Raven nests in San Francisco
This page attempts to attach a number to the question: how many Common Raven nesting pairs are there in San Francisco? The estimate is built on observational data collected from 2010-2017, and tries to determine a territorial size of a San Francisco Common Raven pair. And from that create estimates of how many ravens nest inside the city of San Franciso.
There seem to be a couple parts of the city that don't see many ravens (Potrero Hill and northern part of the city from downtown to the Marina). Potentially this is because of competition from American Crows (there is a large crow roost in the financial district, and they can often be found in numbers at Fort Mason). This could also just be that I have not visited these areas enough.
Another factor of density could be substrate. Common Ravens can be found nesting on a variety of human structures, but only on a few types of trees (Montery Pine mostly, Eucalyptus and some other odd trees). These types of trees can be found all over the city, but not all may be suitable for a variety of reasons (exposure, tree size, tree shape, and other competitive species).
I've expanded the search from year to year. Each year, I've concentrated on different parts of the city trying to go to new places, but also revisit as many previous nests as possible.
One question I had was do ravens nest near where juvenile flocks congregate: The west end of Golden Gate Park, Fort Funston, Visitacion Valley, Sutro Tower, and Hunter's Point. Nests are found close to all of these spots.
|Trees||Mostly Monterey Cypress & Monterey Pine, a few Eucalyptus, and a Redwood.|
|Buildings||Commerical buildings, schools, Catholic churches (so far), etc.|
|Structures||Some examples: a ship, a crane, large signs, etc|
|Unknown||Probably nesting pair but actual nest not found.|
Tools and Methods
- Data stored in a Google Fusion Tables
- Original data was from a Google Map, mostly compiled by myself.
- Map and data on the page were built using a Google Fusion Table. The data is located at this table.
- The script first looks at each nest area, and records the distances to all the other nests
- It takes the top 3 closest nests for each nest, and averages those 3 distances, and creates an array of these averages for all nests. Trying to capture the variability in distances between nests.
- Distance are calculated with the Haversine formula, with a script from this site: Moveable Type
- The script then uses these values to find a median, the shortest, and longest distances, and the average distance between nests.
- San Francisco is routinely described as 49 square miles. So we how many packed circles might fit based on these radii for median, shortest, and longest distance.
- These are a compilation of nests over time. The actual nests change over the years, pairs don't always end up returning to that exact area.
- Distances are not exact because locations are slightly obfuscated, and
- they are also not exact because nesting areas often include more than one nest that can be 10s of meters apart
- Some nesting locations have been removed from the calculations, because they were either
- too far away (where there is likely to be nests in between), or
- nests that were too close and never occupied concurrently by separate pairs. The assumption would be that they are "owned" by nearby nesting pairs.
- How long might one pair occupy the same spot? (the longest known time I've observed a species in the same nesting area is --as of 2017-- 7 years. There's another nest which I think may have been there more like 10 years. This is nothing I can prove, since potentially an original pair might have been replaced, and I would have no way to know for certain.
- What happens to a nesting site after a pair leaves/dies?
- How many new nests are created per year.