NESTING DENSITY AND BREEDING SUCCESS IN THE HERRING GULL LARUS ARGENTATUS

The relationship between nesting density and breeding success of Herring Gulls Larus argentatus was studied on the Isle of May, Scotland, in 1968. Herring Gulls nesting at the most common density started laying earlier in the season than those nesting at lower or higher densities. Therefore, although the overall spacing of nests was uniform, the nest density of birds laying later in the season progressively approached a random distribution. The onset of laying occurred in synchronized groups within the colony. Late-laid clutches were commonly situated on the periphery of the colony where the density of nests was lowest. When the laying period was divided into four time periods, in each period the tendancy was for birds nesting at the most common density to have the highest clutch-size, hatching and fledging success, and to rear the most chicks per pair to fledging. In addition, birds which spaced their nests most uniformly, presumably as a consequence of territorial behaviour, were the most successful parents.     

SEX RATIO,PARENTAL INVESTMENT,AND INTERPARENT VARIABILITY IN NESTING SUCCESS IN A SOLITARY BEE

Observations of uniquely marked females of the solitary, twig-nesting bee, Osmia bruneri, were conducted under greenhouse conditions to test several predictions of sex-ratio and parental-investment theory. In support of Fisher's (1958) theory, we found that the observed sex-ratio of progeny in this dimorphic species did not differ from that expected on the basis of average male and female weights. Investment patterns also exhibited a seasonal component: female parents produced more female than male offspring early in the nesting season but reversed this pattern later. Interfemale variability was large for all nesting parameters examined. Neither female-parent size nor the rate at which females completed cells was significantly related to several estimates of parent fitness. Parent-offspring heritability for size was also low. We found no evidence to support the hypothesis that progeny sex-ratios are influenced by maternal condition. Variance in progeny sex-ratios was large, but the population sex-ratio probably departs frequently from the equilibrium value. The results marginally support Kolman's (1960) prediction of large variance in progeny sex-ratios in large panmictic populations. We conclude that variability among females in investment patterns and variability in size among progenies are probably maintained by such factors as resource heterogeneity and the shape of the adult survivorship curve.     

VARIABILITY IN VEGETATION EFFECTS ON DENSITY AND NESTING SUCCESS OF GRASSLAND BIRDS

Abstract: The structure of vegetation in grassland systems, unlike that in forest systems, varies dramatically among years on the same sites, and among regions with similar vegetation. The role of this variation in vegetation structure on bird density and nesting success of grassland birds is poorly understood, primarily because few studies have included sufficiently large temporal and spatial scales to capture the variation in vegetation structure, bird density, or nesting success. To date, no large-scale study on grassland birds has been conducted to investigate whether grassland bird density and nesting success respond similarly to changes in vegetation structure. However, reliable management recommendations require investigations into the distribution and nesting success of grassland birds over larger temporal and spatial scales. In addition, studies need to examine whether bird density and nesting success respond similarly to changing environmental conditions. We investigated the effect of vegetation structure on the density and nesting success of 3 grassland-nesting birds: clay-colored sparrow (Spizella pallida), Savannah sparrow (Passerculus sandwichensis), and bobolink (Dolichonyx oryzivorus) in 3 regions of the northern tallgrass prairie in 1998–2001. Few vegetation features influenced the densities of our study species, and each species responded differently to those vegetation variables. We could identify only 1 variable that clearly influenced nesting success of 1 species: clay-colored sparrow nesting success increased with increasing percentage of nest cover from the surrounding vegetation. Because responses of avian density and nesting success to vegetation measures varied among regions, years, and species, land managers at all times need to provide grasslands with different types of vegetation structure. Management guidelines developed from small-scale, short-term studies may lead to misrepresentations of the needs of grassland-nesting birds.     

