Heading south for winter, more birds are choosing the Pacific Northwest

Many papers predict that bird ranges will shift northward with a warming climate (Wu et al 2018, Langham et al 2015).

Many studies have already documented that this is happening (Illán et al. 2014, Virkkala, R. and A. Lehikoinen 2014, Hitch and Leberg 2007, and La Sorte and Thompson 2007).

And some have documented poleward range shifts specifically for wintering ranges (Saunders et al 2022, Hampton 2019, Paprocki et al 2017, Prince and  Zuckerberg 2016, and Paprocki et al 2014).

I’ve previously written about an increase in insectivore bird species in winter associated with a warming climate in the Sacramento Valley. As the Putah Creek Christmas Bird Count (CBC) compiler, it was hard not to notice the trends. Cassin’s Vireo, Black-throated Gray and Townsend’s Warblers, and Western Tanagers were becoming more expected in winter. We had crossed a threshold; we didn’t get freezes anymore. My bougainvillea and cape honeysuckle, which previously clung to life in winter, were now growing and blooming year-round. Fruit and insects were available to these birds.

Now in Port Townsend, Washington, we set a local CBC record for Yellow-rumped Warblers last year. This caused me to take a closer look at the data, focusing on Passerines that are rare or uncommon, and at the northern edge of their wintering range. They are: Hermit Thrush, Cedar Waxwing, Lincoln’s Sparrow, White-crowned Sparrow, Orange-crowned Warbler, and Yellow-rumped Warbler. For each of these, the PNW is at the northern limits of their wintering range.

I looked at their numbers and trends on the Portland, Olympia, Seattle, Bellingham, and Vancouver BC CBCs since the 76th CBC (winter 1975-76). I’ve got more notes on my methodology at the end.

Results

All have increased since 1975, generally with the uptick beginning in the 1990s. Here are the results of my inquiry.

The range maps are from eBird’s Abundance Maps. Red=summer; blue=winter; purple=year-round; yellow=migration. The graphs show the birds per party hour across the five CBCs, taking the total number of birds and dividing by the total number of hours across all five counts.

Hermit Thrush

Hermit Thrush has been increasing at a rate of 4.2% per year across all the CBCs. It has been increasing across all five of the counts, most strongly in Vancouver (4.1% annual growth) and most tepid in Seattle (0.6%). The overall average   It is most common on the Portland count, which has averaged 26 Hermit Thrushes per count since 2009.

Cedar Waxwing

Of the six species I focused on, Cedar Waxwing showed some of the most erratic growth, averaging only 2.5% per year. That said, it has been above average 8 of the last 9 years. To illustrate the unpredictable nature of waxwings, they have actually been declining on the Olympia (-2.3%/yr) and Vancouver (-4.1%/yr) counts. They are increasing the most on the Portland count (3.0%/yr).

Lincoln’s Sparrow

Lincoln’s Sparrow has been increasing steadily, from near zero, at an overall rate of 3.6% per year. To put this in perspective, these five CBCs tallied 5 or fewer individuals, summed across all counts, in each of the first five years of this analysis. In each of the last five years, these counts, in aggregate, tallied between 34 and 52 individuals. Growth has been strongest on the Olympia count (4.6%/yr) and weakest on the Bellingham count (1.7%/yr).

White-crowned Sparrow

Despite the eBird map, White-crowned Sparrow is a regular overwintering species in the PNW. The five counts, in aggregate, tally between 100 and 750 individuals each year. They’ve been increasing at a rate of 1.8% per year, strongest in Seattle (3.1%/yr) and weakest in Vancouver (-2.5%/yr, the only count with declining numbers).

Orange-crowned Warbler

Orange-crowned Warbler has seen dramatic increases, averaging 5.0% per year, highest in Olympia (7.2%/yr) and lowest in Bellingham (3.2%/yr). The numbers, however, are still small. Aggregate numbers across all counts were zero five of the first eleven years of this analysis (easily seen on the graph). Double digits were not reached until 1999. The last ten years, however, have averaged 15 individuals across all the counts, making this an expected species in winter now.  

