Shell traits of a marine mussel mediate predation selectivity by crabs and sea stars

Crabs and sea stars are known to preferentially select mussels with morphological traits that diminish the predators' searching or handling times. I compared two distinct morphotype of mussels (Mytilus trossulus; Gould, 1850) from Howe Sound, British Columbia, Canada, by dissection and measurement. Then, I experimentally offered mussels of the two morphotypes on the same patch to crabs (Cancer productus; Randall, 1839) and sea stars (Pisasler ochraceus; Brandt, 1835) to quantify the extent to which these predators select prey based on morphological features. Sea stars preferentially consumed mussels with gaps in shell closures, although these mussels also had larger adductor muscles compared with mussels rejected. Gaps at the shell margin presumably allowed sea stars easier access between shell valves to insert their stomachs and begin digestion. Small crabs preferentially consumed mussels with thin shells, which are easier to crush, whereas, large crabs consumed more thick-shelled mussels, possibly because these mussels were larger and offered greater energetic return. However, overall, crabs and sea stars did not exhibit strong preferences for smaller or larger mussel prey. These findings indicate that morphological features of mussels are important in prey selection by crab and sea star predators. Predator selectivity could cause a trade-off in defense strategies in mussels, and ultimately mediate indirect effects between these predators in the rocky intertidal community

Sex allocation in a monomorphic seabird with a single-egg clutch : test of the environment, mate quality, and female condition hypotheses

Sex allocation theory posits that mothers should preferentially invest in sons when environmental conditions are favorable for breeding, their mates are of high quality, or they are in good body condition. We tested these three hypotheses in rhinoceros auklets (Cerorhinca monocerata), monomorphic seabirds that lay a single-egg clutch, in 2 years that differed in environmental conditions for breeding. Results supported the environment and mate quality hypotheses, but these effects were interactive: offspring sex was independent of paternal traits in the poor year for breeding, while females mated to larger and more ornamented males reared more sons in the better year. Conversely, offspring sex was unrelated to female condition, as indexed by hatching date. We propose that good rearing conditions enable females to rear sons possessing the desirable phenotypic attributes of their mates. Results also supported two critical assumptions of sex allocation theory: (1) dimorphism in offspring condition at independence: daughters fledged with higher baseline levels of corticosterone than sons and (2) differential costs of rearing sons versus daughters: mothers rearing sons when environmental conditions were poor completed parental care in poorer condition than mothers rearing daughters in the same year and mothers rearing either sex when conditions were better. These novel results may help to explain the disparate results of previous studies of avian sex allocation

Dietary segregation between two cohabiting species of sparrows revealed with stable isotope analysis

Fox Sparrows (Passerella iliaca (Merrem, 1786)) and Song Sparrows (Melospiza melodia (A. Wilson, 1810)) cohabit on many islands along the Pacific coast of North America, and previous studies suggest that they rely on similar prey types. We used δ13C and δ15N stable isotope analysis on blood collected from breeding adults of both species in each of two habitats on Triangle Island, British Columbia, Canada, to test the hypothesis that the two species exhibit a consistent pattern (direction) of dietary segregation in different habitat types. Both δ13C and especially δ15N values differed between habitats, indicating that the two habitats were isotopically distinct. As predicted, δ15N values differed consistently between the two species in the two habitats, averaging ~ 1.5‰ higher in the smaller Song Sparrow than in the larger Fox Sparrow in both. We infer that Song Sparrows included more animal matter and less plant matter in their diets than Fox Sparrows, and suggest that fixed traits related to body size might underlie the dietary (trophic) differences. In contrast, δ13C values did not differ between species. We conclude that dietary segregation could help to facilitate the widespread cohabitation of these two species of sparrows

Immune and oxidative stress trade-offs in four classes of Ruffs (Philomachus pugnax) with different reproductive strategies

Immunity and resistance to oxidative stress are two mechanistically related aspects of self-maintenance that are usually not studied together in connection to ecological or evolutionary relevant variables. Whereas many studies compare two sexes, here we use Ruffs (Philomachus pugnax (L., 1758)), a species in which males have three alternative reproductive morphs: independents, satellites, and faeders. Previous work suggested that immune function in Ruffs depends on energetic constraints or potential of injuries. Based on their behaviour and life history, the three male morphs and females can be placed on an ordinal scale with independents at one end and females at the other, and these two explanations predict opposite patterns along this continuum. Innate and cell-mediated immunity decreased along this axis from independents to females, supporting a risk-of injury explanation over the energetic constrains hypothesis. No such pattern was evident for oxidative stress or resistance, and no relationship was detected between immunity and oxidative resistance or stress. Hence, during the breeding season immunity reflected the risk of injury, with faeders located in the immunological continuum between females and other male morphs. Species with alternative reproductive strategies provide particularly useful systems in which to address the evolution and ecology behind physiological mechanisms

Experimental evidence that both timing and parental quality affect breeding success in a zooplanktivorous seabird

