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South shore Cachuma 5 August 2020


Mark Holmgren
 

I tried to cover the major muddy/grassy shores on the S side of the lake yesterday thinking I might find shorebirds.  Last Saturday I walked the trail from Harvey's Cove westward and on Monday I took a boat out on the lake.

 

https://ebird.org/checklist/S72136847  DeVaul Cyn shore

https://ebird.org/checklist/S72140092  Jackrabbit Flats

https://ebird.org/checklist/S72149214  Storke Flats

 

Migrants: Yesterday, juv Snowy Egret, Wilson's Snipe, and a very few Least Sandpipers.  From Saturday's hike I watched 5 dowitchers flying over the lake. An adult Peregrine was in Cachuma Bay on Monday. The Cachuma Naturalistas, Rosey and Kristin, reported 4 Eared Grebes on a couple of trips more than a week ago.

 

Breeding: As the naturalists have documented very well, the large grebes are pumping out young.  I would love to know exactly where they place their nests.  (They nest on open mounds on Santa Margarita Lake in SLO County. See beautiful video by Petra and Jack Clayton here   https://ebird.org/checklist/S72149935)  The 2 juvenile Spotted Sandpipers raised on Storke Flat may be the two juvs now at Jackrabbit Flat.  We could get another brood of Spotted Sandpipers from the breeding pair at Storke.  Before migration got going there were Spotted Sandpipers at a couple other spots on the lake, but breeding was only confirmed at Storke Flat.  37 Killdeer yesterday suggests breeding success, but having just recently practiced distinguishing juvenile from adult Killdeer, I can't speak to the number of juvs involved.   72 Canada Geese include a lot of juveniles, the 24 Mallards counted held a number of juvs.  The only other duck species seen this summer was one pair of adult Gadwall.  I saw 25 total American Coots, but far fewer juveniles than expected. Very few coots were in either Cachuma Bay or Santa Cruz Bay on Monday.  Both on the Monday boat trip and yesterday along the shore I counted only 5 Pied-billed Grebes.  I saw only 1 advanced juvenile among them.  

 

In short, Cachuma underperforms for a lot of insectivorous land birds, diving ducks and puddle ducks, eagles and hawks.  It is not very attractive to shorebirds that must be passing over. Though many Western Grebes and fewer Clark's Grebes are raising young right now, counting these birds, muchless comparing recent to older data, is daunting.  My sense is that there are declines since the early 2000s in, especially, wintering numbers of W and C Grebes, but also in breeding numbers.  

 

I don't know whether the Cachuma trends are seen at other inland reservoirs.  It would be helpful to have the thoughts of fishermen who might evaluate the changes in fish stocking at the lake. How has the effort to recover steelhead affected the resource base in the lake?  Is there a role that pest control agents have played with regard to insects at the lake?  Were the high numbers of birds in the 1980s and 90s a function of fish stocking? And are we seeing a return to a more typical level of support that might have been normal in the late 1950s and 60s?  Is the current situation a product of prolonged drought recovery?  The situation at Cachuma begs study.  


Thanks to Rosey and Kristin for their great interest and efforts to document bird events at the lake. 

 

Mark Holmgren

San Marcos Pass