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Highlights of the Los Angeles Raptor Study 2022

CATEGORY: Preservation · Urban Wild |
On Jan 20, 2023

Background
Launched as the “Griffith Park Raptor Survey” in 2017, we renamed our effort the “Los Angeles Raptor Study” in 2021 to reflect the larger current study area now covering most of Los Angeles exclusive of the north and west San Fernando Valley, South Los Angeles, and the Harbor area. By documenting and tracking raptor nests across Los Angeles, we hope to understand how ecological dynamics change from year to year in the natural and built areas of Los Angeles, in particular how human activity is impacting wildlife here. While a handful of Los Angeles area raptor nesting sites had been documented in the past, the data contained in our annual summary reports represent the first comprehensive dataset of an entire raptor community in the urban core of Los Angeles.

In 2022, we continued monitoring within our 2021 study area (including Sepulveda Basin, Baldwin Hills, and Glendale), and again increased the number of monitored territories for the sixth year of the Griffith Park Nesting Raptor Survey.

As in prior years, we were able to confirm as active many territories by the presence of recently-fledged young and recently-used nests (particularly Cooper’s Hawks), using clues learned while more closely observing known nests.

We again confirmed no active Western Screech-owl or Barn Owl nests, and while we confirmed a single Peregrine Falcon nest, we confirmed no nests of American Kestrel in 2022. As in 2021, these numbers (at least the diurnal species) more closely reflect actual numbers of active nests in the study than those in surveys prior to 2020 when our efforts reflected lower numbers.

Location
The Study Area originally centered on Griffith Park, was expanded in 2020 to include additional portions of the San Fernando Valley and coastal plain that were not covered in prior years. As of 2022, the Study Area extends to the 405 Freeway/Sepulveda Pass in the west (with an extension to include Sepulveda Basin), Sherman Way/
Vanowen Blvd. in the north, Slauson Ave. in the south, and East Los Angeles in the east. A handful of raptor nests just outside this area were monitored by volunteers (e.g., Encino, Pasadena, Calabasas), but we did not specifically search for nests in these areas.

Nest Use, Re-use and Success
By 2022, our rate of finding new nests within the study area has slowed from 2020 and 2021, such that we added new nests in 2022 as follows: Red-tailed Hawk (6), Red-shouldered Hawk (4), Cooper’s Hawk (26), and Great Horned Owl (7). These numbers don’t include new (for 2022) territories where nests were suspected, or known territories where we had pairs (or even juveniles) in the past but only discovered physical nests in 2022. Looking at territories (some of which had nests, but not all), we located additional territories for 27 new Cooper’s Hawk pairs, 13 for Great Horned Owl, 9 for Red-tailed Hawk, and 5 for Red-shouldered Hawk (not including species switches, such as an owl taking over a hawk nest). This may be contrasted with 2021, when we found 35 new Red-tail nests alone.

This year (2022), we re-analyzed our data from all six years of territory re-use and success for the three focal hawk species and for Great Horned Owl in order to present these results in a more consistent, quantitative manner. Importantly, our nest-searching effort increased greatly starting in 2020, so the years 2017-19 may be thought of as preliminary compared to the years 2020-22. In particular, we searched for and located few urban Cooper’s Hawk nests in the San Fernando Valley or mid-City area prior to 2020, before we learned some of the tricks to finding them there.

We intend to analyze nest structure re-use in the future… as the multiple years of the study are presenting unexpected analytical challenges; for example, determining how to best consider a nest structure re-used, particularly if a pair skipped using it for  a year or more, then returned to use it, re-built nests, and those built a few feet away in the same tree.

Looking at each species, we found that Red-tailed Hawks maintained fewer active territories in 2022 than the year prior, and 2022 saw the fewest fledged nests over the past three years, with several dozen pairs either absent from known territories, or abandoning nests midway through the season.

Cooper’s Hawks also maintained fewer territories in 2022 than in the two prior years, and fledged fewer young than in 2021, but proportionately more of these 2022 nests fledged young than in prior years.

For Red-shouldered Hawk, 2022 saw more active territories, but roughly the same number of active nests fledged young, resulting in proportionately lower fledging rate per nest.

For Great Horned Owl, 29 active territories were monitored (the most since the start of the project), and as in prior years, many (28) were deemed to have fledged. However, most owl territories were identified­ by the presence of young; we did not attempt to search for owls during the study.

Nest Productivity – Factoring in Precipitation
Across all focal species, nest productivity peaked in 2020, but the differences between years was slight, and likely not statistically significant. Cooper’s Hawk consistently fledged the highest mean number of chicks per successful nest, with an average of 2.43 young from 2018-2022. Red-tailed Hawk had the next-highest rate (1.92), followed by Great Horned Owl (1.89), and Red-shouldered Hawk (1.6).

There are many ways to measure nest productivity – one is the proportion of single-chick nests (nests where the maximum number of chicks was believed to be just one, versus all other nests where chicks were produced), which could indicate a shortage of food that year. Assuming that Red-tailed Hawks would be most sensitive to change in precipitation (since they take more native prey species from wildland areas than, say, Cooper’s Hawk, which are well-distributed in urban areas), we examined the relationship between precipitation the prior year, and the proportion of 1-chick nests in Red-tailed Hawk.

Evidently, the years following the driest winters (i.e., 2018, 2021 and 2022) all saw >20% of Red-tailed Hawk nests with single chicks (no such pattern was observed with Cooper’s Hawks), which conforms to recent findings looking at precipitation and Northern Goshawk (Accipiter gentilis) nests.

Failed Nests
As in past years, it may be instructive to review why the few failed nests did so. We summarize all Red-tailed Hawk nests believed to have failed, that were never initiated (within known territories), and that were apparently abandoned mid-season. Unlike last year, when three Red-tailed Hawk pairs had their nest trees removed, only one Red-tailed Hawk nest tree had been trimmed prior to the start of the 2022 season.

We were also notified of the illegal trimming and removal of a long-standing Red-tailed Hawk nest in the Mt. Washington area shortly after fledging…and of another trimming incident involving an active Cooper’s Hawk nest in Los Feliz…

Raptor Mortality, Rescue, and Rehabilitation
One of the leading causes of death for raptors in urban areas are collisions.

Interested in volunteering for the 2023 LA Raptor Study?
We will need community scientists to help monitor nests.
2022 LA Raptor Study Results

~Original report by Dan Cooper, Courtney McCammon, and Nurit Katz

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