fixed effects.
- Spatial synchrony of 183 breeding bird species did not increase on North American BBS routes in 1967–2024, while summer temperature synchrony at the same routes increased
Shows that BBS observer changes can strongly bias route trends; we use within-observer growth rates for this reason.
- Spatial synchrony of 168 North American breeding bird species did not increase in 1970–2024 BBS data at 0–1000 km, although winter temperature synchrony on the same route pairs increased
Shows that BBS observer changes have large effects on trends; we use within-observer intervals for this reason.
- Spatial synchrony of breeding bird population growth on 1,299 North American BBS routes did not increase from 1969–1996 to 1997–2024, while annual air temperature synchrony increased
Shows that BBS observer changes affect trends; motivates same-observer growth rates.
- usesMore passing vehicles lower stop-level BBS counts in 318 of 382 species (3.8% per car), with more loss for low songs, but 1998–2024 traffic change alters trends by under 0.1%/yr for 375
We reuse its BBS to BirdTree name match for song peak frequency.
- supportsIn BBS route counts (1966–2024), the extra variance at an observer change did not increase with outgoing tenure, so drift from an observer's first survey does not explain same-observer variance growth
Our species-year-centred tenure curve shows declining counts within observer tenures in BBS and Swedish data, in agreement with its within-observer declines.