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Project description

Impacts and management of roof rats (Rattus rattus) in riparian areas in California. (01XN025)
Program Exotic Pests and Diseases Research Program
D.A. Whisson, Wildlife, Fish and Conservation Biology, UC Davis
A. Engilis Jr, Wildlife, Fish and Conservation Biology, UC Davis
Host/habitat Riparian Area
Pest Black Rat; Roof Rat; Ship Rat Rattus rattus
Discipline Wildlife Biology
Natural Systems
Start year (duration)  2001 (Three Years)
Objectives Determine the impacts of roof rats on nesting success of migratory songbirds nesting in riparian areas in California.

Develop and test potential management techniques for roof rats in riparian areas.

Predation of artificial nests was similar to previous years with 100%, 100%, and 70% of nests depredated in the Tall Forest (CRP1), Orr Ranch (CRP2), and Bobelaine Sanctuary (BOB) respectively. Roof rats were the most frequent nest predator, depredating 88% of nests. Rats were abundant at all sites, but most pervasive in the Consumnes River Preserve sites. A major flood resulted in lower rat abundance (less than 5% trap success) at both CRP sites in April, but populations recovered quickly with 15 to 25% trap success recorded in August. We completed laboratory testing of an additional commercial rodenticide (Rozol rat and mouse pellets, active ingredient 0.005% chlorophacinone). This bait was well accepted by rats, and was therefore chosen for the baiting program that was implemented in October 2003. A grid of 49 bait points spaced at 50m intervals was established in a 10ha section of the Tall Forest (CRP). Bait (200g) was placed in each station and the stations checked at four-day intervals for a six-week period. Bait consumption from stations was high. Because of imminent flooding of the site, bait stations were removed at the end of November. A population census conducted in the last week of the baiting program suggested that baiting had successfully reduced the population in the treated area (from 18% to 1% in the treated area, and from 5% to 13% in an adjacent untreated area). A census in January (when flood waters had receded) suggested that rat populations were recovering in the treated area. We, therefore, decided to proceed with additional baiting once conditions became suitable. Baiting will occur from April through June 2004, during the most intensive bird nesting period.

In 2002, artificial nest predation was high at all sites except the Kern River Preserve where only 3 nests were predated. Predators were identified from photos at 27 of 30 predated nests. Roof rats were the most common predator in both forest tracts of the Cosumnes River Preserve, and in the Bobelaine Sanctuary. Other predators at those sites included a raccoon (1 nest, Cosumnes River Preserve), western gray tree squirrels (2 nests, Bobelaine Sanctuary), and a ringtail (1 nest, Bobelaine Sanctuary). At the Kern River Preserve, 2 nests were predated by Bullock's orioles and 1 nest by a striped skunk. Rats were most numerous at the two Cosumnes River Preserve sites with approximately 10% trap success recorded during trapping sessions (June/July, September and November). Trap success varied between 2 and 6% at Bobelaine. Pregnancies were recorded in all trapping sessions suggesting that conditions for breeding are favorable year round in riparian habitats. Roof rats were not present at the Kern River Preserve. This may be due to the high number of wood rats at the Preserve (a native competitor), and a temporal water supply. We completed laboratory and field testing of 4 commercial rodenticide formulations that might be used to manage rat populations in important riparian areas. The baits that performed the best were tested in bait stations in the field in November 2002. Acceptance of bait was relatively poor in the laboratory but slightly higher in the field. Remote camera units indicated that rats were the only animals to enter bait stations. We will be conducting additional laboratory bait acceptance tests next month in an attempt to find a better-accepted bait.

Placement of artificial nests at all 4 sites was delayed as a result of the late receipt of funds. Most efforts have focused on obtaining necessary permits and selecting sites. Ten nests with remote camera units were placed in the Tall Forest area of the Cosumnes River Preserve (CRP1) on April 18. To date, only two nests have been predated: 1 by a rat (Rattus rattus) and another by an avian predator (photos yet to be developed). We have visited the Bobelaine Sanctuary and Orr Ranch tract of the Cosumnes River Preserve (CRP2) to select locations for artificial nests. Nests will be placed in those sites on May 3. A trip to the Kern River Preserve (KRP) to select sites and place nests has been scheduled for May 20.

To date, 17 rats have been trapped for laboratory studies of bait acceptance and efficacy. Preliminary testing with 10 of these animals has shown high acceptance of bait blocks made from paraffin and oats (no toxicant added for initial testing of acceptance). This bait base is currently being tested at CRP1 for field acceptance by R. rattus.

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