Yellow-rumped Warbler

Yellow-rumped Warbler wins the award for poster child of species increasing in winter at the northern edge of their wintering range. They’ve been increasing at a rate of 5.3% per year. Interestingly, this growth is concentrated in the south. Portland (3.7%/yr), Olympia (3.4%/yr), and Seattle (6.1%/yr) have seen the most growth, while Bellingham (-0.5%) and Vancouver (-5.0%) have seen declines. Perhaps those Fraser River winds are too cold for warblers. 

Methodology

The data includes bird per party hour for the Portland, Olympia, Seattle, Bellingham, and Vancouver BC Christmas Bird Counts from the 75th count (winter 1975-76) to the 120th count (winter 2019-20). The 121st count was impacted by the pandemic.

CBC (and Breeding Bird Survey) data is uniquely advantageous for looking at long-term trends such as climate change, as they both go back many decades with generally similar effort over time (for certain well-established counts). Nevertheless, there were some issues with this data:

  • I did not use the Portland data from the 76th thru the 82nd count, due to aberrantly low party hours relative to later counts.
  • The following data was missing entirely from the Audubon CBC database: Olympia 76th, 77th, 78th, 84th, 104th, and 110th counts; and Seattle 91st count.
  • The following counts had no (or obviously incorrect) data for party hours: Portland 104th count; Bellingham 111th, 112th, and 119th counts. Because they did have bird numbers, I approximated the party hours based on their counts in nearby years. I used 230 party hours for the Portland count and 200 party hours for the Bellingham counts.

Other climate-related bird changes in the Pacific Northwest

I’ve previously blogged about climate change and birds in the Pacific Northwest:

The invasion of the Pacific Northwest: California’s birds expand north with warmer winters looks at northward range expansions of Great Egret, Turkey Vulture, Red-shouldered Hawk, Anna’s Hummingbird, Black Phoebe, Townsend’s Warbler, and California Scrub-Jay, with some discussion of others as well. Note that Townsend’s Warbler, as a migrant that winters rarely in the PNW, fits with the group of birds described in this post.

The song of the Lesser Goldfinch: Another harbinger of a warming climate looks at increasing records in the PNW in summer.

Mapping the expansion of the California Scrub-Jay into the Pacific Northwest looks at the steady range expansion of this non-migratory species.

References

Hampton, S. 2019. Avian responses to rapid climate change: Examples from the Putah Creek Christmas Bird Count. Central Valley Birds 22(4): 77-89.

Hitch and Leberg. 2007. Breeding distributions of North American bird species moving north as a result of climate change. Conservation Biology 21(2): 534-9.

Illán et al. 2014. Precipitation and winter temperature predict long-term range-scale abundance changes in Western North American birds. Global Change Biology, 20 (11), 3351–3364.

Langham et al 2015. Conservation status of North American birds in the face of future climate change. PLoS ONE 10(9): e0135350.

La Sorte, F.A., and F.R. Thompson III. 2007. Poleward shifts in winter ranges of North American birds. Ecology 88(7):1803–1812.

Paprocki et al. 2014. Regional Distribution Shifts Help Explain Local Changes in Wintering Raptor Abundance: Implications for Interpreting Population Trends. PLoS ONE 9(1): e86814.

Paprocki et al. 2017. Combining migration and wintering counts to enhance understanding of population change in a generalist raptor species, the North American Red-tailed Hawk. The Condor, 119 (1): 98–107.

Prince, K. and B. Zuckerberg. 2016. Climate change in our backyards: the reshuffling of North America’s winter bird communities. Global Change Biology 21(2): 572-585.

Saunders et al. 2022. Unraveling a century of global change impacts on winter bird distributions in the eastern United States. Global Change Biology

Virkkala, R. and A. Lehikoinen 2014. Patterns of climate-induced density shifts of species: poleward shifts faster in northern boreal birds than in southern birds. Global Change Biology 20: 2995–3003.

Wu et al. 2018. Projected avifaunal responses to climate change across the U.S. National Park System. PLOS ONE 13(3): e0190557.

I try to maintain an updated list of references at the Birds and Climate Change Facebook group. At that page, click on Files to find the list.

Helping forests migrate: Planners race to plant trees adapted to the future climate

Researchers from UC Davis collect acorns in arid west Texas to plant on their campus in northern California. They estimate their climate in 2100 will be similar to that of Barstow or even Phoenix today. City staff from a town near Portland, Oregon travel to California and Arizona for seedlings they can take home and plant along their city streets. They are preparing for Portland’s weather to become like Sacramento today.

