Suzann G. Speckman

Prey density and the behavioral flexibility of a marine predator: The common murre ( Uria aalge )

Flexible time budgets allow individual animals to buffer the effects of variable food availability by allocating more time to foraging when food density decreases. This trait should be especially important for marine predators that forage on patchy and ephemeral food resources. We examined flexible time allocation by a long-lived marine predator, the Common Murre (Uria aalge), using data collected in a five-year study at three colonies in Alaska (USA) with contrasting environmental conditions. Annual hydroacoustic surveys revealed an order-of-magnitude variation in food density among the 15 colony-years of study. We used data on parental time budgets and local prey density to test predictions from two hypotheses: Hypothesis A, the colony attendance of seabirds varies nonlinearly with food density; and Hypothesis B, flexible time allocation of parent murres buffers chicks against variable food availability. Hypothesis A was supported; colony attendance by murres was positively correlated with food over a limited range of poor-to-moderate food densities, but independent of food over a broader range of higher densities. This is the first empirical evidence for a nonlinear response of a marine predators time budget to changes in prey density. Predictions from Hypothesis B were largely supported: (1) chick-feeding rates were fairly constant over a wide range of densities and only dropped below 3.5 meals per day at the low end of prey density, and (2) there was a nonlinear relationship between chick-feeding rates and time spent at the colony, with chick-feeding rates only declining after time at the colony by the nonbrooding parent was reduced to a minimum. The ability of parents to adjust their foraging time by more than 2 h/d explains why they were able to maintain chick-feeding rates of more than 3.5 meals/d across a 10-fold range in local food density.

Temporal variability in abundance of Marbled Murrelets at sea in southeast Alaska

-We examined effects of season, time of day, tide stage, tidal oscillation, and sea surface temperature on Marbled Murrelet (Brachyramphus marmoratus) abundance and distribution at sea. We also evaluated whether constraining surveys to specific time periods or tide stages would reduce temporal variability in counts. Murrelets were surveyed daily from small boats and from shore in Auke Bay and Fritz Cove, Alaska, from May through August in 1992 and 1993. Murrelet numbers were high before egg-laying, declined by more than half during egg-laying and incubation and were highly variable during chick-rearing and fledging. Murrelet numbers were highest in early and late morning and declined throughout the day, sometimes increasing slightly in the evening. Peak murrelet numbers occurred on high or falling morning tides, especially in shallow areas where Pacific sand lance (Ammodytes hexapterus) were abundant. Differences between years in murrelet abundance and breeding phenology probably resulted from interannual differences in the pattern of seasonal warming and subsequent effects on production at lower trophic levels. We recommend that surveys for trends in abundance in Southeast Alaska be conducted in early morning, in June, at high or falling tides. Power analyses indicated that surveys conducted in this manner would minimize the number of years required to detect a significant change in abundance. Received 6 April 2000, accepted 3June 2000.

Deciphering the Social Structure of Marbled Murrelets from Behavioral Observations at Sea

Abstract We surveyed Marbled Murrelets (Brachyramphus marmoratus) daily from small boats in Auke Bay and Fritz Cove, Alaska, from May through August 1992 and 1993. Differences in numbers of juveniles and in the timing of their presence in the study area between the two years indicated that breeding phenology was late and productivity was low in 1992 compared to 1993. This difference was consistent with variability in the physical environment. Of 99 fish identified in the bills of fish-holding adult murrelets, 81 (82%) were Pacific Sand Lance (Ammodytes hexapterus). Counts of fish-holding adult murrelets were significantly higher in the evening than at any other time of day. Time of day had no significant effects on counts of fledglings, indicating that juveniles were moving into and out of the study area during the day. Murrelets were predominantly found in groups of two or more, even during incubation, suggesting that murrelets incur an appreciable benefit, such as increased foraging efficiency, from foraging in groups. For both summers, there was no correlation between counts of murrelets on the water and numbers of murrelet detections in the adjacent forest. We suggest that many behavior patterns of the Marbled Murrelet (displaying, choosing of mates, and pair-bonding, finding of nest sites and successful foraging of juveniles) may be socially facilitated.

SPATIOTEMPORAL PREDICTABILITY OF SCHOOLING AND NONSCHOOLING PREY OF PIGEON GUILLEMOTS

Abstract Low spatiotemporal variability in the abundance of nonschooling prey might allow Pigeon Guillemots (Cepphus columba) to maintain the high chick provisioning rates that are characteristic of the species. We tested predictions of this hypothesis with data collected with beach seines and scuba and hydroacoustic surveys in Kachemak Bay, Alaska, during 1996–1999. Coefficients of variability were 20–211% greater for schooling than nonschooling prey on day, seasonal, and km scales. However, the proportion of schooling prey in chick diets explained relatively little variability in Pigeon Guillemot meal delivery rates at the scale of hours (r2 = 0.07) and weeks (r2 = 0.19). Behavioral adaptations such as flexible time budgets likely ameliorate the negative effects of high resource variability, but we propose that these adaptations are only effective when schooling prey are available at distances well below the maximum foraging range of the species. Previsibilidad Espacio-Temporal de Presas que Forman y No Forman Cardúmenes de Cepphus columba Resumen. La baja variabilidad espacio-temporal en la abundancia de presas que no forman cardúmenes podría permitir que Cepphus columba mantenga las altas tasas de aprovisionamiento de los pichones que caracterizan a esta especie. Evaluamos las predicciones de esta hipótesis con datos colectados mediante redes barredoras de playa y buceo y muestreos hidro-acústicos en la Bahía Kachemak, Alaska, durante 1996– 1999. Los coeficientes de variabilidad fueron 20– 211% mayores para las presas que forman cardúmenes que para las que no forman cardúmenes a las escalas diaria, estacional y de km. Sin embargo, la proporción de presas que forman cardúmenes en la dieta de los pichones explicó relativamente poca variabilidad en las tasas de entrega de alimento de C. columba a la escala de horas (r2 = 0.07) y semanas (r2 = 0.19). Las adaptaciones de comportamiento como presupuestos de tiempo flexibles probablemente corrigen los efectos negativos de la alta variación de recursos, pero proponemos que estas adaptaciones son sólo efectivas cuando las presas que forman cardúmenes están disponibles a distancias bien por debajo del rango máximo de forrajeo de la especie.