Flock size and habitat-dependent food and energy intake of foraging Goldfinches

Summary During the breeding season Goldfinches (Carduelis carduelis L.) feed on milky ripe seeds of about 20 food plants. Individual Goldfinches joining a flock reduce the time spent vigilant with increasing flock size. Therefore birds feeding in flocks get an increased intake of kernels per time unit. This was measured for five different food plants (Dactylis glomerata (Gramineae), Knautia arvensis (Dipsacaceae), Senecio vulgaris, Taraxacum officinale, Tragopogon pratensis (Compositae)). In large-sized flocks, birds fed up to 2.3 times more kernels, than when feeding solitarily. In addition, visibility in the vegetation leads to a further increase of kernel intake. Thus feeding under good conditions as in recently mown areas, can raise kernel intake to the seven fold per time unit as compared to solitary feeding. The maximum ingestion rate of kernels was 98 per min which implies a head up-and-down movement every 0.6 s. The calculated energy intake of birds per time unit is lowest in Senecio and highest in Tragopogon. Thus the birds, when feeding on Tragopogon in larger flocks, can gain an energy intake 16 times higher than that reached when feeding on Senecio, despite of a higher kernel intake rate. The energy intake individual Goldfinches gain at the particular plant species is markedly increased with flock sizes up to eight birds, with larger flocks the intake increases only slightly.     

Seasonal clutch size variation of multi-brooded bird species: comparisons between breeding season and latitudes

Single- and multi-brooded species of birds differ in their seasonal patterns of clutch size. Single-brooded species start with a maximum clutch size that declines continuously as the season progresses, whereas the clutch sizes of multi-brooded species usually increase to a mid-season maximum peak and then decrease progressively until the end of the breeding season. Previous studies have shown that multi-brooded migrant species present seasonal patterns that are similar to single-brooded species at high latitudes but similar to multi-brooded non-migratory species at lower latitudes. We studied the Greenfinch Carduelis chloris and Goldfinch C. carduelis populations in eastern Spain (Sagunto, Valencia) between 1975 and 2002 to compare seasonal variations in clutch sizes between years with early and late starts to the breeding season. The period over which clutch sizes increase was longer when the breeding season started earlier. The Goldfinch population showed no pattern of initial increase in clutch size when there was a late start to the breeding season: a late start shortens the season giving them less time to breed, and may also coincide with maximum food availability. Thus, the pattern of single-brooded species was observed. In the Greenfinch population, a trend toward the seasonal pattern of single-brooded species was also observed when the following indices were compared: clutch size increase, modal timing, initial slope and timing of maximum clutch size. We have also compared the seasonal patterns of clutch size of both species in eastern Spain with the patterns observed in Britain. Our results show that for both Goldfinches and Greenfinches, the non-migrant southern populations of Sagunto in eastern Spain do not tend towards a more multi-brooded seasonal pattern of clutch size than the migrant Goldfinches of Britain.     

