Re: Banded warblers at Tilden Regional Park
Thu, 17 May 2001 22:10:29 -0700
From: Larry Tunstall
Hi EBBirders,
I am forwarding the following message from Bill Gilbert to the list. [The note he mentions at the beginning was actually sent to Erik, not posted to the list.]
Larry Tunstall
To: East Bay Birders� Circle
From: Bill Gilbert
Date: 17 May, 2001
Subject: Wilson�s Warbler sighting, correction on biographical info, and info on WIWA and OCWA nest predation.Alan Kaplan handed me a note, appearing in the East Bay Birders Circle Digest, related to my work with Wilson�s Warblers at Tilden Regional Park. In response, initially let me correct info provided by Larry Tunstall. Although I received my doctorate from University of California, Davis, I have not been professionally associated with University of California, Berkeley. My studies of Wilson�s Warblers (WIWA) and Orange-crowned Warblers (OCWA) are a personal project. I co-authored the BNA [Birds of North America] account for the OCWA, and also co-authored the WIWA account, which appeared in early 2000. I do not have a website, but thanks for the suggestion.
The bird sighted by Erik Stromberg was a female (female caps of west coast WIWAs are comparable to males, making field separation difficult). I color banded the bird on 4 May 2001 (F&WS aluminum band left leg, red/black right leg). Erik likely sighted it along the Lower Packrat Trail in the Tilden Nature Area, close to its south end (near parking lot). There the bird and mate (orange/black left leg, aluminum right leg) have a nest which hopefully will fledge young about 20 May. I say hopefully because WIWA fledging is all too infrequent at Tilden. This brings up the topic of nest predation at Tilden, which may interest some digest subscribers.
Of nine WIWA nests I have located so far this spring, all were subsequently depredated and/or were likely replacement nests following depredation of initial nests. One pair now is on its fourth nesting effort. Considering that earliest nest building does not start until between week one and two of April, this failure rate is impressive. The nest mentioned above has been only one of two to make it to the nestling stage, and the other was lost early in the nestling stage.
Rates of nest success at Tilden are typically low for both WIWAs and OCWAs, but lower for WIWAs (16% for years 1995 to 1998 for WIWAs versus 38% for OCWAs in early 1990s). However, nest success for WIWAs in 1999 was 58% (see below). The pattern of predation differs for WIWAs and OCWAs. WIWAs, which always nest in low shrubs at Tilden, typically have an egg loss rate of over half (56%), but also a high loss rate for young (67% of eggs hatched). About 85% of these losses is attributable to nest predation. OCWAs, however, which usually nest on the ground at Tilden, have an egg loss rate of only 15%, but a 56% nestling loss rate. Most losses are attributable to nest predation. In other words, WIWAs get hit hard in both the egg and nestling stages most years, while OCWAs usually get hit hard just in the nestling stage. I surmise that a majority of nest predation in the egg stage is from jays (visual predators), while predation in the nestling stage may be more equally distributed between jays and nocturnal mammals (e.g., raccoons, scent predators). Other predators, such as garter snakes, also take eggs and young, but their contribution to nest mortality probably is small.
As I mentioned, 1999 was exceptional for WIWAs, with a 58% nest success rate (I have no data for OCWAs in 1999). Loss rate for eggs laid was just 23%, and loss rate for nestlings was just 36%. All nest failure in 1999 was attributable to predation. When I first realized that nest success in 1999 was uncharacteristically high, I began (on 9 May) to take a crude jay survey, counting the number (mostly Steller�s) seen or heard within about 100 meters during my rounds. From 9 May through the summer of 1999 I counted an average of about one jay per hour. This year (I took no data in 2000) I have counted about 5 to 10 jays per hour during my rounds. Since the disparity in numbers between 1999 and 2001 has held up even through the overlap survey period (9 to 17 May), I don�t anticipate that the difference between 1999 and 2001 will prove to result from a timing difference in surveys. However, I will survey jays through this summer to test this.
If differences in jay populations between 1999 and 2001 prove significant (which is anticipated), this will suggest that jays may be a important source of WIWA nest predation at Tilden. It also might suggest that excessive numbers of (corn chip- and hotdog-fed) jays at Tilden may be a source of excessive WIWA nest predation at Tilden. Why Jay populations at Tilden apparently were low in 1999 is unknown, but higher numbers appear to be the norm.
As a final note on nest predation, there is concern among songbird field researchers that their own activity can affect nest predation rates. In addition to potentially promoting songbird mortality, such effects can skew one�s data. My concern for this potential effect was heightened in hearing that (before inspection methodologies were altered) jays at Point Reyes Bird Observatory had learned to get breakfast by following researchers on their rounds of nest checks. I since have tested the possibility of induced jay nest predation by conducting exaggerated, mock examinations of defunct nests when jays were present in trees above. Invariably, the jays simply moved on; they apparently were not interested to my activities. Additionally, I never have observed jays converge on a nest containing eggs after I have examined it, even after I have moved a distance away. I have concluded that random, abbreviated nest exams likely do not attract jay predation, at least at Tilden Park.
Nocturnal mammalian nest predation may be a different matter. I suspect mammals such as raccoons and skunks sometimes follow human scent trails and subsequently smell young at visited nests. The danger may be more severe when a trail leads directly to a nest site, and back out (rather than passing along side the nest site). For these reasons I do not visit nests after eggs hatch, unless they can quickly be observed while passing on a main park trail.
I hope this information is of interest. There is some information that East Bay Birder�s Circle participants might provide for me. I am interested in knowing where other East Bay breeding populations of WIWAs may be located (in addition to the Wildcat Creek watershed). Museum specimens indicate that WIWAs once bred commonly along creeks to the southwest of the Berkeley-Oakland ridgeline. With channelization of the creeks, however, I suspect that few if any of these populations persist. If anyone knows of any, can you please let me know (a Breeding Bird Atlas map might be helpful in this regard, but I have been unable to obtain one).