Benjamin H. Becker

COMBINING DEMOGRAPHIC AND COUNT-BASED APPROACHES TO IDENTIFY SOURCE-SINK DYNAMICS OF A THREATENED SEABIRD

Identifying source-sink dynamics is of fundamental importance for conservation but is often limited by an inability to determine how immigration and emigration influence population processes. We demonstrate two ways to assess the role of immigration on population processes without directly observing individuals dispersing from one population to another and apply these methods to a population of Marbled Murrelets (Brachyramphus marmoratus) in California (USA). In the first method, the rate of immigration (i) is estimated by subtracting local recruitment (recruitment from within the population due to reproduction) estimated with demographic data from total recruitment (f; recruitment from within the population plus recruitment from other populations) estimated using temporal symmetry mark-recapture models developed by R. Pradel. The second method compares population growth rates estimated with temporal symmetry models (lambdaTS) and/or population growth rates estimated from counts of individuals over multiple sampling periods (lambdaC) with growth estimates from a stage-structured projection matrix model (lambdaM). Both lambdaTS and lambdaC incorporate all demographic processes affecting population change (birth, death, immigration, and emigration), whereas matrix models are usually constructed without incorporating immigration. Thus, if lambdaTS and lambdaC are > or = 1 and lambdaM < 1, the population is sustained by immigration and is considered to be a sink. Using the first method, recruitment estimated with temporal symmetry models was high (f= 0.182, SE = 0.058), the mean adult birth rate, as estimated using the ratio of juveniles to > or = 1 year old individuals (observed during ship-based surveys) was low (bA = 0.039, SE = 0.014), and immigration was 0.160 (SE = 0.057). Using the second method, murrelet numbers in central California were stable (lambdaC = 1.058, SE = 0.047; lambdaTS = 1.064, SE = 0.033), but were projected to decline 9.5% annually in the absence of immigration (lambdaM = 0.905, SE = 0.053). Our results suggest that Marbled Murrelets in central California represent a sink population that is stable but would decline in the absence of immigration from larger populations to the north. However, the extent to which modeled immigration is due to permanent recruitment or temporarily dispersing individuals that simply mask population declines is uncertain.

DIET–FEATHER STABLE ISOTOPE (δ15N AND δ13C) FRACTIONATION IN COMMON MURRES AND OTHER SEABIRDS

Abstract We measured the fractionation of stable nitrogen (δ15N) and carbon (δ13C) isotopes in the breast and primary feathers of 11 Common Murres (Uria aalge) maintained on a diet of capelin (Mallotus villosus). Diet–feather δ15N fractionation from delipidated capelin muscle to murre feathers was 3.6‰ ± 0.2‰ in breast feathers and 3.7‰ ± 0.2‰ in primary feathers. Fractionation of δ13C was 2.5‰ ± 0.2‰ in breast feathers and 1.9‰ ± 0.3‰ in primary feathers. Prey–feather fractionation (for delipidated, muscle-only prey samples) for nine other species of seabirds ranged from 3.0‰ to 4.6‰ for δ15N and 0.1‰ to 2.5‰ for δ13C. Studies that did not remove lipids from prey samples showed higher δ15N and δ13C fractionation, and those that used whole prey items rather than muscle tissue alone showed higher δ15N fractionation. We suggest that: (1) prey samples be delipidated to facilitate interpretation of δ13C fractionation, (2) high interstudy and interspecific variation in δ13C makes species-specific studies essential, and (3) use of muscle tissue rather than whole bodies of fish will minimize unexplained variation in δ15N fractionation.

At-sea density monitoring of Marbled Murrelets in central California : Methodological considerations