THE ADAPTIVE RADIATION AND FEEDING ECOLOGY OF SOME BRITISH FINCHES

The habitats and foods of ten species of finches studied near Oxford, England, are described in detail, and differences in feeding habits between them are related to differences in bill- and leg-structure, body-weight and behaviour. The Hawfinch, Siskin, Redpoll and Crossbill live primarily in woodland, the Greenfinch, Linnet and Goldfinch primarily in farmland, while the Bullfinch and Chaffinch breed, and occur throughout the year, in both these habitats. Habitat differences between species were probably more marked in primeval England than now, and the problems of studying animals in man-modified environments are discussed in this context. Diet was studied by observation and by gut-analyses of live and dead finches. Before roosting, finches usually accumulate food in their gullet, which is expandable but otherwise undifferentiated (the term “crop” is thus inappropriate). The Lesser Redpoll, the smallest species studied, can store up to about 0.8 c.c. of food in its gullet, but the Greenfinch, one of the largest finches, up to 2.5 c.c. Among the Carduelis finches, the Greenfinch lives mainly on the large seeds of various trees, cereal crops and agricultural weeds; the Goldfinch on the seeds of Compositae; the Siskin on alder and conifer seeds; the Lesser Redpoll on birch and other small seeds; and the Linnet on the seeds of various weeds of farmland. The Hawfinch and Bullfinch eat a greater proportion of buds and seeds from fleshy fruits than do other finches; but the Hawfinch prefers larger ones than does the Bullfinch. The three European crossbills feed almost entirely on conifer seeds; specializing on different tree species. In summer, the Chaffinch and Brambling live largely on invertebrates but in winter on beechmast or small seeds taken from the ground. In winter, Chaffinches reaching Britain from northern Europe tend to feed in large flocks in open areas and to roost communally, while residents tend to remain near their breeding places and roost singly. The migrations and distribution of finches in Europe are described briefly. At any time of winter, closely related species with similar feeding habits are often concentrated in different parts of a common wintering range. Furthermore, in some species, such as the Goldfinch and Chaffinch, the sexes winter mainly in different areas. In Oxford, food was least readily available during hard (winter) weather and there was then complete segregation in diet between different species, but at other times their diets often overlapped considerably. The overlap was much more pronounced in the species living in farmland (which is an artificial habitat) than in those living in natural or seminatural woodland; some tentative explanations for this are offered. The fringilline finches, the Chaffinch and Brambling, feed their young entirely on invertebrates, are territorial in the breeding season and collect food mostly within their territories. The cardueline finches, on the other hand, feed their young mainly, or entirely, on seeds, often nest in “loose colonies” and usually gather food communally at some distance from their nests. The fringilline finches carry food in their bills to the young, which therefore receive small, but frequent, meals; whereas the carduelines feed their young by regurgitation and at longer intervals. Among the carduelines, the Hawfinch is exceptional in that seeds are first swallowed and regurgitated to the nestlings, whereas insects are carried in the bill. The Bullfinch develops in the breeding season special buccal pouches in which food is carried to the young. In general, most finches select diets for their young which differ from their own diets in the breeding season. The Crossbill and Linnet often feed their young on seeds alone, and some other species may do so occasionally. The breeding of these carduelines is therefore independent of a supply of invertebrates, but always coincides with an abundance of seeds. They breed at seasons when other birds do not; the most extreme cases are the three crossbills, which, given an abundance of suitable seeds, will nest at any time of year. The process of seed-husking is described; the strength of the jaws, the extent of the gape and the width of the palatal grooves limit the size and hardness of seeds which a given species can take. No two species have bills of the same shape and size, and differences in diet between species are connected with differences in the structure of their bills and the way the bill is used. In general, the deeper the bill of a finch, the more numerous the types of seeds in its diet. The Greenfinch, Linnet and Redpoll have relatively short broad bills and feed more than do other Carduelis finches from plants whose seeds are relatively exposed (e.g. grasses) or enclosed within pods or capsules; they also pick up many fallen seeds from the ground, but each species concentrates on different sizes of seeds. The Goldfinch and Siskin have reIatively long, narrow bills and obtain much of their food by probing into cones and into the seed-heads of Com-positae; unlike other finches, they also use a gaping movement while feeding. The Goldfinch has a longer bill and eats more Compositae seeds than the Siskin, while tlie latter eats more tree seeds than the Goldfinch. Further, male Goldfinches have slightly longer beaks than females, and hence can obtain teasel seeds more efficiently; females feed only rarely from these plants. The Bullfinch and Hawfinch obtain much of their food by crushing fruits, seed-heads, etc. in their bills and peeling off the outer layer to get the seeds. The methods used by the crossbill species to open cones of varying degrees of hardness are described in detail. The Chaffinch and Brambling obtain almost all their food by rapid pecks; they show none of the behaviour patterns used by carduelines to open various types of seed-heads. Species also differ in the proportion of food obtained from the ground, from vegetation, or on the wing. These differences are related to the various positions in which each species is able to feed. Those species (Goldfinch, Siskin, Redpoll and Crossbills) which are most adept at clinging and hanging feed mainly from trees and herbaceous plants, while those that are least adept at this (Chaffinch and Brambling) obtain almost all their food from the ground or from firm horizontal twigs. The remaining species are intermediate in the proportion of time spent feeding from the ground or from vegetation; and only the Chaffinch, Brambling and Bullfinch obtain an appreciable amount of food on the wing. Differences between species in the frequency with which various positions are adopted during feeding can be related to differences in their average body weights, and (to a lesser extent) in their leg-structure; the most agile species are either light in weight or have relatively short strong legs. Body-weight, leg-proportions and bill-size (and hence feeding habits) may vary considerably between different populations of the same species. In Appendix 2, the leg-proportions of certain English finches are compared with those of their German and Russian counterparts. Some finch species use their feet in coordination with their bill to obtain food. The extent to which various species do this in the wild is discussed in relation to the ease with which they can be taught experimentally to pull up food suspended on the end of a long thread. The ecology, structure and behaviour of finches is discussed in relation to some current ideas on speciation and the ontogeny of feeding behaviour. The young of several species of finches respond initially to a similar range of potential food-items (both in size and appearance) and all attempt to deal with them in similar ways. However, each individual soon learns to concentrate on those foods it can obtain most efficiently with its own particular structures and behaviour patterns. It is thus the structural and behavioural differences between closely-related species which lead the individuals of each to prefer different foods. Further, some of the behaviour patterns themselves probably develop mainly as a result of learning conditioned by structure, so that some of the behavioural differences between species that affect food-choice are in turn dependent on structural differences. This implies that, from an evolutionary viewpoint, two forms need diverge only in structure (and not necessarily “psychologically”) for some ecological segregation to be achieved. There are, however, certain differences in feeding behaviour between the fringillines and carduelines that cannot be attributed merely to structural differences between the two groups, so presumably some “psychological factor” is also important in the development of these particular behaviour patterns. A close relationship between the structure, feeding behaviour and food of an animal is usually taken for granted, but it is often difficult to decide which of these came first in evolution. Among finches, however, there are sometimes indications that a structural change was necessary before a new behaviour pattern or feeding habit could evolve, though further modification of structure may then occur through adaptation to the new foods thereby made available. Some evolutionary trends among finches are described, and the role of competition in the evolution and maintenance of ecological differences between species is discussed. The effect of social experience on feeding behaviour is also discussed, and some examples are given of the spread of novel food-habits among finches, and of presumed traditional differences in feeding behaviour between different populations of the same species. Finally, some recent changes in food, and habitat-selection by finches are discussed in relation to problems of conservation.     

