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Raptor Study Final Report

CATEGORY: Featured · In the News · Urban Wild |
On Jan 19, 2024

Of the many scientific surveys and studies Friends of Griffith Park (FoGP) has sponsored over the last decade-plus, the Los Angeles Raptor Study is the standout in both the scale of data collected and the duration of study period. Many thousands of data-points have been collected by trained nest monitor volunteers and amassed into a master database over the seven years of the Study.


The 2023 Final Report was recently presented by ecologists that have been driving this important study which represents the first comprehensive dataset of an entire breeding raptor community within the urban core of Los Angeles. The ecological importance of raptors such as apex predators cannot be overstated, particularly in our complex urban setting with human influences that alter ecological balances. The Report aspires to gain a better understanding of the dynamics at play from year-to-year with our local raptors.

Residential streets and backyards are highly productive sectors for hawks, particularly Cooper´s hawks. The other two local hawk species, Red-tailed hawks and Red-shouldered hawks, are better associated with native prey of wild areas, including parks and open space areas. Great horned owls, rounding out our top four raptor species, are widely distributed. Sometimes their nests are not found at all and rather, the fledged owlets are the telltale confirmation of a nesting pair´s location.

How was the ‘community science’ project organized in 2023?

– Applications to participate as a nest monitor are taken year-round. This form is on FoGP´s Raptor Study webpage, and confirmation of acceptance is sent near the beginning of the new season, with an initial training which takes place in February.

~ A virtual training session, followed by a field training workshop, is mandatory. Also, an optional second virtual refresher and field session focused on late-season nester, Cooper´s hawk, were both well-attended this year. Our raptor volunteer force totaled well over 200 participates for 2023!

~ The Raptor Study leads (Dan Cooper, Courtney McCammon, Nurit Katz, and Gerry Hans) conducted opportunistic nest surveys to document the status of known and suspected raptor nests, and also identify new territories. These surveys continued through the spring and summer, with experienced volunteers contributing, as well.

~ All volunteers were assigned an initial nest to monitor. Volunteers received additional assignments when Cooper´s hawk territories became active. GPS coordinates were provided, and Study leads assisted new volunteers at their assigned nest locations, as needed.

~ Final results were presented via virtual Zoom in September, with a celebratory volunteer dinner in Griffith Park held a few days later. The final presentation is available on YouTube and linked on the LA Raptor Study website.

Study Result Highlights in 2023

A total of 549 raptor territories were rechecked or newly discovered during the season, not all of them active. The breakdown included: 222 Cooper´s hawk territories, 184 Red-tailed hawk territories, 55 Red-shouldered hawk territories, 84 Great horned owl territories, as well as 4 American Kestrel territories and 2 Peregrine falcon territories. Of the four top species above, all numbers represented an increase versus 2022. However, the increase in Cooper´s hawk territories did not result in more active nests for 2023. Surprisingly, the number doubled for great horned owls. This species most often moves nesting sites year-to-year, and the Study Team does not attempt to specifically search for owls or owl nests, unlike the more thorough efforts in accounting for hawks species.

Fledge Rates

Red-tailed hawks: While maintaining more active territories (138), the fledging success rate was 63%, identical to 2022. It is notable that the last two years produced the lowest fledging rate versus all other years of the study.
Cooper´s hawks: Although more territories were discovered, active territories (84) were down versus all of the last three years. However, the fledging success rate was slightly higher at 90%, versus 2022.
Red-shouldered hawks: The number of active territories (35) was significantly higher with just a minor increase in the rate of fledging success (57%), versus 2022.
Great horned owls: The active territories (42) yielded 79% fledging rate, the lowest of the last three years.

Territory Re-occupancy

This year again, Red-tailed hawks maintained a stellar territory re-occupancy rate, steady and in-line with rates calculated since 2018, at nearly 80%! The other two hawks´ re-occupancy rates have been more variable over the years of study, the lowest being for Cooper´s hawks, with roughly 50% territory re-occupancy in 2023. Only the hawk species were included in this analysis.

Nest Productivity

Across all four focal species, nest productivity rate, as measured in the mean number of chicks hatched from active nests, peaked in 2019. There is no definitive explanation, but the cumulative impacts of drought years could be in play. Over the seven years of the Study (2017-2023), Cooper´s hawks consistently fledged the highest mean number of chicks per nest with an average of 2.54 young. Red-tailed hawks had the next-highest rate (2.12), followed by Great horned owls (2.01), and Red-shouldered hawks (1.79).

Another way to measure nest productivity is the proportion of single-chick nests, which could indicate a shortage of food that year. Red-tailed hawks were chosen for this analysis, as they would likely be the most sensitive to change in precipitation. They take more native prey species from wildland areas than, for example, Cooper´s hawks, which are well-distributed in urban areas. The Study data was examined for a relationship between prior year precipitation figures against the proportion of 1-chick nests of Red-tailed hawks. Over the seven years, nesting seasons following the driest winters (2018, 2021 and 2022) all saw jumps in the proportion of Red-tailed hawk nests with single chicks. However, this interesting relationship did not hold after the extreme rainfall of 2022-23! The reason may possibly be attributed to the destruction of multiple nests from winds and flooding. More data collection over the years may reveal a better picture of this relationship.

Our Special Birds of Prey

Let´s not forget about our rarer raptor species, American Kestrels, Peregrine falcons, and Western Screech owls.

A pair of Peregrines slipped into Griffith Park to nest during its COVID closure in the spring of 2020. They´ve returned each year since, and produced four young both in 2022 and 2023! The nesting site is protected from disturbance with the help of Recreation and Parks staff. The Study team is on the verge of identifying more active Peregrine nest sites, especially near the top of high-rise buildings.

Kestrels and Screech owls, as cavity-nesters, are not only scarce, but difficult to find. Yet, a few were observed. In particular, a kestrel pair fledged two young in Griffith Park (Bette Davis Park).

A Difficult Year — Rescues, Mortalities, and Nest Disturbances

Great horned owlets were falling out of nests, left and right, more than we ever experienced in past years, due to 2023´s stormy weather. LA Animal Services SMART team was a godsend, as they re-nested or transported nestlings. Their skilled climbers scaled tall trees in some instances. Study Team member Nurit Katz transported quite a few hawks, both juveniles and adults, to the Ojai Raptor Center where a good relationship has been established. When recuperated, Nurit releases the birds near where they were rescued. City traffic and building window collisions were common causes of raptor injuries. If you observe an injured raptor you can text or call Nurit at (818) 384-9493.

Rodenticide continues to be a major threat to local raptors and Friends of Griffith Park continued to sponsor lab testing on raptors found dead in the Study area in 2023. Consistently, multiple rodenticides are detected in their system — likely the cause of death — if not conclusive. This year at least one juvenile owl perished needlessly from anticoagulant rodenticides as evidenced by lab results.

The 2023 Final Report includes a table that highlights the extensive number of nest disturbances. Tree trimming continues to be a major issue adversely affecting reproductive success. We advise keeping the CALTIP hotline number in your smartphone, (888) 334-2258. It´s an effective way of reporting violations to California Fish and Wildlife on a timely basis — if you catch disturbance in action, you can call this 24-hour hotline.

The enthusiasm and comradery of the entire Raptor Study family of participants thrills the FoGP board, as well as our membership. We want to thank our volunteers, our science team, and the public agencies that provided access and services. Finally, we want to thank all the residents living within the Study area for providing tips regarding raptor nests and active territories!­

~Gerry Hans, FoGP President

For more information on the L.A. Raptor Study, go here

Photo courtesy: Nurit Katz / Cooper´s hawk shedding its down.



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