Raymond Pierotti

Diet Choice in the Herring Gull: Constraints Imposed by Reproductive and Ecological Factors

Studies of diet choice in carnivorous animals typically concentrate on maximizing rates of energy intake, and rarely emphasize either constraints on choice or alternate currencies. We examined diet choice in a generalist carnivore, the Herring Gull, Larus argentatus, in relation to both nesting habitat and reproductive performance over a 2—yr period. During prelaying and incubation periods 75—80% of Herring Gulls specialized on either intertidal organisms, human refuse, or other seabirds, while only 20—25% had generalized diets. Specific types of specialists tended to nest in particular habitats. Foraging tactics associated with each of three diets were related to time budgets and ecological constraints, i.e., levels of predation or intraspecific competition in specific habitats. Variation in diet choice was also strongly related to individual breeding performance. Intertidal specialists laid eggs earlier, produced larger and heavier clutches, and had higher rates of hatching than generalists and other specialists. This pattern held true over both years of the study even though weather conditions, and food accessibility, differed between years. Despite differences in performance related to laying and hatching, there were no significant differences in fledging rates, either between years or among diets. These results suggest that (a) choice of nesting habitat may constrain diet choice, (b) diet choice may have major effects on breeding performance, and (c) energy may not be the proper currency for examining diet choice in carnivores subject to a variety of constraints.

LONG-TERM REPRODUCTIVE OUTPUT IN WESTERN GULLS: CONSEQUENCES OF ALTERNATE TACTICS IN DIET CHOICE

Numerous studies reveal strong, positive skews in long-term breeding performance among free-living animals, yet few studies explore the mechanisms underlying such variation. We examine the results of a 12-yr study of a population of Western Gulls, Larus occidentalis. Of 112 pairs for which we have either long-term (≥5 yr) or lifetime reproductive output, 44% bred for only a single year, and an additional 25% bred for only 2–3 yr. A few pairs bred successfully for 6 –12 yr and showed higher average clutch size, hatching success, and fledging success within any single season than did less successful breeders. The principal trait influencing both survival and reproduction was individual diet, which consisted of a mix of human refuse and fish. A strong, positive relationship existed among the amount of fish taken, breeding life-span, and reproductive performance. Birds with short life-spans took little or no fish on an annual basis. Birds with breeding life-spans >10 yr and high breeding success took >60% fis...

Habitat Selection and Its Effect on Reproductive Output in the Herring Gull in Newfoundland

The breeding biology of the Herring Gull (Larus argentatus) was studied on Great Island, Newfoundland for part of the 1976 breeding season, and for two succeeding complete breeding seasons (1977 and 1978). Gulls nesting in three different habitats (designated puffin, rocky, and mead- ow) were compared with regard to several reproductive parameters. More pairs were found to nest in rocky habitat, and fewer pairs were found to nest in puffin habitat, than would be expected from a random distribution. In 1977, a year of low food availability, pairs in rocky habitat laid and hatched eggs significantly earlier than in the other two habitats. In 1978, when food was more abundant, gulls in rocky habitat laid heavier eggs than their counterparts in the other habitats. In both 1977 and 1978, chicks from rocky habitat grew at the fastest rate and were heavier than chicks in the other habitats. Finally, results of an experiment to test the egg-production capacity of females demonstrated that female gulls in rocky habitat were capable of producing significantly more eggs than their counterparts in meadow and puffin habitats. Despite the apparently better condition of gulls in rocky habitat, however, Herring Gull pairs in puffin habitat fledged as many chicks per nest as pairs in rocky habitat in 1977 and 1978, and even fledged more chicks in 1976. In all 3 yr of study, gulls in meadow habitat fledged the fewest chicks per nest. Within habitats, chick survival was strongly correlated with early laying dates and high rates of growth. However, there was also a large residual effect which was probably due to differences in the habitats. These differences are probably due to crowding and socially induced mortality in rocky habitat, and to the presence of predatory Great Black-backed Gulls in meadow habitat. The results support the theoretical models for habitat selection and dispersion developed by Fretwell and Lucas (1970), which suggest that increasing density in a preferred habitat can create a situation whereby fitness may actually be greater in a less-preferred habitat.