广西扶绥黑叶猴主要食物种类能量含量的初步研究

能量对灵长类食物选择的影响是营养生态学中重要的组成部分。为了研究黑叶猴食物能量及其对食物选择的影响,采用焦点动物取样法和连续记录法在广西扶绥观察记录野生黑叶猴的觅食行为;每月观察结束后采集被取食的植物部位样本,利用全自动氧弹热量计测定其能量。结果表明,黑叶猴食物中嫩叶最高（58.5%）,其次为果实（12.4%）,其他种类依次为成熟叶（12.1%）、种子（10.6%）、花（4.3%）和树皮（2.4%）。在不同的生活型食物中,乔木占48.5%,灌木占34.6%,藤本和草本分别占19.4%和17.2%。各种食物平均能量为17.09 kJ/g,月平均能量在16.52 -17.61 kJ/g之间变化。种子的能量最高,达19.52 kJ/g,最低为成熟叶,仅为16.84 kJ/g。不同生活型植物中,灌木能量最高,达17.52 kJ/g,草本植物最低,仅为15.73 kJ/g。不同植物种类之间能量有显著的差异（P < 0.001）。取食量最多的15种和最少的15种植物之间平均能量有显著差异（P < 0.001）,但在月份间、雨季和旱季间则无显著差异（P > 0.05）。取食不同种类的时间与其平均能量之间无显著相关关系（P = 0.40）,食物能量含量并非影响黑叶猴食物选择的主要原因,其觅食策略需要进一步的研究。     

Macrogeographic variations in food choice of mainland citril finches Carduelis [citrinella] citrinella versus insular Corsican (citril) finches Carduelis [citrinella] corsicanus