Avian breeding success generally declines with later laying because of seasonal reductions in food supply, late laying by less capable pairs, or both. To understand the direct fitness consequences of breeding time requires distinguishing between these two possibilities. We used egg removal and re-laying experiments to evaluate how date and parental quality affect breeding success in a zooplanktivorous seabird, Cassin's Auklet (Ptychoramphus aleuticus). Egg laying began at the same time in all 5 years of study at Triangle Island, British Columbia, but, compared with a cold-water year, the population laid later and less synchronously in 4 warm-water years in which prey populations peaked earlier. As a result, Cassin's Auklets were less successful in years in which they laid later. Within seasons, early-laying females whose breeding attempt we delayed did not follow the population-wide seasonal declines in hatching success. This indicates a strong role for parental quality at the egg stage, probably because early, high-quality birds maintained more constant incubation. By contrast, the experimental females followed the population-wide seasonal declines in nestling survival and fledging mass. This indicates a strong role for date at the offspring-provisioning stage, which accords well with a previous study that found that success while raising nestlings is largely determined by the degree of temporal (mis)matching with the copepod Neocalanus cristatus. Our results offer novel insight into the causes of seasonal declines in avian breeding success, indicating that date and parental effects can be differentially involved, depending on the stage of breeding. Received 21 October 2008, accepted 16 May 2009

Seasonal variation in the foraging ecology of a zooplanktivorous seabird assessed with stable isotope analysis

We used δ15N and δ13C analyses on blood drawn from zooplanktivorous Cassin's auklet (Ptychoramphus aleuticus Pallas, 1811) on Triangle Island, British Columbia, over four years to address two questions: (1) do these seabirds systematically alter their trophic level and foraging habitat from one breeding stage to the next; and (2) do auklets select prey at the same trophic level and from the same foraging habitat to self-feed and provision offspring? Adult trophic level (inferred from δ15N values) tended to be higher prior to egg-laying than during incubation, perhaps reflecting an abundant zooplankton biomass in early spring. Season-long declines in δ13C values suggested that the auklets foraged on the continental shelf early in the season, but thereafter increasingly used habitats seaward of the continental shelf-break to obtain Neocalanus cristatus copepodites. Similarity in the δ15N and δ13C values of provisioning adults and nestlings suggested that adults consumed the same prey from those oceanic habitats to both self-feed and to provision offspring at the nest, a behaviour that may save time and energy. The results of this study show the dynamic nature of seabird foraging ecology and the utility of stable isotope analysis for assessing seasonal variation in the foraging ecology of oceanic species

Are yolk androgens adjusted to environmental conditions? : a test in two seabirds that lay single-egg clutches

It is widely believed that female birds strategically allocate androgens to yolk in the manner that best equips offspring for feeding conditions during their development. Because most avian studies have focused on multi-egg clutch species, and interpreted results within the framework of sibling competition, we still know little about how yolk androgens might be allocated in direct response to environmental conditions. Most oceanic birds are long-lived and lay single-egg clutches, and their breeding success is tightly linked to highly variable marine production. That combination: a variable breeding environment, long lives, and single-egg clutches, makes oceanic birds good subjects to test hypotheses about yolk androgen allocation strategies. We measured concentrations of two yolk androgens, androstenedione (A4) and testosterone (T), in the single-egg clutches laid by early-laying Cassin’s (Ptychoramphus aleuticus) and rhinoceros (Cerorhinca monocerata) auklets at Triangle Island, British Columbia, Canada, in 2002–2004. Environmental conditions including sea-surface temperatures and the timing and intensity of marine primary production varied over the 3 years, and in response, both the timing and success of seabird breeding varied. As in other avian species, concentrations of A4 and T varied markedly among individual eggs in both species (by factors of 3–8), yet contrary to expectation, little of the variation could be attributed to year effects. The high interindividual variation and the lack of interannual variation suggest a non-adaptive explanation for yolk androgen deposition relative to environmental conditions in these species

Trophic responses to the hatching of offspring in a central-place foraging seabird

We used δ15N and δ13C stable isotope analysis on blood drawn from adult and nestling Rhinoceros Auklets (Cerorhinca monocerata) in 4 years to answer two questions: (1) do adults tend to feed at higher trophic level while provisioning offspring than they did prior to breeding or while incubating eggs across a range of environmental conditions (years), and (2) do adults select prey at a similar trophic level and in similar habitats to complete the temporally overlapping tasks of self feeding and provisioning? Mean δ15N values in adult blood were higher during provisioning than prior to laying or during incubation in all 4 years. Thus, irrespective of environmental conditions, adults increased trophic level after their offspring hatched. Answers to the second question were more equivocal. However, there was support for models in which adult and nestling δ15N and δ13C values did not differ, suggesting that adults did tend to take prey from similar trophic level and habitat to self-feed and load to deliver to offspring. We propose that the two behaviours are related and can be explained as a strategic response by central-place foragers: small, low trophic-level prey (zooplankton) suitable for self-feeding are not as well suited as large, high trophic-level prey (forage fish) for loading in the bill to deliver to offspring, and adults can save time by self feeding and loading in the same habitats