Gary W. Kaiser

Norway rats as predators of burrow-nesting seabirds : Insights from stable isotope analyses

Introduced species to oceanic islands can cause tremendous declines and extinctions of native avifauna. On Langara Island. British Columbia, Canada, the burrow-nesting ancient murrelet (Synthliboramphus antiquus) has declined from all estimated original population of 200,000 pairs to 14,600 pairs ill 1993. Previously, causes of this decline were unknown, but the introduction of Norway rats (Rattus norvegicus) has been implicated as a major factor and has resulted in a recent rat eradication program. A major obstacle in the investigation of the effects of rats on seabirds, here and elsewhere, has been the inability to accurately assess the importance of seabirds to the diets of rats that also consume a variety of plants and invertebrates. We used stable-carbon (δ 13 C), nitrogen (δ 15 N), and sulphur (δ 34 S) isotope analyses of muscle and liver tissues of rats and prey organisms from 3 regions of Langara Island to evaluate evidence for marine foods, including seabirds, in the diets of rats. Rats were segregated into 3 isotopic groups corresponding to upland, littoral, and seabird nesting areas on the island. We interpret these groups to represent individuals consuming predominantly C-3 terrestrial foods (x δ 15 N = 5.4‰; x δ 13 C = -24.9‰; n = 24), intertidal invertebrates (x δ 15 N = 8.9‰; x δ 13 C = -14.3‰; n = 21), and ancient murrelet adults, chicks, or eggs (x egg δ 15 N = 13.2‰; x δ 13 C = -17.6‰; n = 8). We found strong correlations between liver and muscle isotope values for both 13 C and 15 N, suggesting that dietary preferences within individuals remained relatively constant. Stable-sulphur isotope values of rat liver were less useful in segregating marine and terrestrial dietary inputs, possibly because sources of sulphur to tile terrestrial food web were of marine origin pooled x δ 34 S value = 17.8‰; n = 20). Our measurement of liver and muscle tissues gave dietary estimates based on relatively short- and long-tern integrations and revealed tile 3 groups of rats remained isotopically segregated at least over the 2-month period of ancient murrelet breeding on Langara Island. Our results have important ramifications for dietary investigations of introduced fauna and their effect on native scabirds on oceanic slands.

Intraspecific Variation in Commuting Distance of Marbled Murrelets (Brachyramphus marmoratus): Ecological and Energetic Consequences of Nesting Further Inland

Abstract Radio transmitters were deployed on Marbled Murrelets (Brachyramphus marmoratus) at Desolation Sound, British Columbia, Canada, during the 1998 breeding season to assess individual variation in distance birds nested from foraging areas, and potential energetic and ecological consequences of commuting those distances. Radio-tracking from a helicopter was used to locate nests, and tracking from the air and boats was used to locate murrelets on the water. Twenty-three nests were found, with active incubation at 16, and active chick-rearing at 12. A minimum of 3 nests fledged chicks, 9 were failures, and 11 were unknown. Nests were at an elevation of 806 ± 377 m and a distance of 39.2 ± 23.2 km (range 12–102 km) from locations on the water. Birds spent an estimated 1.2 ± 0.7 h per day commuting to and from nests (range 0.3–3.5 h per day). It was estimated that birds expended 3,883 ± 2,296 kJ (range 1,200–10,144 kJ) over the breeding season when commuting to those nests, which was 5–41% of their estimated field metabolic-rate during the breeding season. There was no relationship between distance to nests and breeding success. Either Marbled Murrelets can accommodate that additional energy expenditure, or reduce commuting costs by modifying their foraging behavior. They may forage closer to nest sites when provisioning chicks, thereby reducing commuting costs with a payload, or alter nest visitation rates in relation to distance they nest from foraging areas. Nests further inland may also confer advantages that compensate for the added commuting, or birds might replenish body reserves at the end of the breeding season.

SEX RATIOS OF DUNLIN WINTERING AT TWO LATITUDES ON THE PACIFIC COAST

Abstract Latitudinal clines in sex ratio during the nonbreeding season occur in some shorebirds of the Scolopacidae. We compared populations of nonbreeding Dunlin (Calidris alpina pacifica) from two latitudes along the Pacific flyway: the Fraser River Delta, British Columbia, and Bolinas Lagoon, California, to determine whether, and to what degree, they exhibited sex ratios consistent with a latitudinal cline. Dunlin are plumage monomorphic, so we used a maximum likelihood model to estimate overall and monthly sex ratios for each population based on culmen length distributions. Sex ratios in the Fraser River Delta were corrected for sex differences in habitat use. Monthly sex ratios were similar at the two sites but varied throughout the winter, likely reflecting differences in seasonal movement patterns between the sexes. Both populations showed an overall bias toward males (Fraser = 61% males, Bolinas = 65% males). Since there is no evidence to support the possibility of a skew toward males in C. a. pacifica as a whole, our data are consistent with some form of latitudinal cline in the sex ratio of C. a. pacifica. However, additional data from the Oregon coast, southern California, and Mexico are required to resolve this question. We also tested the hypothesis that mean body size within each sex is larger at the higher-latitude site (Fraser River Delta), but this did not appear to be the case.

DO TWO MURRELETS MAKE A PAIR? BREEDING STATUS AND BEHAVIOR OF MARBLED MURRELET PAIRS CAPTURED AT SEA

Abstract Marbled Murrelets (Brachyramphus marmoratus) observed at sea usually are in pairs throughout the year. Although it has been assumed that these pairs are mates, this assumption has not been formally examined. Using data from three study sites during the breeding seasons of 1997–2001, we found that 92% of the birds that were paired at capture were of male-female pairs, and that paired females were more likely (73%) to be producing eggs than were single females (8%). Fourteen of fifteen pairs were tracked to a single nest location per pair. No pair members caught at sea were found breeding at separate nest sites. One pair was caught in two successive seasons, suggesting that at least some pairs are long lasting. Notably, pair members breeding together and radio tracked throughout the summer were detected without their breeding partners for 77% of the time. Thus, while pairs of Marbled Murrelets observed at sea most likely are members of a breeding pair, single murrelets observed at sea should not be assumed to be unpaired or nonbreeders.

Morphometric Variation in Marbled Murrelets, Brachyramphus Marmoratus, in British Columbia

Morphometrics (culmen length, bill height, bill width, wing chord length, and tarsus length) were taken on 664 marbled murrelets at Desolation Sound and Mussel Inlet, British Columbia, during 1994 to 1997, in order to assess morphological differences within and between populations and the accuracy of a discriminant function analysis to identify the sex of birds. An assessment of interand intra-observer variability in measurements was also made. Significant inter-observer effects and some intra-observer effects were found. Data from recaptured murrelets indicated they had decreased in size with age, which was attributed to interobserver effects. Wing chord length had the highest measurement error (66.8%) among observers and tarsus length had the lowest error (36.8%). Deviations of measurements from the mean were compared among years and sites. No inter-annual differences were detected in any morphometric at Desolation Sound. Significant differences in culmen length, wing chord length, and tarsus length were found between birds from Mussel Inlet and Desolation Sound, which might indicate discrete populations. The degree of sexual dimorphism in this species was small (measurements of females average 98% of corresponding measurements of males) and discriminant function analysis revealed only about a 70% success rate in allocating birds to sex; therefore, its widespread use in this species is not recommended. Future studies of marbled murrelets, or other avian species, which involve large numbers of personnel, should incorporate extensive training of all observers, with data continually cross-checked in order to minimize intraand inter-observer differences in measurements.