We studied variations in diet composition in sub-populations of mainland citril finches Carduelis [citrinella] citrinella in European mountain chains, compared to closely related insular Corsican (citril) finches Carduelis [citrinella] corsicanus on several Mediterranean Islands with a special emphasis on key food plants used during the breeding period. We found that citril finch sub-populations of the Pre-Pyrenees and the Black Forest rely mostly on mountain pine and dandelion seeds as main food resources during the breeding season. In contrast, insular Corsican finches on the Mediterranean islands of Capraia and Corsica fed mainly on various herb species. Finches in Corsica fed predominantly on shepherd’s purse, while birds from Capraia were choosing rosemary as their main food plant during the breeding period. These observations confirmed the previously expected difference in food choice between the two forms due to the observed increased niche breadth on Corsica. However, Corsican finches on Sardinia showed a food choice more similar to that of the citril finch populations, in that they fed predominantly on black pine seeds during breeding season. Thus we were not able to find clear behavioural differences in diet composition between all studied sub-populations of citril and Corsican finches. To conclude, food choice of both forms is highly adaptive and, in the different breeding areas, distinct key plants may be of special importance during the breeding season. The preference for these plants is likely to be linked to their abundance and their energetic and nutritional content.     

Why water birds forage at night: a test using black‐tailed godwits Limosa limosa during migratory periods

Many migratory water birds are known to feed both during day and night outside the breeding season, but the underlying factors and mechanisms determining this foraging pattern are poorly understood. We addressed this topic by comparing both diurnal and nocturnal foraging activity (FA) and metabolizable energy intake rate (MEIR) in migrating black‐tailed godwits Limosa limosa staging in two different habitats, rice fields and coastal salt pans. Black‐tailed godwits staging in rice fields during pre‐breeding migration fed on rice seeds, and only foraged during the daylight period (FA: 81.89 ± 3.03%; MEIR: 1.15 ± 0.03 kJ · min^?1). Daily energy consumption (DEC) of godwits relying on seeds was enough to meet the theoretical daily energy expenditure (DEE). In contrast, black‐tailed godwits staging in salt pans during post‐breeding migration fed on chironomid larvae, and they foraged during both daylight (FA: 67.36 ± 4.30%; MEIR: 0.27 ± 0.01 kJ · min^?1) and darkness (FA: 69.89 ± 6.89%; MEIR: 0.26 ± 0.00 kJ · min^?1). Nocturnal energy intake contributed 31.7% to DEC, the latter being insufficient to fully meet DEE. Our findings give empirical support to the view that diurnal foraging is the norm in many migratory water birds outside the breeding season, and nocturnal foraging occurs when the daily energy requirements are not met during the daylight period, supporting the supplementary food hypothesis.     

The influence of rainfall on diet and reproduction in four African rodent species

Arvicanthis nitoticus was found, during two seasons of the study, to begin breeding some two to three months after the start of the rains; at this time weed seeds first appeared in the stomach contents. Breeding continued during the rains and into the early part of the dry season while weed seeds and cereals formed the major part of the diet. Breeding declined during the early part of the dry season, and fat was laid down while weed seeds and cereals were still present but less abundant in the diet. Later in the dry season, breeding ceased and the diet was switched dramatically, first to the leaves and stems of dicotyledonous plants then almost exclusively to grass, and fat deposits were rapidly utilized. Arvicanthis living in an area where cereals were artifically supplied bred almost continuously. Mastomys natalensis also bred when seeds and cereals were most plentiful, but, by ranging widely over open ground, often managed to continue to find these foods, and insects, during the dry season. Supplied cereals failed to produce a reproductive response in Mastomys –instead, unprecedented fat deposits developed. Unseasonal reproduction in this species was, however, observed in an unseasonal wheat crop. Rhabdomys pumilio bred when cereals were plentiful but was also found breeding when its diet was mainly clover–a crop that is noted for its high crude protein content. The herbivorous Otomys angoniensis bred throughout the year and was unaffected by supplied cereals. It seems clear that the food supply of grani-vorous species is dependent on rainfall, and that prolongation of the normal rainy season may generate excess food and cover, resulting in and extended breeding season and unusually large numbers of these rodents.     