The range of Arizona oak. For one town near Portland, Oregon, the list of potential future street trees includes this species, as well as California buckeye, California laurel, and silverleaf oak.

With these regions breaking new heat records annually – Sacramento just topped 90 degrees for the 110th day (and counting) in 2020—and given that trees take decades to mature, the race is on. Birds can fly, mammals can walk, but trees expand their ranges very slowly. Most acorns from an oak end up within a few hundred yards from their home tree.

Climate velocity, the speed at which ecotones are shifting north, is much faster than that. Our climate is changing ten to one hundred times faster than during a global warming event 55 million years ago known as the Paleocene-Eocene Thermal Maximum (PETM). During that “rapid” spike, palm trees successfully migrated to the Arctic circle, but they had thousands of years to make it there.

Dead blue oaks in Fresno County, California. They experienced excessive mortality during the 2012-16 drought. These hills may revert to grassland. Researchers want to use the genes of the survivors as stock for the future in the north. For a full presentation of blue oak gene-assisted migration see this presentation by the California Department of Fish and Wildlife.

While trees can’t walk, they can die. Range contraction of trees along their southern xeric (dry) edge is happening in the American West right with the speed of climate change. Blue oak die-offs are widespread in the southern third of their range. From California to Colorado, conifers such as Ponderosa pine and Douglas-fir are disappearing from lower elevations. To quote Davis et al (2019), “In areas that have crossed climatic thresholds for regeneration, stand-replacing fires may result in abrupt ecosystem transitions to nonforest states.” When people talk about California becoming Arizona, the cleanup hitter in that process may be fire, but the first batters are heat, drought stress, and bark beetles. After fires, decreased soil moisture and increased vapor pressure deficit (VPD) associated with climate change are leading to reduced probability of regeneration (Davis et al 2019). In short, many forests are not coming back.

Ponderosa pines are disappearing from lower elevations of the Sierra in California. This has been documented in Colorado as well.

Range expansion of trees northward has been documented, but the pace is anemic, insufficient to keep up with the changing climate. One study in the east found that ranges in adult trees expanded north less than 150 yards per year (Sittaro et al 2017). They concluded, “our results add to the body of evidence suggesting tree species are mostly limited in their capacity to track climate warming…”

Recent mega fires include many of the drought-killed conifers in the southern Sierra. Research suggests regeneration may be imperiled due to a warming climate.

Researchers have discussed facilitating tree migration due to climate change for over a decade (Aitken et al 2008). For over a hundred years, botanists have recognized regional differences within the same plant species, and simple garden experiments have shown that local varieties do better. The standard rule of thumb has always been that local varieties are best; they are adapted to the local ecological niche. Now that is changing.

Recent research is showing that trees are now in the wrong places; the climate has shifted past them. Valley oaks, white fir, Douglas fir, ponderosa pine, Western hemlock, and lodgepole pine seedlings all do better when removed from their original home and moved north (Aitken and Bemmels 2015).

The local trees are becoming misfits in a world that is changing around them. Many researchers are hesitant to fully embrace assisted migration; introducing non-native species has a horrid track record. But they are beginning to study “assisted gene flow”, moving hardy trees from the southern end of a species’ range to the north end. Cities, on the other hand, are beginning to see trees as more than just aesthetically pleasing; they are critical infrastructure, providing shade and reducing urban temperatures. So the cities and towns are moving faster, boldly cultivating trees from the dry Southwest into the Pacific Northwest.

This photo from Aitken and Bemmels (2015) shows a series of Sitka spruce, all eight years old, planted together in British Columbia. The trees from the south, adapted for a warmer and drier environment, are out-competing the locals.

Tree migration is also critical for the range expansion of animals. Without the trees and other vegetation, many birds, mammals, and other forms of life have no habitat rungs on the ladder to enable them to move north as well. Anna’s Hummingbirds now winter in Canada and even Alaska, largely due to ornamental plantings. The Oak Titmouse, on the other hand, is dependent on oaks, tightly constraining its ability to expand north. It may be that, in the coming decades, oaks and other tree species planted in cities and towns will provide critical refugia for a wide variety of birds and insects seeking cooler climes.