SELECTIVE PREDATION BY TWO SWIFTS AND A SWALLOW IN CENTRAL AMERICA

Samples of flying insects were made at three sites and during dry and wet seasons in Costa Rica and at two sites during the wet season in Panama. Compared with these were the diets of three aerially foraging birds: the Band-rumped Swift Chaetura spinicauda and Rough-winged Swallow Stelgidopteryx ruficollis in Costa Rica and the Short-tailed Swift Chaetura brachyura in Panama. Among insects flying, Diptera accounted for between 52 and 75%, with smaller numbers of Hymenoptera (mean = 15%) and Coleoptera (10%). In bird stomach contents, however, Hymenoptera predominated (59–81%), Coleoptera were less common (mean = 18.5%) and Diptera least common (3.4%) of the three orders. The discrepancy between diets and samples of flying insects could not be accounted for solely on the basis of differential digestion. The composition of the swift and swallow diets was similar; there were no significant seasonal differences in the taxonomic composition of the diet of Chaetura spinicauda. Size distributions of beetle prey fitted log-normal distributions in every case. Mean size of prey was significantly larger than that of insects available in all comparisons. Although there was no seasonal difference in the mean size of prey of Chaetura spinicauda, the variance in prey size, used as a measure of niche width, was significantly larger in the wet than in the dry season. Although insect densities may be higher, foraging time is reduced and the wet season is likely the worse season. This result therefore supports Emlen's hypothesis of narrower niches under optimal conditions and broader niches under suboptimal conditions. For increasing altitude above ground published data show a decrease in the density of insects flying, a decrease in the proportions of taxa favoured by the birds, and a significant decrease in the mean size of beetles. None of these trends can explain the differences between the birds' diets and the samples of flying insects. Preference of prey by these birds is concluded not to be a function of prey size alone, but to involve interaction among prey size, ease of capture, and local density. Because of different flight abilities, insect taxa will differ in ease of capture and, consequently, in mean size preferred by the predator, as well as proportions in the diet. This factor favours Hymenoptera and Coleoptera over Diptera as prey. Local concentrations of winged ants, bees and social wasps, and fig wasps make these prey differentially easy to exploit and explain the preference of Hymenoptera over Coleoptera. Large species of swifts are hypothesized to be the evolutionary consequence of the tendency of aerially foraging birds to exploit local concentrations of single prey species.     

AGE, EGG-SIZE AND BREEDING SUCCESS IN THE HERRING GULL LARUS ARGENTATUS

The breeding biology of the Herring Gull Larus argentatus was studied in 1969, 1970 and 1972 on Skokholm, Pembrokeshire. The distribution of laying dates in three years is given; median laying date was always about 10 May. Laying dates of individual pairs were fairly constant between two years, but females up to nine years of age apparently advanced their laying date in successive years. In 1969 there was a decline in clutch-volume index during the season but this was not found in 1970 or 1972. Females laid clutches of similar volume in successive years, although the volume did vary with age, increasing up to 7 or 8 years of age and then declining. Hatching success was highest in the earliest of four periods in 1970 (69%) and lowest in the last (51%); in 1972 the corresponding figures were 71% and 63% and the seasonal trend was not significant. Chick mortality was higher in 1970 (69%) than in 1972 (59%); in 1970 there was significant seasonal variation in chick survival but in 1972 there was not. In 1970 chick survival was positively correlated with hatching weight (and hence egg-size) and chick survival was also related to growth which was itself correlated with hatching weight. There were no such correlations in 1972. Mean chick production in 1970 was 0.60 per pair and in 1972 0.70. In both years later breeders were less successful, but the seasonal range of mean success was less in 1972. In the discussion it is shown that some of these differences between years could be due to changes in the age structure of the population and it is suggested that the correlation between egg-size and chick survival in 1970 may be an artefact of the age structure.     