Foraging ecology of the South Australian glossy black‐cockatoo (Calyptorhynchus lathami halmaturinus)

The endangered South Australian glossy black‐cockatoo (Calyptorhynchus lathami halmaturinus Mathews 1912) feeds almost exclusively on the seeds of the drooping sheoak (Allocasuarina verticillata), and shows marked preferences for individual trees. This field study investigated foraging ecology and tree selection through observations of foraging birds and measurements of trees and seed cones. The cockatoos spent the vast majority of their foraging time (94%) handling seed cones, and handling behaviour was highly stereotyped. Handling time per cone was correlated primarily with cone size, while seed intake rate was correlated primarily with seed mass per cone. The cockatoos fed mostly in trees with signs of previous feeding. They tended initially to sample trees with large seeds, and to stay for long feeding bouts in trees with high ratios of seed‐to‐cone mass. As a result of these biases, feeding was concentrated in trees with high seed mass per cone. Preferred trees were also larger, with higher ratios of seed‐to‐cone mass and larger seeds containing more lipid and protein. By feeding from selected trees the cockatoos increased both their seed intake rate and the nutritional quality of the seeds ingested, thereby increasing their energy intake rate by an estimated 28%. They did not discriminate against trees that had re‐grown from basal shoots after fires. Insect larvae were present in some seed cones but the cockatoos did not appear to actively seek them. Males foraged 19% more efficiently than females, resulting in greater daily food intake. The characteristics of individual A. verticillata trees that determined the cockatoos’ feeding rates were also correlated with their distribution on a regional scale. This suggests that the distribution of this endangered cockatoo depends not only on the presence of food trees, but also on their regionally varying feeding profitability.     

The adaptive value of energy storage and capital breeding in seasonal environments

Timing of reproduction in a seasonal cycle is a life history trait with important fitness consequences. Capital breeders produce offspring from stored resources and, by decoupling feeding and reproduction, may bend the constraints caused by seasonality in food or predation. Income breeders, on the other hand, produce offspring from concurrent food intake, with the disadvantage of less flexibility, but with high efficiency and no inventory costs of carrying stores. Here, we assess relative profitability of capital and income breeding in herbivorous zooplankton inhabiting seasonal, high-latitude environments. We apply a state-dependent life history model where reproductive values are used to optimise energy allocation and diapause strategies over the year. Three environmental scenarios were modelled: an early, an intermediate, and a late feeding season. We found that capital breeding was most important in the early season. Capital breeding ranged from 7–9% of the eggs produced but, because of the high reproductive value of early eggs, capital breeding ranged from 9–30% when measured in terms of reproductive value. The main benefit of capital breeding was reproduction prior to the feeding season – when the reproductive value of an egg peaked. In addition, capital breeding was also used to increase egg production rates at times of income breeding. For individuals born late in the season the model predicted a two-year cycle instead of the typical annual life cycle. These individuals could then reap the benefits of early reproduction and capital breeding in their second year instead of income breeding late in the first year. We emphasize the importance of evaluating reproductive strategies such as capital and income breeding from a complete life cycle perspective. In particular, knowing the seasonality in offspring fitness is essential to appreciate evolutionary and population-level consequences of capital breeding.     

Measuring the impact of Leptoglossus occidentalis (Heteroptera: Coreidae) on seed production in lodgepole pine using an antibody-based assay

