Picnics At The Beach

WET OR DRY FOOD?

   A laboratory study published in 2005 [1] attempted, through examination of California condor bones and feathers, to compare the types of food eaten by condors in historical versus modern times. At the time, I wrote a review of the paper, questioning both the methods and the conclusions. Because the paper is still being quoted as pertinent to current condor recovery efforts, I'm repeating my 2006 comments here, with some updated information.


Their Methodology

The researchers started from the hypothesis "that the restriction of the range of condors to the Pacific coast after the Pleistocene megafaunal extinction was largely controlled by the presence of a 'fall-back' food source, marine mammals," and that "the switch to terrestrial foods in historical condor populations may reflect the reduction in pinnipeds and whale populations due to commercial hunting." To test the hypothesis, they obtained feather samples from 12 "modern" condors (birds that died between 1993 and 2001) and 50 "historical" condors (museum specimens collected 1797 to 1965), and analyzed these for changes in carbon and nitrogen isotopes. They analyzed bone collagen from 10 Pleistocene condors, 10 historical birds (1904-1965), and 10 modern ones (1993-2001). From their samples, they felt they could tell differences between the relative amounts of land-based and ocean-based food that the condors had been eating.

   The researchers' basic technique seems a proper one. The problems are with sample size, type, and distribution. The researchers must have assumed that their sample of condor materials adequately covered all aspects of condor distribution and behavior that would have affected their food use. That assumption is incorrect. The range of "historical" specimens (1797 to 1965 for feathers, and 1904 to 1965 for bones) is much too broad to take in many aspects of condor distribution, and the available samples of feather and bone are not adequate to measure each of the known historical changes in distribution. For example, only one condor specimen is available from before 1805 (and that was collected in 1792, not 1797 as noted in the publication). It was collected near the coast. I think there is only one specimen in existence from Oregon that could have been in their sample, collected in 1835 in Clackamas County; by location alone, that condor could have been either a "coastal" or an "inland" bird. [The reported sampling of one or more condor specimens from the Columbia River between 1877 and 1885 (page 16710) is an error. There are only a few condor records from north of California after 1835, and none involved specimens in existence now.]

   From 1836 to 1890, there are approximately 35 condor specimens preserved. At least 24 of those were collected near the coast; of the other eleven, some are from unknown locations, and some were from far enough inland that they might or might not have had access to marine life. Between 1890 and 1900, perhaps a third to a half of the 60 existing specimens were from coastal counties. After 1900, there were few California condors left near the Pacific Ocean, and only a few of 85 condors now available as specimens would have had regular access to marine mammals.

   [This specimen information is from my compilation [2], the only attempt at a complete list ever done. The paper does not identify which of these specimens the researchers used in their study.]

 

Their Conclusions

   The authors concluded (page 16709) "that marine mammals were an important component of the diets of Pleistocene condors, and that historical condors ate terrestrial land mammals." Considered at face value, it seems safe to say that marine mammals were an important component of the diets of some Pleistocene condors (say, those living within 100 miles or so of the seacoast). However, the research does not support the conclusion that the diet of condors changed markedly from marine to terrestrial mammals from Pleistocene to Recent times. Neither does it show that the marine mammal food supply in any way controlled the post-Pleistocene distribution of condors, nor that the presumed switch from marine to terrestrial foods was a function of fewer marine mammals.
   As I noted above, their samples were not adequate to measure what they were trying to measure. For example, all the researchers' Pleistocene samples came from Rancho La Brea, in what is now the western portion of the Los Angeles metropolitan area. The sample results of four birds (bones) that showed strong marine food characteristics and six with strong terrestrial affinities seems logical. Rancho La Brea is only a few miles from the Pacific Ocean (the source of sea mammals), but in the Pleistocene it was also in the midst of an abundant supply of land mammals. Condor remains were found in the La Brea tar pits because the condors came to feed on land mammals that were trapped in the tar; the condors in turn became mired. Some likely came to the pits from great distances, and we have no way of knowing whether individual birds flew in from the coast (where marine mammals would have been available), or came from the interior (where land mammals would be their main food). The data only tell us that Pleistocene condors near the coast were feeding on both marine and terrestrial mammals.

   The extinction of much of the Pleistocene megafauna, followed by severe drought during the early Holocene,  almost certainly had a major effect on the distribution of condors and other scavengers. Probably, adequate food became very difficult to find in the increasingly arid interior Southwest. But if condors disappeared from much of their former habitat because of lack of food, why were they able to survive through the arid middle Holocene along the Pacific fringe of the continent? The researchers contended that it was only because they still had marine mammals to eat. That conclusion ignores the fact that terrestrial food supplies for condors in California did not "disappear" after the Pleistocene.  

