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Winter Rains…Spring Surveys

Southern Tarplant blooming after the winter rainfall

With roughly a quarter of the wet season still ahead, most of California has received greater than normal year to date rainfall amounts, and many areas have received greater than normal whole-year amounts, some twice over. As of March 1, 2023, Bishop already had seen 207 percent of an entire water year’s average rainfall (San Francisco Chronicle). See the map below for regional variation.

The Precipitation map during winter rainfall in California
Oregon State University
The average percentage of winter rainfall in California.

This level of precipitation is good news to many of California’s rare, threatened, and endangered plant and wildlife species. 

Due to the increased rainfall so far in the 2022-2023 winter, the timing of focused surveys for many protected species can now be scheduled with reasonable confidence of credible results. ECORP biologists and botanists are ready to take advantage of the wet winter and spring – conducting surveys, assessments, reports, and providing other environmental needs.

Below are examples of a few highlighted species groups. Please see the table listing a number of surveys that may be required for projects and their mandatory start dates.

Southern Tarplant blooming after the winter rainfall
Southern Tarplant, CNPS 1B.1

Rare Plants

The California Department of Fish and Wildlife (CDFW) website states that California hosts approximately 6,500 species, subspecies, and varieties of plants that occur naturally here. Of these species, over 200 are formally listed as rare, threatened, or endangered by state law, and an additional 2,000 are of conservation concern.

California’s native rare plants reproduce in direct response to rainfall. Annuals, plants that complete a lifecycle during a single year, germinate from seed and begin to grow following winter and spring rains, flower, produce seeds, and then die. When rainfall is insufficient, the seeds may not germinate, in which case the plants cannot be detected. Perennials, plants which live for three or more years, tend to be more conspicuous than annuals during drought conditions, but not always. Many-stemmed dudleya, a rare perennial that exists as an underground corm during drier months, sends up leaves and blossoms in response to winter rains. But if the rains do not come, the plant may remain underground, until conditions are better. Mariposa lilies, many of which are listed at the federal, state, or local level as rare, threatened, or endangered, likewise can “wait out” periods of drought.

Federal, state, and local guidelines for rare plant surveys note that surveys conducted during a drought year may need to be repeated during a year when conditions are better. It looks like this is one of those years.

Vernal Pool Fairy Shrimp collected after the winter rains
Vernal Pool Fairy Shrimp

Vernal Pool Species

Vernal pools provide habitat, not only for a variety of rare plants, but also for threatened or endangered tadpole shrimp and five species of fairy shrimp. Without adequate rainfall, vernal pools do not fill up or do not retain water long enough to support the hatching and growth of tadpole and fairy shrimp. Wet season surveys often cannot be conducted or may not yield satisfactory results under drought conditions. Wet season surveys for these species have already started this season and, so far, are producing good data.


Amphibians require water, especially for the beginning stages of their lifecycles, and many live out their adult lives within a very small area near their breeding pools. Although they can live on dry land or hiding under rocks, logs, or leaf litter, precipitation is needed to create the pools, ponds, and streams for breeding and for larval stages to grow and develop.

Several of California’s amphibians are threatened or endangered, and specifically timed surveys are often required for projects where suitable aquatic habitats are present. These species include California red-legged frog and arroyo toad, among others. The California tiger salamander (pictured to the right) is also dependent on breeding conditions being suitable in these ephemeral pools. Several other species, such as the western spadefoot, are similarly constrained by the variances of annual rainfall.

California Tiger Salamander discovered after the winter rains
California Tiger Salamander
Desert Tortoise in its natural habitat
Desert Tortoise

Desert Species

Even desert species are not immune from the effects of drought. Desert tortoises are herbivores and respond to seasonal rains, in part because that is what their food supply does. When rainfall is short, food plants are less abundant, and tortoises spend less time outside their burrows. In fact, the U.S. Fish and Wildlife Service (USFWS) equation used to estimate the number of tortoises within a given survey area factors in those that were underground and not detected during a survey based partially on rainfall amounts from the preceding winter. The number of underground tortoises is, predictably, higher when rainfall has been low. 

Many of the desert’s threatened or endangered small mammals for which pre-project surveys can be required remain active and detectable during periods of drought; however, this not always the case. One exception is the state-listed Mohave ground squirrel, which lives in the desert and relies on seasonal rainfall for the growth of its food plants. The Mohave ground squirrel spends the hotter months and early winter aestivating in an underground burrow and, if winter and spring rains are insufficient, it forgoes reproductive efforts and quickly returns to aestivation. For this reason, required surveys for Mohave ground squirrels are not recommended during drought years.

Riparian Birds

Riparian habitat is associated with the margins of rivers, lakes, and streams or is found where water is near the ground in drainage features. According to a report published during the 1980s by Thomas Oberbauer, a well-respected ecologist in San Diego County, the statewide riparian habitat decline since pre-European settlement was estimated at greater than 80 percent. Although progress has been made in riparian conservation since that time, declines in the extent of intact riparian habitat have continued as development has increased.

The least Bell’s vireo, southwestern willow flycatcher, and western yellow-billed cuckoo are federal and stated listed threatened and/or endangered bird species that are all dependent on riparian habitat. The extent and health of riparian habitats and, by proxy, these species, can be affected by drought through a reduction in insect prey and habitat loss when the water sources supporting their nesting habitat declines. Although the trees and shrubs that have died from drought are not replaced in just one year, this year’s above-average rainfall should help recover riparian habitat suffering from the drought. Surveys for these species are required for projects where suitable habitat is present

Bells Vireo in a riparian habitat after the winter rains
Bells Vireo

Other Nesting Birds

The Migratory Bird Treaty Act and California state law protect all migratory birds and make it illegal to pursue, hunt, take, capture, kill, or sell some 800 bird species in the United States, including destroying their active nests. Although most birds continue to nest regardless of the amount of winter rainfall, the length of the nesting season and reproductive effort (number of young in a brood and number of broods raised) are often increased during years of abundance brought on by abundant rain. Increased rain leads to increased plant growth, which provides more food for insects that serve as a food source to raise hungry chicks. Nesting bird surveys are usually done in the spring and summer, when most birds are breeding, and are required a few days in advance of habitat disturbance by a project. Considering the especially rainy winter, nesting activity in 2023 is likely to be prolific.

Burrowing owl chicks at a nest after the winter rains
Burrowing owl chicks at a nest

Burrowing Owls

Burrowing owls are small fossorial owls that favor open habitats in towns, rural areas, agricultural fields, and elsewhere. They occupy burrows year-round but tend to move around and change burrows more frequently or even migrate away from nesting areas during the fall and winter months. When the breeding season arrives in February and March, the owls require burrows for nesting and a source of insect and small mammal prey. Owls depend on the abundance of prey, which is related to seasonal rainfall, among other factors, and when prey is lacking, nesting efforts tend to be less successful, if at all. 

Burrowing owls are protected under the Migratory Bird Treaty Act and are protected by special requirements by the CDFW. Projects occurring in suitable habitat within the species’ range likely require a habitat assessment and pre-project surveys, especially during the spring and summer. The current survey protocol for these owls requires that one of the four breeding season survey visits be conducted between February 15 and April 15, so the time to mobilize is now.

Surveys timeline
Burrowing owl chicks at a nest

For more information or to schedule surveys, please contact Kristen Wasz in the Redlands Office at (909) 307-0046, or Peter Balfour in the Rocklin Office at (916) 782-9100.

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