We measured the impact of Leptoglossus occidentalis on seed production in lodgepole pine, Pinus contorta variety latifolia Engelmann, using an antibody marker developed to detect residual saliva in fed-on seeds. Nymphs, adult females, and adult males were caged on cones during early, mid- and late season cone development. Individual analysis of 12,887 seeds extracted from 365 cones revealed that 37.3% seeds tested positive for seed bug saliva. The antibody assay was 38 times more effective than radiography at detecting seed bug damage. Radiography can detect partially emptied seed but cannot discriminate between aborted seeds and those emptied by seed bugs. The antibody marker was least sensitive in detecting early season damage compared with mid- and late season damage. We hypothesize that residual saliva in seeds fed on early in the season was either absorbed by the damaged seed or degraded over time. Early season feeding resulted in the greatest number of seeds fused to cone scales and the extraction efficiency for cones exposed to feeding during this time was reduced by 64% compared with control cones. Adding fused seeds to antibody-positive seeds raised the proportion of damaged seeds to 48.3%. At all stages of cone development, adult females were the most destructive life stage, damaging up to two seeds per day late in the season. When seed losses were adjusted to damage per degree-day, female damage was greatest early in the season, while males caused the same amount of damage regardless of cone development period. The results of the antibody assay provide baseline data for developing damage prediction formulae, and establish L. occidentalis as a potentially serious pest in lodgepole pine seed orchards.     

Effects of extra food on Peromyscus and Clethrionomys populations in the southern Yukon

Summary If food supply limits density in rodent populations, the addition of supplemental food ought to increase population size. From May to September we added food on 2-hectare areas of white spruce forest in the southern Yukon. In 1977, we used oats with no measurable impact. In 1978 and 1979 we used sunflower seeds and doubled or tripled the population densities of Peromyscus maniculatus and Clethrionomys rutilus. Supplemental feeding with sunflower seeds increased juvenile production in both species, probably by increasing early juvenile survival. The breeding season ended at the same time on experimental and control areas, and the onset of maturity in juveniles was not affected by feeding. Survival of adult rodents was not improved by feeding. Immigration into sunflower seed areas was very high, and was the major factor producing the increase in population density. Body weight was not changed by feeding. Food supply is thus one factor limiting population density in these subarctic small mammals, but we do not know why supplemental food produces no more than a 2- to 3-fold effect on densities.Dedicated to Dr. Springer and Prof. Evenari     

Habitat use, foraging ecology and diet of Turtle Doves Streptopelia turtur in Britain

Since 1968, Turtle Doves Streptopelia turtur in Britain have declined in number by 70% and in breeding range by at least 25%. This study was undertaken during the 1998–2000 breeding seasons at two sites in East Anglia to examine habitat use and foraging ecology. Home ranges of radiotagged Turtle Doves were 0.3–1130 ha in size and contained more woodland than expected from availability. Radiotagged birds undertook foraging trips of up to 10 km. Turtle Doves were recorded feeding primarily at 'man-made' sites (i.e. spilt grain, animal feed and grain stores) and were only infrequently recorded feeding at 'natural' sites. Diet analysis showed that wheat and rape seed averaged 61% of the seeds eaten by Turtle Doves. This contrasts with a study undertaken in Britain in the 1960s, when the seeds of wild plants (weeds) constituted over 90% of those eaten, with wheat and rape seeds making up only 5%. The current arrangements for set-aside and agri-environmental schemes provide a framework for the establishment and maintenance of weed-rich areas on farmland as a source of wild food.     

Feeding ecology of Lilian’s Lovebird Agapornis lilianae in Liwonde National Park,Malawi

Lilian's Lovebird Agapornis lilianae is a small, near-threatened parrot resident in mopane Colophospermum mopane woodlands. We investigated its diet and foraging behaviour in Liwonde National Park, Malawi. We expected that Lilian's Lovebirds would show little specialisation for a particular food source but generally feed on available seeds, fruits, flowers and other items as observed in other lovebirds. Lilian's Lovebirds fed on 30 different plant species. Lilian's Lovebirds were observed feeding in six habitat types in Liwonde National Park and adjacent areas during the wet season, and four in the dry season. In the wet season lovebirds (23% of observations) foraged in grassy wetland (dambo) areas the most, whilst in the dry season they foraged in grasslands with tree cover (18%) the most. In mopane woodland, foraging flock sizes differed significantly between the wet (mean = 19.8 ± 1.0 lovebirds) and dry season (mean = 33.6 ± 2.3 lovebirds). Grass seeds were lovebirds’ main food source from December to June. The nutritional analysis of preferred foods showed that grass seeds have a relatively high protein and energy content. Grass seed availability is reduced with savanna burning and so early season burning (before May–June) in areas in and outside the park is not recommended.     