Diet and Reproductive Output in Seabirds

In recent years, behavioral ecology has placed considerable emphasis on understanding the adaptive basis of diet choice and factors that influence which foods are selected from an available array (Stephens and Krebs 1986). Most of this research, however, has concentrated on estimating how animals might maximize the rate of caloric intake, and there is some evidence that animals are capable of such choices (Kamil et al. 1987, Stephens and Krebs 1986). In contrast, few studies have focused on the relationship between diet and reproductive performance (Kamil et al. 1987). This relationship is important, because reproductive performance is likely to be a better indicator of individual fitness than ability to maximize caloric intake (Pierotti and Annett 1987). Excellent data exist on reproductive performance in many species of birds and mammals. These data demonstrate that individuals vary considerably in reproductive performance, considered both within single breeding seasons and over reproductive lifetimes (Clutton-Brock 1988). Only rarely, however, do these studies identify specific ecological factors, such as individual differences in diet

BOUNDED HYBRID SUPERIORITY IN AN AVIAN HYBRID ZONE: EFFECTS OF MATE, DIET, AND HABITAT CHOICE

Abstract There has been considerable debate in the study of hybrid zones as to whether hybrids may be superior to parental types within the area of contact (bounded hybrid superiority). In birds, naturally occurring hybridization is relatively common, and hybridization within this group always involves mate choice. If hybrids are superior, females choosing heterospecific mates should be expected to show higher fitness under the conditions prevalent in the hybrid zone. Hybrid superiority under these circumstances would reduce reinforcement and thereby help to maintain the hybrid zone. To examine this issue, we studied reproductive performances of hybrids and parental species of gulls (Larus occidentalis and Larus glaucescens) at two colonies within a linear hybrid zone along the west coast of the United States. This hybrid zone contains predominantly gulls of intermediate phenotype. Previous studies indicated that hybrids were superior to one or both parental types, but provided no data on possible mechanisms that underlie this hybrid superiority. Using a hybrid index designed specifically for these species, we identified to phenotype more than 300 individuals associated with nests, including both individual males and females within 73 pairs in the central portion of the hybrid zone and 74 pairs in the northern portion of the hybrid zone. There was little evidence of assortative mating, and what little there was resulted solely because of pairings within intergrades. In the central hybrid zone, females paired with hybrid males produced larger clutches and hatched and fledged more chicks compared with females paired to western gull males. This was a result of heavy predation on eggs in sand habitat, where male western gulls established territories. In contrast, many hybrid males established territories in vegetated cover that was less vulnerable to predation. In the northern part of the hybrid zone, clutch size did not differ among pair categories, however, there were differences in hatching and fledging success, with females paired to hybrid males showing better success compared to females paired to glaucouswinged gull males. Hybrids showed better hatching and fledging success in the north because hybrids are more like western gulls than glaucous‐winged gulls in foraging behavior, taking a higher percentage of fish in their diet, which enhances chick growth and survival. This is believed to be the first documentation of bounded hybrid superiority that delineates the mechanisms that underlie hybrid superiority.

Intergenerational conflicts in gulls

Abstract Gulls (genus Larus) are known both to adopt and to attack and kill unrelated offspring wandering through their territories. The tendency of adult gulls to accept unrelated offspring appears to counter standard Darwinian natural selection, and has been interpreted as kin selection, altruism, or reproductive error. Another alternative, not generally advanced, is that there is an intergenerational conflict in which gull chicks receiving inadequate parental care seek parental care from unrelated adults, and the adults try to prevent this from happening. In this study, chicks that abandoned their natal territories were being fed at low rates or were growing slowly compared to siblings, or were the recipients of siblicidal aggression. Of the chicks that abandoned their parents, around 20–30% became successfully adopted, and a similar percentage were killed by conspecific adults. Selection should favour the chicks winning the conflict, since if they are not adopted they will perish, whereas the cost to an adult that adopts an unrelated chick will be measured in lost time and energy, which may have little or no effect on future reproductive potential. The concept of an intergenerational conflict in which adults care for unrelated offspring may be of general applicability to those species that have precocial offspring.