不同繁育系统的榕树雌花的花柱长度和繁殖率比较

多年来, 不同繁育系统的榕树(Ficus spp.)的进化问题引起生物学家们极大的兴趣。前人通过对不同繁育系统榕树雌花的花柱长度、传粉榕小蜂产卵器长度和繁殖率的比较, 推测榕树的雌雄异株是由雌雄同株进化而来的。为验证这一推论, 选取雌雄同株的垂叶榕(Ficus benjamina)和钝叶榕(F. curtipes)以及雌雄异株的斜叶榕(F. tinctoria)和鸡嗉子榕(F. semicordata), 进行了雌花花柱长度、传粉榕小蜂产卵器长度及繁殖率的比较。研究结果显示: 1)两种雌雄同株榕树的传粉榕小蜂(Eupristina koningsbergeri和Eupristina sp.)的产卵器长度, 显著长于两种雌雄异株榕树的传粉榕小蜂(Liporrhopalum gibbosae和Ceratosolen gravelyi)产卵器的长度, 且雌雄同株榕树雌花花柱长度的变异大于雌雄异株雌花花柱长度的变异; 2)两种雌雄同株榕树的雌花花柱长度呈单峰分布, 且花柱长度的变异大于传粉榕小蜂产卵器长度的变异; 两种雌雄异株榕树花序的雌花花柱长度呈双峰分布, 雌花花柱长度的变异也大于传粉榕小蜂产卵器的长度变异; 3)两种雌雄同株榕树的传粉榕小蜂产卵器长度能到达雌花子房的比例更高, 可更有效地产生瘿花, 而雌雄异株榕树产生种子的优势更强。研究表明: 在传粉榕小蜂产卵器长度及花序雌花花柱分布方面, 雌雄异株榕树有着明显的优势, 但在繁殖率方面没有绝对优势。因此, 前人从雌花花柱分布及繁殖率比较而做出榕树进化的推测并不正确。要真正解决榕树的进化问题, 需要多学科、多指标的综合分析和深入研究。     

BREEDING SYSTEM EVOLUTION IN LEAVENWORTHIA: BREEDING SYSTEM VARIATION AND REPRODUCTIVE SUCCESS IN NATURAL POPULATIONS OF LEAVENWORTHIA CRASSA (CRUCIFERAE)

Populations of Leavenworthia crassa (Cruciferae) studied for 3 years exhibited among- and within-population genetic variation for a suite of floral and reproductive traits (flower width, petal length, anther position, ability to set seeds in the absence of pollinators, time to first flowering) associated with breeding system. We used electrophoretic markers to show that a population with small, monomorphically colored flowers with introrse anthers had a significantly lower outcrossing rate (t = 0.03) than did a population with larger, polymorphically colored flowers with extrorse anthers (t = 0.33). In the more-outcrossing population the correlation between higher maternal plant outcrossing rate and the suite of six traits approached significance (P < 0.067), with greater petal size, greater flower width, and reduced ability to set seeds in the absence of pollinators contributing significantly. Plants in selfing populations had a generally higher reproductive success, with a higher number of flowers per plant, a smaller proportion of unfertilized ovules, a smaller proportion of fertilized ovules aborted, a higher rate of fruit set, and overall a larger number of seeds matured than did plants from the more outcrossing populations. Pollen limitation did not appear to account for lower reproductive success in outcrossing populations. Resource limitation did not differ substantially between populations. However, within-ovary patterns of fertilization, abortion, and seed weight were significantly less random in outcrossing populations than in selfing populations, suggesting that differential gamete and embryo success may be responsible for lower reproductive success in outcrossing populations.     

BREEDING SUCCESS AND POPULATION GROWTH IN A COLONY OF HERRING AND LESSER BLACK-BACKED GULLS LARUS ARGENTATUS AND L. FUSCUS