   Even though areas west of the Sierra Nevada and Cascade Range felt the effects of the middle Holocene drought, the effects were moderated by proximity to the coast. “Climatically influenced changes in terrestrial environments along the coast during the Middle Holocene appear to have been less drastic than in the interior, particularly in central and northern California” [3]. The California coast, San Francisco Bay, and the Sacramento-San Joaquin delta have been considered “climatically complacent,” implying they were likely to have been less affected by changing climatic conditions than more interior locations [4]. It is probably not coincidental that much of the evidence of human occupation of California between 8,500 and 5,500 B.P. has been found in coastal and near-coastal locations [5].

   Artiodactyls (deer, elk, pronghorn) appear to have become considerably less common in interior western California during the middle Holocene. However, their rapid increase following the establishment of a more temperate climate regime suggests that a significant population survived through the drought [6]. Along the central California coast, there may have been little or no reduction in abundance of black-tailed deer [7]. Even in the Pacific Northwest, both coastal and inland archaeological sites show evidence of local populations of deer, elk, pronghorn, sheep and bison throughout the Holocene [8]. Avian scavengers on the Pacific Coast might not have had the quantity or variety of terrestrial foods they had during the late Pleistocene, but every indication is that they had much more than did the condors of the interior Southwest.

   The earliest European explorers to reach the Pacific Coast in the late 1500s and early 1600s reported seeing large numbers of big game; many similar reports followed in the 1700s. Declines in artiodactyl populations may have occurred in the middle Holocene due to climate, but they were clearly back in abundance by the 17th century.

 

Condors did not survive past the Pleistocene in California because they had marine mammals as a "fall-back" food supply. They survived because there was much more food (of all kinds) available in California than in the rest of the Southwest. Those that lived within a moderate distance of the coast were "lucky enough" to have both marine mammals and terrestrial food species. The presumed "switch" from marine mammals to terrestrial mammals did not occur. Less marine food showed up in later samples because there were few condors living within foraging distance of the coastline.

 

 Notes

1. C. P. Chamberlain, J. R. Waldbauer, K. Fox-Dobbs, S. D. Newsome, P. L. Koch, D. R. Smith, M. E. Church, K. L. Sorenson, and R. Risebrough. 2005. Pleistocene to recent dietary shifts in California condors. Proceedings National Academy of Sciences 102(46):16707-16711.

2. Wilbur, S. R. 2012. Nine feet from tip to tip: the California condor through history. Volume 2. Gresham,, Oregon: Symbios Books.

3. Kennett, D. J., B. J. Culleton, J. P. Kennett, J. M. Erlandson, and K. G. Cannariato. 2007. Middle Holocene climate change and human population dispersal in western North America. Pages 531-557 in: D. G. Anderson, K. A. Maasch, and D. H. Sandweiss (editors), Climate change and cultural dynamics: a global perspective on mid-Holocene transitions. Academic Press.

4. Moratto, M. J., T. F. King, and W. B. Woolfenden. 1978. Archaeology and California’s climate. Journal of California Anthropology 5(2):147-161.

5. Glassow, M. A. 1992. Archaic cultural development in California. Revista de Arqueología Americana 5:201-229.

6. Broughton, J. M., D. A. Byers, R. A. Bryson, W. Eckerle, and D. B. Madsen. 2008. Did climatic seasonality control late Quaternary artiodactyl densities in western North America? Quaternary Science Reviews 27(19-20):1916-1937.

7. Jones, T. L., J. F. Porcasi, J. W. Gaeta, and B. F. Codding. 2008. The Diablo Canyon fauna: a coarse-grained record of trans-Holocene foraging from the central California mainland coast. American Antiquity 73(2):289-316.

Codding, B. F., J. F. Porcasi, and T. L. Jones. 2010. Explaining prehistoric variation in the abundance of large prey: a zooarchaeological analysis of deer and rabbit hunting along the Pecho Coast of central California. Journal of Anthropological Archaeology 29(1):47-61.

8. Butler, V. L., and S. K. Campbell. 2004. Resource intensification and resource depression in the Pacific Northwest of North America: a zooarchaeological review. Journal of World Prehistory 18(4):327-405.

9. For compiled references to early wildlife observations, see especially pages 95-98 in: Burcham, L. T. 1981. California range land. Center for Archaeological Research at Davis, Publication Number 7. Davis, California: University of California.

 

 

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