From south-bay-birds-bounces+south-bay-birds-archive=[[email protected]] Fri Jan 24 10:34:10 2003 Received: from www.plaidworks.com (localhost [127.0.0.1]) by plaidworks.com (8.12.6/8.12.2) with ESMTP id h0OIVh4p000930 for <[[email protected]]>; Fri, 24 Jan 2003 10:31:43 -0800 (PST) Received: from merlin.arc.nasa.gov (merlin.arc.nasa.gov [128.102.219.21]) by plaidworks.com (8.12.6/8.12.2) with ESMTP id h0OIV7SE000888 for <[[email protected]]>; Fri, 24 Jan 2003 10:31:07 -0800 (PST) Received: from merlin.ARC.NASA.GOV by merlin.ARC.NASA.GOV (PMDF V6.2 #30665) id <[[email protected]]> for [[email protected]]; Fri, 24 Jan 2003 10:31:03 -0800 (PST) Date: Fri, 24 Jan 2003 10:31:03 -0800 (PST) From: [[email protected]] To: [[email protected]], [[email protected]] Message-id: <[[email protected]]> MIME-version: 1.0 Content-type: TEXT/PLAIN; CHARSET=us-ascii cc: [[email protected]] Subject: [SBB] Nuttall's Woodpecker Range Expansion (more) X-BeenThere: [[email protected]] X-Mailman-Version: 2.1 Precedence: list List-Id: South Bay Birding List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: south-bay-birds-bounces+south-bay-birds-archive=[[email protected]] Errors-To: south-bay-birds-bounces+south-bay-birds-archive=[[email protected]] Folks: I have some comments on the interesting thread on Nuttall's Woodpecker range expansion on [pen-bird] which I will also post to [SBB] as it is of interest to both groups. This species was not found in northwestern Santa Clara County until the early 1970s, when a range expansion there and into southeastern San Mateo County was noted, much as numerous observers have related as occurring currently for other areas of San Mateo County, San Francisco County, and Santa Cruz County on the recent thread. Christmas Bird Count data and the Palo Alto Summer Bird Count (SBC) data provide an excellent source of data to examine this species population trends. Looking at only birds/party-hour, here are some useful statistics where I've looked at data that after a log-normal transformation: COUNT YEARS Pop. Change p San Jose CBC 1956-2000 +2.3% <0.0000 Palo Alto CBC 1968-2000 +17.9% <0.0000 Palo Alto SBC 1981-2001 +6.4% <0.0000 Mt. Hamilton CBC 1978-2000 -0.5% * *not significant The Palo Alto CBC data extends back to 1959, but no birds were recorded before 1968. The trends shown here are extraordinarily strong for biological systems where a p value of 0.05 is considered significant (details of this analysis of CBC data can be found on the SCVAS web site ). Although I think that initial disperants or invaders are typically found in fall and winter, I believe this slight bias does not affect the trend data from CBC counts. I have overplotted the Palo Alto CBC and SBC counts and the trend data are indistinguishable. It is interesting to note that the San Jose CBC circle, which has always had this species, also shows an increase, essentially a doubling of birds in 50 years. This suggests that within its historical range the bird is doing very well and can act as a source population for dispersal. Although forest maturation is a relatively fast process, perhaps measured in half centuries, the Palo Alto CBC circle population went from roughly 0 birds in 1970 to 70-90 birds (now possibly stable) by 1990. This, like in many species invasions, seems relatively fast. There is no indication that Nuttall's has expanded through niche competition with Downy or Hairy Woodpeckers. I've examined the correlation between Nuttall's and Downy, and Nuttall's and Hairy and there is no significant correlation (I would expect a negative correlation if Nuttall's were outcompeting Downy or Hairy). Interestingly, however, there has been at least one hybrid pair, that is Nuttall's x Downy. This is considered a rare occurrence for this species pair. A comparison of the atlas maps for the three congeners for Santa Clara County shows that Downy and Hairy appear to be more specific in their habitat needs than Nuttall's. Downy is most often found at locations on the valley floor and in riparian areas, while Hairy is found at higher elevations. Nuttall's, by contrast, appears to be distributed the most widely. One implication of these distributions is that Nuttall's is more of a generalist. Prior to the atlas years, with far too much time on my hands, I looked at all of the California CBC data to see if the peninsular invasion was repeated elswhere. At all borders of Nuttall's California distribution I looked for CBC pairs--one within the historical range and one outside. It was an interesting excercise that I never finished, but my tentative conclusions were: 1. Things are always more complicated than they first appear. 2. In the southeast of its range, Nuttall's is in conflict with Ladder-backed. There was no change there. 3. In most of its Central Valley range there were no changes. However, in all of these cases, Nuttall's seemed stopped by fairly severe altitudinal gradients. 4. On the central coast, only the San Francisco Peninsula was showing large changes. It is interesting to speculate on changes that have influenced this range expansion. Most of these, however, are difficult to test. Bill _______________________________________________ Do not post admin requests to the list. They will be ignored. south-bay-birds mailing list ([[email protected]]) Help/Unsubscribe/Update your Subscription: http://www.plaidworks.com/mailman/options/south-bay-birds/south-bay-birds-archive%40plaidworks.com This email sent to [[email protected]]