Variation in the effect of profitability on prey size selection by the lacertid lizard Psammodromus algirus

Maximizing the average rate of energy intake (profitability) may not always be the optimal foraging strategy for ectotherms with relatively low energy requirements. To test this hypothesis, we studied the feeding behaviour of captive insectivorous lizards Psammodromus algirus, and we obtained experimental estimates of prey mass, handling time, profitability, and attack distance for several types of prey. Handling time increased linearly with prey mass and differed significantly among prey types when prey size differences were controlled for, and mean profitabilities differed among prey taxa, but profitability was independent of prey size. The attack distance increased with prey length and with the mobility of prey, but it was unrelated to profitability. Thus, lizards did not seem to take account of the rate of energy intake per second as a proximate cue eliciting predatory behavior. This information was combined with pitfall-trap censuses of prey (in late April, mid-June and late July) that allowed us to compare the mass of the prey captured in the environment with that of the arthropods found in the stomachs of sacrificed free-living lizards. In April, when food abundance was low and lizards were reproducing, profitability had a pronounced effect on size selection and lizards selected prey larger than average from all taxa except the least profitable ones. As the active season progressed, and with a higher availability of food, the number of prey per stomach decreased and their mean ize increased. The effect of profitability on size selection decreased (June) and eventually vanished (July–August). This variation is probably related to seasonal changes in the ecology of lizards, e.g. time minimization in the breeding season as a means of saving time for nonforaging activities versus movement minimization by selecting fewer (but larger) prey in the postbreeding season. Thus, the hypothesis that maximizing profitability could be just an optional strategy for a terrestrial ectothermic vertebrate was supported by our data.     

Impact of Leptoglossus occidentalis (Hemiptera: Coreidae) on Douglas-fir seed production

We investigated the effect of feeding by the western conifer-seed bug, Leptoglossus occidentalis Heidemann, on seed production in developing cones of coastal Douglas-fir, Pseudostuga menziesii (Mirb.) Franco, with respect to seed bug life stage and sex (nymphs, adult females, and adult males) and timing of feeding (early, mid-, and late season cone development). Feeding by females on caged cones for a 2-wk period during late season cone development reduced the proportion of full seeds in cones by approximately 70% compared with caged control cones. There was no significant difference among nymphs, adult females, and adult males with respect to the proportion of empty or partially fed-upon seeds produced during the same feeding period. Feeding by nymphs for 2 wk early in the season resulted in a threefold increase in the number of unextractable seeds fused to cones compared with the control. Weight measurements of harvested seeds indicated that radiography is an accurate tool to distinguish among Douglas-fir seeds that have sustained light, moderate, or severe damage. Determining the full impact of L. occidentalis on conifer seed production will require the development of a reliable method to distinguish between naturally aborted seeds and seeds emptied through feeding by seed bugs.     

Balancing Energy Budget in a Central-Place Forager: Which Habitat to Select in a Heterogeneous Environment?

Foraging animals are influenced by the distribution of food resources and predation risk that both vary in space and time. These constraints likely shape trade-offs involving time, energy, nutrition, and predator avoidance leading to a sequence of locations visited by individuals. According to the marginal-value theorem (MVT), a central-place forager must either increase load size or energy content when foraging farther from their central place. Although such a decision rule has the potential to shape movement and habitat selection patterns, few studies have addressed the mechanisms underlying habitat use at the landscape scale. Our objective was therefore to determine how Ring-billed gulls (Larus delawarensis) select their foraging habitats while nesting in a colony located in a heterogeneous landscape. Based on locations obtained by fine-scale GPS tracking, we used resource selection functions (RSFs) and residence time analyses to identify habitats selected by gulls for foraging during the incubation and brood rearing periods. We then combined this information to gull survey data, feeding rates, stomach contents, and calorimetric analyses to assess potential trade-offs. Throughout the breeding season, gulls selected landfills and transhipment sites that provided higher mean energy intake than agricultural lands or riparian habitats. They used landfills located farther from the colony where no deterrence program had been implemented but avoided those located closer where deterrence measures took place. On the other hand, gulls selected intensively cultured lands located relatively close to the colony during incubation. The number of gulls was then greater in fields covered by bare soil and peaked during soil preparation and seed sowing, which greatly increase food availability. Breeding Ring-billed gulls thus select habitats according to both their foraging profitability and distance from their nest while accounting for predation risk. This supports the predictions of the MVT for central-place foraging over large spatial scales.     

