By Bob Lewis

This year will be the 113^th year of Christmas Bird Counts in North America.  Golden Gate Audubon sponsors two counts in our region, centered in San Francisco and Oakland.  Although Christmas counts are great fun and an opportunity to meet other birders, see a variety of birds and get a good understanding of what birds are available in our area, they also have a more serious side.

The National Audubon Society has made the results of all counts available to everyone at www.audubon.org/bird/cbc/hr/index.html. This Citizen Science data is used by environmentalists and ornithologists to understand population changes, and consider actions that might help to minimize the loss of endangered species.

Recently I took a look at data from the Oakland Count to see what it might say.

For a variety of reasons, it was easy for me to look at the data from 1974 to 2011.  This is a 38-year period.  I divided it in half, and compared the years 1974-1993 with 1994-2011.  Over this period, the population of Alameda County increased over 40 percent.  Another development that potentially affected bird populations was the Oakland Hills fire in 1991, just before the beginning of the second period.

I averaged the count data over each period, and applied some statistical tests to the results.  Here’s what I found, for the top 20 increases and decreases:

Birds with increasing populations			Birds with decreasing populations
Species	% incr.	Avg(1)	Species	% decr.	Avg(2)
Common Raven	1258	146	White-winged Scoter	95	199
Red-shouldered Hawk	978	29	Bonaparte’s Gull	94	164
Pygmy Nuthatch	362	95	Northern Pintail	90	1599
American Crow	302	386	Red Knot	88	117
Common Merganser	276	87	Horned Lark	87	45
Hairy Woodpecker	222	23	Pine Siskin	84	891
Black Phoebe	177	186	Ruddy Turnstone	77	22
Townsend’s Warbler	171	125	California Quail	77	432
Greater Yellowlegs	158	60	Glaucous-winged Gull	76	1386
Black-necked Stilt	121	162	Loggerhead Shrike	75	26
Wood Duck	116	17	Brandt’s Cormorant	74	107
Cinnamon Teal	110	40	Red-throated Loon	72	44
Marsh Wren	99	10	American Pipit	68	196
Acorn Woodpecker	97	39	Canvasback	65	1356
Nuttall’s Woodpecker	85	88	European Starling	64	6154
Rock Pigeon	78	2124	Wilson’s Snipe	63	33
Gadwall	76	152	Wrentit	60	445
Bufflehead	65	2093	Brewer’s Blackbird	60	1805
Brown Creeper	55	74	Burrowing Owl	58	10
Turkey Vulture	37	183	Red-breasted Merganser	57	90

 

(1)   This is the average number of birds seen per year during the second period, 1994-2011.

(2)   This is the average number of birds seen per year during the first period, 1974-1993.

Keep in mind that statistical figures can be misleading.  The Red-shouldered Hawk increase of almost 1,000 percent reflected an increase of two dozen individuals – from about three during the first period to 29 during the second period. (That is still a significant increase for hawks, which require a lot of territory.) The Northern Pintail reduction of 90 percent, on the other hand, reflected a decline from an average of 1,599 to 153.

Percentages can mean different things, depending on whether you’re talking about increases or reductions: Going from 10 to 20 birds is a 100 percent increase, while going from 20 to 10 birds is a 50 percent reduction.

Each entry in the table provokes a question – why the change?  Ravens and Crows, two of the species with the greatest increase, are attracted to human refuse as scavengers.  Perhaps our increasing population is part of the cause.

Pygmy Nuthatches were uncommon in most of the count circle until after the Oakland fire, when roasted pine cones provided a food source.  They came and stayed.  Nuttall’s, Acorn and Hairy Woodpeckers have all increased.  Buffleheads are now one of the most common winter ducks on the bay, while Surf Scoters, which didn’t quite make the table, have declined 52 percent and scaup species by 38 percent.  More recent years show sharper declines.

California Quail and Wrentits have declined markedly, as has acreage of chaparral, their preferred habitat.  The decline in Northern Pintail mirrors the nationwide drop in this species, perhaps due to loss in breeding habitat.

One Christmas Count data set does not provide enough data for many conclusions, but coupled with data from many counts, stories of success and worry begin to emerge.  The data is available because we have volunteers in the field every year, adding to our knowledge.  It’s getting close to count time!  The signup forms will become available on the GGAS web page in the second half of October.

Won’t you join us?  Sign up as a feeder watcher if you’re not able to spend the day in the field – it only takes a little time to check your feeder to see what’s coming, and it’s fun to do.

Oakland’s CBC will be on Sunday December 16^th.  San Francisco’s will be held on Thursday December 27^th.

Sign up on the GGAS web site at https://goldengateaudubon.org/birding-resources/christmas-bird-counts/.