This paper describes the results of investigations into the factors affecting breeding success of the Herring and Lesser Black-backed Gulls Larus argentatus and L. fuscus, in the large colony on Walney Island, northwest Lancashire, between 1962 and 1965. These investigations were concerned with the incubation period, and the first ten days after hatching. The survival of chicks to ten days is 67% in Herring Gulls, and 56% in Lesser Black-backs. Most of these losses occur in the period just after hatching and are due to “cannibalism” by other gulls. This form of predation does not appear to be masking any effects from starvation or disease. The following factors contribute to egg or chick mortality:breeding too late (and, to a much smaller extent, too early); breeding in the open, as opposed to amongst cover; the facts that eggs in small clutches have a lower hatching success than those in large ones and that Herring Gull (but not Lesser Black-back) chicks in small broods are less likely to survive to ten days than are those in large broods. Chick mortality after the first ten days is not certainly known. About 30% of the eggs laid gave rise to fledged young— or about one fledged chick per pair. In the Herring Gull, the average clutch size (2.56) is lower than that of the Lesser Black-back (2.76). Both species show a seasonal decline in clutch size—this occurs earlier in the Herring Gulls than in the Lesser Black-backs. The Walney population, which stood at about 700 pairs in 1950, had reached 12,000 in 1957, and is at present about 18–19,000 pairs. It is suggested that this increase may be linked to the greater availability, or exploitation, of human garbage in the Morecambe Bay area. The population explosion between 1950 and 1957 must have been partly due to massive immigration and could not have come about through natural increase alone. The possible influences of the gulls' behaviour on the population growth are discussed. There is no evidence of any “shock disease”, although the Walney colony is very crowded. “Cannibalism” is regarded, not as evidence of a failing food supply, but as an extension of the normal hunting behaviour of these omnivorous gulls; it will be an economical means of obtaining food only in a large, dense colony, such as Walney. It may be offset by increased breeding efficiency due to social factors.     

CYTOGEOGRAPHY OF PANICUM VIRGATUM IN CENTRAL NORTH AMERICA

Mc Millan , Calvin . (U. Texas, Austin.), and John Weiler . Cytogeography of Panicum virgatum in central North America. Amer. Jour. Bot. 46(8): 590–593. Illus. 1959.—For 124 clones of Panicum virgatum L. representing 44 local populations from Manitoba and eastern Montana to Texas, the chromosome number was determined. Most of the clones were grown in a transplant garden at Lincoln, Nebraska. Among the earlier-flowering clones from the northern and western area, a preponderance were tetraploids (n = 18). Among the later-flowering clones, the Iowa material was predominantly tetraploid, while those from Nebraska presented a diversity of types (n = 18, 27, 36) within each population. A series of polyploids was common in population samples from southern Kansas, Oklahoma, and Missouri. Through the complex breeding patterns resulting from the common occurrence of a range of polyploidy and the possible action of apomixis, variability within the population is maintained, and with it, genetic insurance for survival in highly unpredictable prairie habitats.     

遗鸥繁殖生境选择及其繁殖地湿地鸟类群落研究

本文就遗鸥(Larus relictus)在内蒙古鄂尔多斯高原对其繁殖生境的选择和该生境下所出现的湿地鸟类种类、数量、居留习性等作了记述,并对该地湿地鸟类的区系成份、生境类型与种的生态分布及数量配比关系、伴生种问题及东方白鹳(Ciconia boyciana)在鄂尔多斯的发现等作了分析和探讨。     

BREEDING SUCCESS IN SOME AFRICAN EAGLES RELATED TO THEORIES ABOUT SIBLING AGGRESSION AND ITS EFFECTS

Brown, L. H., Gargett, V. & Steyn, P. 1977. Breeding success in some African Eagles related to theories about sibling aggression and its effects. Ostrich 48:65-71. Previous explanations for fatal inter-sibling strife in eagles (lack of food, extra aggressiveness which enhances survival, and an expression of the innate aggressiveness or territoriality of raptors) can not be supported by recent evidence. The latest theory, that the second egg acts as a “reserve”, is examined. If so, eagles normally laying two eggs should have better reproductive success than eagles laying a single egg. However there was no significant difference between the mean reproductive success of five African eagle species that lay two eggs and that of five African eagle species laying one egg. Even excluding inequalities due to sample size, and other factors, the overall finding is the same. In Verreaux's Eagle Aquila verreauxi for which the largest number of records is available there was a significantly higher total loss in clutches recorded as c/l than in c/2 clutches, but the reasons for the laying of a one-egg clutch, and its success or failure, have not been investigated. In three out of 120 c/2 the second-laid egg acted as a “reserve”, but in 110 cases the second-laid egg was “wasted”. Direct evidence and a comparison of reproductive success between two-egg and one-egg African eagles does not support the latest theory, therefore inter-sibling strife which effectively reduces the clutch of two to one, is still inexplicable.