Reproductive Consequences of Dietary Specialization and Switching in an Ecological Generalist

The foraging behavior of animals in complex environments is affected by conflicting demands (cf. Sih 1980, Martindale 1982, 1983, Cerri & Fraser 1983) and may be dependent on more than one aspect of prey quality (Pulliam 1975, Westoby 1978, Nicotri 1980, Breitwisch et al. 1984). Various currencies have been suggested that could affect the dietary selection of a foraging animal. These include energy intake per unit of time (Schoener 1971), maximizing intake of an essential nutrient (Goss-Custard 1981), and mixing nutrient intake to ensure adequate nutrition (Westoby 1978, Nicotri 1980).

THE INTERACTION OF HABITAT FRAGMENTATION, PLANT, AND SMALL MAMMAL SUCCESSION IN AN OLD FIELD

We compared the density and spatial distribution of four small mammal species (Microtus ochrogaster, Peromyscus maniculatus, Sigmodon hispidus, and P. leucopus) along with general measures of an old field plant community across two successional phases (1984–1986 and 1994–1996) of an experimental study of fragmentation in eastern Kansas. During the early phase the plant community was characterized by little spatial or temporal variance across patch size, consistent with spatially neutral models of succession. In contrast, there was a strong, species-specific effect of patch size on small mammal species distribution and abundance. The lack of variance in vegetation structure across patch size during the early seres suggests that small mammal distributions were responding in large part to features of the system other than variance in vegetation structure and composition across patch size. As succession proceeded, the colonization of the system by woody plant species precipitated a series of patch size effect...

Infanticide Versus Adoption: An Intergenerational Conflict

Considerable attention has been paid to the phenomenon of infanticide in recent years. Five functional categories of infanticide have been defined. Here I concentrate on those that either have been described as the outcome of possible competition for limited resources or have by default been classified as the result of social pathology. Many of the species that show infanticide of this nature also show adoption of unrelated young at fairly high frequencies. I suggest that the possibility of caring for nonfilial offspring creates an intergenerational conflict, or arms race, whereby offspring separated from their parents or receiving parental care of substandard quality (insufficient for their survival) should be selected to solicit care from adults other than their parents and the potential adoptive parents are selected to avoid giving such care. Evidence suggests that most examples of supposedly pathological infanticide, or resource-based infanticide, are the result of potential foster parents killing unrelated offspring when these offspring can clearly be identified as nonrelatives. Support for this idea comes from observations that (1) such infanticide is most common in group-living or colonial species, where chances of encountering wandering offspring are high; (2) infanticidal individuals come almost exclusively from the sex that bears the primary costs of adoption; (3) such infanticide occurs only under conditions where victims can clearly be identified as nonfilial; and (4) in species with little or no cost to adoption, adoption is common, but infanticide is nonexistent.

ISOLATING MECHANISMS IN SEABIRDS

In most species of birds, differences in plumage coloration or song structure act as isolating mechanisms. In seabirds, plumages are generally drab, and vocal repertoires are limited so that other phenotypic attributes must act as isolating mechanisms. One classic study of gulls suggests that the contrast between eye color and head color acts as a reproductive isolating mechanism, but this idea has been largely refuted by widespread evidence of hybridization in gulls in the last 20 years. An examination of more than 100 species of seabirds that breed sympatrically with congeners reveals that species with bills and feet similar in color hybridize in all areas where they breed sympatrically. Species that have bills or feet of different colors either do not hybridize or they produce rare hybrids that are unable to obtain mates as adults. This suggests that bill and foot coloration act as the primary isolating mechanisms in all surface‐nesting seabirds and some burrow nesters. It may be a general pattern in birds that foot color acts to supplement bill color in reproductive isolation and mate choice.