The importance of food quantity and quality for reproductive performance in alpine water pipits (Anthus spinoletta)

Studies relating reproduction to food availability are usually restricted to food quantity, but ignore food quality and the effects of habitat structure on obtaining the food. This is particularly true for insectivorous birds. In this study we relate measures of reproductive success, time of reproduction and nestling size of water pipits (Anthus spinoletta) to biomass, taxonomic composition and nutritional content of available food, and to vegetation structure and distance to feeding sites. Clutch size was positively correlated with the proportion of grass at the feeding sites, which facilitates foraging. This suggests that water pipits adapt their clutch size to environmental conditions. Also, pipits started breeding earlier and produced more fledglings when abundant food and a large proportion of grass were available, probably because these conditions allow the birds to gain more energy in less time. The number of fledglings was positively correlated with the energy content of available food. No significant relationships were found between feeding conditions and nestling size or the time that nestlings took to fledge. This suggests that water pipits do not invest more in individual nestlings when food conditions are favourable but rather start breeding earlier and produce more young. Taxonomic composition and nutritional content of prey were not correlated with any of the reproductive parameters, indicating that profitability rather than quality of food affects reproductive success. Received: 31 May 1996/Accepted: 12 August 1996     

Ecological and behavioral consequences of digestion in frugivorous animals

The postingestional effect of seed size and mass and nutrient composition on fruit profitability are reviewed. It is emphasized that profitability results from the interaction between fruit characteristics and the physiological and morphological traits of frugivores. The processes by which frugivores regurgitate or defecate seeds and the differential processing of the nutrient rich pulp and ballast in fruit are strongly dependent on the interaction between frugivore gut morphology and seed size. Euphonias that lack a functional gizzard defecate seeds, whereas tanagers that have a delete a functional gizzards regurgitate seeds. Some frugivores separate seeds from pulp and exocarp in the gizzard. It is hypothesized that the gizzard plays an important role in determining the postigestional fate of different pulp components. Although fruit nutrient content is often invoked as a determinant of frugivore feeding choices, studies that rely on proximal nutrient analysis, have often failed to find clear nutrient composition-preference correlations. It is argued that a partial reason for this failure is that proximal nutrient analysis ignores the complexities of fruit digestion. The ecological and physiological correlates of lipid and sugar assimilation are used to identify the limitations of traditional proximal nutrient analyses in fruit-frugivore studies. We suggest that recognizing the intricacies of the digestive characteristics of frugivores may reveal a much richer patterning in the interaction of frugivores with plants than has been previously hypothesized.     

Seed-specific expression of a bacterial phosphoenolpyruvate carboxylase in Vicia narbonensis increases protein content and improves carbon economy

An ambitious aim in plant breeding and biotechnology is to increase the protein content of crop seeds used for food and feed. Using an approach to manipulate assimilate partitioning, we succeeded in elevating the protein content in legume seeds up to 50%. Transgenic bean plants were generated which express a Corynebacterium glutamicum phosphoenolpyruvate carboxylase (PEPC) in a seed-specific manner. The bacterial enzyme was not feedback inhibited by malate. Transgenic seeds showed a higher [14C]-CO2 uptake and about a threefold increased incorporation of labelled carbon into proteins. Changed metabolite profiles of maturing cotyledons indicated a shift of metabolic fluxes from sugars/starch into organic acids and free amino acids. These changes were consistent with an increased carbon flow through the anaplerotic pathway catalysed by PEPC. Consequently, transgenic seeds accumulated up to 20% more protein per gram seed dry weight. Additionally, seed dry weight was higher by 20%-30%. We conclude that PEPC in seeds is a promising target for molecular plant breeding.