Winter management of Californian rice fields for waterbirds

Published source details Elphick C.S. & Oring L.W. (1998) Winter management of Californian rice fields for waterbirds. Journal of Applied Ecology, 35.
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Summary

Legislation designed to reduce air pollution has restricted Californian rice-farmers from burning rice stubble and straw after harvest. Intentional flooding of fields during winter to speed stubble decomposition is becoming more common as an alternative to burning. The potential for flooded fields to act as a surrogate for lost wetland habitat may be an additional benefit in this region which hosts a large proportion of North America's wintering waterbirds. The degree to which waterbirds use flooded fields and whether the method of flooding affects their use was investigated.

Study site: Data was collected from sites three areas (Richvale Biggs, Sutter and Princeton) in Sacramento Valley, California, USA, during the winters of 1993/94 and 1994/95.

In the first winter, 53 rice fields (37 flooded – total 797 ha, 16 unflooded – total 487 ha) were studied, and in the second 40 fields (25 flooded – total 699 ha, 15 unflooded – total 398 ha). Flooded fields were designated those intentionally flooded to enhance rice stubble and rice straw decomposition and/or attract birds.

Experimental design: The experiment was designed to test whether waterbird (i.e. Podicipediformes, Ciconiiformes, Anseriformes, Gruiformes, Charadriiformes) use was greater in intentionally flooded fields than in unflooded fields, differed among flooded fields receiving different rice straw manipulations, and varied with water depth. Fields representing six management methods were sampled:

i) Flooded without straw management

ii) Rolled to flatten straw and stubble, then flooded

iii) Rolled after flooding to increase extent that straw stuck in the mud

iv) Chopping straw ti increase surface area and then flooding

v) Disked or chiselled to cut up and partially bury the straw

vi) Flooded after straw removal by burning or bailing

Bird censuses: Fields were censused about once every 10 days. Birds within field boundaries were identified and counted.

Water depth: Most fields were sub-divided into 'checks' by narrow earth levees. Water depth varied in each so depth was recorded at randomly located points. Twenty five measurements were made in the first winter, and 15 in the second.

A total of 46 waterbird species were recorded. The intentionally flooded rice fields had significantly greater use by 24 of the 31 bird species for which data could be analysed. Only great blue heron Ardea herodias and sandhill crane Grus canadensis were significantly more common in unflooded fields. Geese densities did not differ between flooded and unflooded fields.

There were no differences in the densities of most species in flooded fields that received manipulations to improve rice straw decomposition rates. Exceptions included four small shorebirds (killdeer Charadrius vociferous, dunlin Calidris alpina least sandpiper C.minutilla long-billed dowitcher Limnodromus scolopaceaus) which occurred at highest densities in fields where straw was incorporated into the soil.

Species, unsuprisingly, differed in their use of different water depths. For 14 species we tested whether preferred depths, suggested in the literature, received disproportionately higher use. Most of these species were more likely to be encountered within the suggested depth ranges. Depth, however, was a poor predictor of bird density. Depths of 15–20 cm resulted in frequent use by the greatest number of species.

Conclusions: In the Sacramento Valley, flooding rice fields increased suitable habitat for most, but not all, waterbird species. Different rice straw manipulation methods had little effect on most species, with the exception of four waders (Charadriiformes) which occurred at highest densities in fields where straw was incorporated into the soil. Water depth, was important in determining species occurrence. During the first half of the winter, water depths were greater than the median depths used by most species.

Note: If using or referring to this published study, please read and quote the original paper. The original paper can be viewed at: http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-2664.1998.00274.x