We conducted at-sea line transect surveys for Marbled Murrelets (Brachyramphus marmoratus) to determine density off the coast of central California and to explore the utility of various survey protocols. Surveys were designed to compare line versus strip transect methods, and reveal the effects of distance from shore, viewing conditions and seasonal trends on density estimates. On consecutive days, we conducted 12 paired (24 total) at-sea line and strip transect surveys that were 20 km long at 400 m and 800 m from shore. We also performed nine surveys that were 10 km long and at distances of 400 m, 900 m, 1,400 m, 2,400 m, 3,400 m and 4,400 m from shore. Density estimates calculated using line transects were significantly greater than estimates based on strip transects of 100 m and 200 m widths. Marbled Murrelet density ranged from 2.4-39.4 birds km -2 at 400 m from shore, and from 0.0-16.5 birds km -2 at 800 m from shore. Density was higher on the 400 m than on the 800 m survey on 22 of 24 survey days. Densities measured on consecutive days were highly correlated on the 400 m transect but not on the 800 m transect. Line transect densities on the 400 m transect were higher when conducted under better viewing conditions. Line transects had higher statistical power to detect trends than strip transects. Statistical power analyses indicated only a 24% chance of detecting a population declining by 5% per year over 5 years when surveying line transects 20 km in length five times a year. Power to detect a 5% annual change increased to 57% when surveying line transects five times per year over 10 years. Survey design should strive to minimize variability in bird density in order to maximize likelihood of detecting population trends. An increase in the number of surveys per breeding season, length of a transect, or duration of monitoring effort should increase power to detect trends in murrelet density. We suggest that at-sea surveys should focus on detecting trends in density rather than population size.

INDIVIDUAL AND TEMPORAL VARIATION IN INLAND FLIGHT BEHAVIOR OF MARBLED MURRELETS: IMPLICATIONS FOR POPULATION MONITORING

Abstract We studied the inland flight behavior of 46 radio-marked Marbled Murrelets (Brachyramphus marmoratus) in 2000 and 2001 in central California to determine how the frequency of inland flights varied among individuals and over time. All breeding murrelets regularly flew inland (mean 82% of daily surveys), but we observed considerable variation in the inland flight behavior of non-nesters. Non-nesters that were physiologically in breeding condition (potential breeders) regularly flew inland (90% of individuals; mean 41% of daily surveys), but non-nesters that were not in breeding condition (nonbreeders) rarely flew inland (20% of individuals; mean 1% of daily surveys). The mean percentage of surveys on which individual murrelets flew inland increased from 20% in 2000 to 61% in 2001, which was partly due to an increase in the percentage of breeders from 11% in 2000 to 50% in 2001. The frequency of inland flights was greatest during the incubation and chick- provisioning stages (100% in both stages), and lowest during the pre- and postbreeding stages (70% and 78%, respectively). Although the mean percentage of flights increased dramatically between years, the regional population estimate from at-sea surveys increased only 28% from 496 to 637 individuals during the same period, indicating that monitoring techniques such as radar that count inland flights are more likely to reflect annual variation in breeding effort than changes in regional population size. Moreover, the inland flight behavior of potential breeders indicates that radar surveys will overestimate breeding population size, even though the lack of inland flights by nonbreeders indicates that radar surveys will underestimate regional population size. Variación Individual y Temporal en el Comportamiento de Vuelo Tierra Adentro de Brachyramphus marmoratus: Implicancias para el Monitoreo de Poblaciones Resumen. Estudiamos el comportamiento de vuelo tierra adentro de 46 individuos de Brachyramphus marmoratus marcados con radio transmisores durante el 2000 y 2001 en California central para determinar cómo la frecuencia de vuelos tierra adentro varió entre individuos y a lo largo del tiempo. Todos los individuos reproductivos de B. marmoratus volaron regularmente tierra adentro (media 82% de los muestreos diarios), pero observamos considerable variación en el comportamiento de vuelo tierra adentro en los individuos que no nidificaban. Los individuos que no nidificaban pero que se encontraban fisiológicamente en condición reproductiva (reproductores potenciales) volaron regularmente tierra adentro (90% de los individuos; media 41% de los muestreos diarios), pero los individuos que no nidificaban y que no se encontraban en condición reproductiva raramente volaron tierra adentro (20% de los individuos; media 1% de los muestreos diarios). El porcentaje medio de los muestreos en los cuales los individuos de B. marmoratus volaron tierra adentro incrementó de un 20% en el 2000 a un 61% en el 2001, lo que se debió parcialmente a un incremento en el porcentaje de individuos reproductivos de un 11% en el 2000 a un 50% en el 2001. La frecuencia de vuelos tierra adentro fue mayor durante las etapas de incubación y suministro de alimento a los pichones (100% en ambas etapas), y fue menor durante las etapas pre- y post-reproductivas (70% y 78%, respectivamente). Aunque la proporción media de vuelos incrementó dramáticamente entre años, la población regional estimada a partir de muestreos en el mar incrementó sólo 28% de 496 a 637 individuos durante el mismo período, indicando que las técnicas de monitoreo como el radar, que cuentan los vuelos tierra adentro, tienen una mayor probabilidad de reflejar la variación anual en el esfuerzo reproductivo que en los cambios de tamaño poblacional regional. Más aún, el comportamiento de vuelo tierra adentro de los reproductores potenciales indica que los muestreos con radar sobrestimarán el tamaño poblacional reproductivo, a pesar de que la falta de vuelos tierra adentro por parte de individuos no reproductivos indica que los muestreos con radar subestimarán el tamaño poblacional regional.

