 Following winter rainfall, new foliage grew in abundance (Below), favoring nesting songbirds. 
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Overall, the primary cause of nest failure is predation, and the rate of predation is strongly
influenced by the quality of habitat. Factors like high herbacous cover or the number and
variety of plant species where nests can be placed can dramatically increase the number of
nests that fledge young. Our data from Palomarin, site of PRBO's long-term study of
terrestrial birds, have shown that more rainfall increases Song Sparrow nesting success,
apparently by providing more grass for concealment (see Observer 94, Fall 1992). The Vision fire removed most if not all of the vegetation that songbirds use to conceal and support nests. Prime nesting substrates such as sword ferns were left as charred rootballs. Most of the shrubs favored by birds, such as coyote bush, blackberry, and young willow, had been burned to the ground. Returning migrants would have little luck finding concealed sites in which to place nests. And because of their strong insistence on breeding in the same place year after year, we knew they would stay in virtually the same territories despite the new lack of vegetation. With our state-of-the-art song bird monitoring protocols, I was confident that we would document some of the lowest songbird productivity ever. |
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| But I was wrong! With the help of Point Reyes National Seashore and data from an ongoing study by the Kern River Research Center, and using an expanded PRBO mist- netting program, we found exactly the reverse.The number of young produced at Muddy Hollow, as indexed by our mist nets, was the highest of any site we monitored in West Marin County in 1996. Indices of productivity obtained at burned sites at Muddy Hollow and Coast Trail surpassed even the highly productive (and, of course, unburned) Pine Gulch on Bolinas Lagoon. Palomarin appeared to have the lowest such index in our comparison. |
Nests located within the Vision burn zone had a success rate of over 50%-- 15% more than in unburned and grazed riparian. |
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These results were backed up by the nest data. In the spring of 1996, over 250 nests of
open-cup nesting birds were found at six different riparian sites: Muddy Hollow and Coast
Trail (burned), Olema Creek and Bear Valley (unburned), and Kehoe and Abbotts Lagoon
(grazed). Nests located within the Vision burn zone had a success rate of over 50% (Figure 1) --15% more than in unburned and grazed riparian.
In real numbers of birds, this translates to an average of 50 more young per 100 nests in a
burned area. Even the census data showed that the number of individuals between 1995 and 1996 actually increased in burned areas, and we observed no real differences in numbers or species diversity between burned and unburned areas (Figure 2). In fact, riparian habitat that had been grazed measured the lowest in all of these measures of avian communities' vitality. Some of the species that apparently benefited from the burn, showing higher productivity in 1996 than their counterparts in unburned habitats, were Allen's Hummingbird, Black- headed Grosbeak, Common Yellowthroat, MacGillivray's Warbler, Wilson's Warbler, Orange-crowned Warbler, and Song Sparrow. Only Wrentit and Swainson's Thrush appear to have been negatively affected by the changes. One of the mechanisms for these improved conditions, contrary to our forecasts in the wake of the fire, was more herbaceous cover for hiding nests. Winter rains produced diverse and vigorous growth in the burn zone of native plants such as hedge nettle and cow parsnip, where birds could place their nests. The temporary elimination of numerous nest predators such as snakes and woodrats also contributed to birds' reproductive success. Our data show that even nests high in the canopy with relatively little cover had excellent nest success. |
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"Stirring the Pot"These results are remarkably similar to what we have observed along the Sacramento River following catastrophic floods in 1994. Bird productivity increased dramatically after floods scoured out the dominant weedy vegetation. As with the fire at Point Reyes, the flooding process in the Central Valley stimulated new and more diverse vegetation, especially in the understory. This in turn increased productivity of most bird species by providing better nest cover, more diverse nesting substrates, and lowered predator density.As results of PRBO's various long-term studies bear fruit, we are seeing, across ecosystems, that annual fluctuations (some very pronounced) are the norm. Even in so- called stable and mature habitats, we observe extreme variations in density, number of species, number of nests fledging young, and even species composition. Events such as fire and flood, though catastrophic in appearance, are normal and natural processes. If these natural processes are suppressed and habitats remain unaffected by fluctuations - or are subject to the same pressure year after year (e.g., grazing) - then the ecological community stagnates and is dominated by a few species. With a regular "stirring of the pot," most temperate-zone organisms are likely to respond favorably and biodiversity increases. |
Events such as fire and flood, though catastrophic in appearance, are normal and natural processes. . With a regular "stirring of the pot," most temperate-zone organisms are likely to respond favorably and biodiversity increases. |
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The constant "unbalance of nature," in fact, is likely the key to maintaining healthy
ecosystems. Devising management strategies that work with natural processes, and open
up ecosystems to fluctuations that renew them, may hold a promise for cost-effective
maintenance of healthy and diverse populations of birds. Acknowledgments: To be able to respond quickly to unpredictable events requires a timely and generous response from our many supporters. We would like to thank the following for their rapid response to our appeals: Point Reyes National Seashore Association, The San Francisco Foundation, Esprit, Ms. Doris Leonard, Conservation Associates, Mrs. Dorothy Hunt, and the many generous members of Point Reyes Bird Observatory. We would also like to thank the Kern River Research Station and staff of the Point Reyes National Seashore for the use of their data and for assisting in all aspects of this study. Prbo staff biologists and field biologists who assisted in this project include Stacy Small, Tom Gardali, Grant Ballard, and Anne Kiener. |
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