AGE RATIOS AS ESTIMATORS OF PRODUCTIVITY: TESTING ASSUMPTIONS ON A THREATENED SEABIRD, THE MARBLED MURRELET (BRACHYRAMPHUS MARMORATUS)

Abstract The ratio of hatch-year (HY) to after-hatch-year (AHY) individuals (HY:AHY ratio) can be a valuable metric for estimating avian productivity because it does not require monitoring individual breeding sites and can often be estimated across large geographic and temporal scales. However, rigorous estimation of age ratios requires that both young and adult age classes are sampled in an unbiased manner, an assumption that is rarely tested. We estimated HY:AHY ratios for Marbled Murrelets (Brachyramphus marmoratus), a threatened seabird, using at-sea surveys and captures to assess whether age-specific differences in behavior and distribution result in biased productivity estimates in central California. AHY and HY Marbled Murrelets were distributed similarly at sea, and HY individuals did not congregate in nursery areas. Moreover, dispersal by radiomarked AHY Marbled Murrelets out of our survey area occurred at a low rate, and AHY densities were constant over the survey period, which suggests that AHY immigration and emigration did not significantly bias productivity estimates. HY density increased linearly over the survey period as expected if little dispersal occurred, which suggests that productivity estimates were not significantly biased by HY dispersal. Finally, simulation analyses indicated that annual variation in the timing of breeding resulted in only small biases in HY:AHY ratios. HY:AHY ratios were corrected for the proportion of AHY Marbled Murrelets that were incubating and the proportion of HY individuals that had not fledged at the time of sampling. Mean corrected HY:AHY ratios were low on the basis of both at-sea surveys conducted from 1996 to 2003 (0.032; SE = 0.011) and captures conducted from 1999 to 2003 (0.037; SE = 0.028), implying that productivity was poor in central California. Estimating age ratios may be an effective way of monitoring changes in reproductive success and identifying environmental factors that affect Marbled Murrelet populations, though tests of assumptions are needed in other regions. Le Rapport des Âges comme Estimateur de la Productivité: Tester les Hypothèses avec un Oiseau de Mer Menacé, Brachyramphus marmoratus

Effects of Rapid Flight-Feather Molt on Postbreeding Dispersal in a Pursuit-Diving Seabird

Abstract Breeding seabirds have been well studied but seabird ecology during the nonbreeding season is poorly understood because many species disperse far from breeding colonies to molt at sea. We characterized the timing of prebasic molt and postbreeding dispersal, described postbreeding dispersal movements, and estimated changes in body mass during molt for Marbled Murrelets (Brachyramphus marmoratus; Alcidae) in central California, 1999–2004. According to mark–recapture and at-sea surveys, 248–315 of 496–637 individuals (43–50%) used Año Nuevo Bay, located immediately adjacent to nesting areas, for their prebasic molt in August–October. Long-distance dispersal (≥ 100 km) from Año Nuevo Bay by radiomarked Marbled Murrelets was low during breeding (9–13%, n = 46), but was greater for individuals radiomarked at the end of the breeding season (69–90%, n = 20). The mean dispersal dates were 18 May and 21 October for the breeding and postbreeding samples, respectively, and postbreeding dispersal occurred an average of two weeks after molt completion. Mean dispersal distances were 184 km and 256 km in the breeding and postbreeding periods, respectively. Of 12 long-distance dispersers, all moved south except one. Marbled Murrelets gained mass during molt (n = 184), except during a moderate El Niño event in 2002 when mass remained constant. However, birds did not take longer to molt in 2002, which suggests that individuals allocated more energy reserves to molt processes in that year. Apparently, sufficient prey resources were available in Año Nuevo Bay for both basic metabolic requirements and the demands of molt, even when water was moderately warm. Efectos de la Muda Rápida de las Plumas de Vuelo Sobre la Dispersión Post-Reproductiva en un Ave Marina que se Zambulle para Perseguir Presas