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Clutch Size Variation (clutch + size_variation)
Selected AbstractsGeographical differences in seasonal clutch size variation in multi-brooded bird speciesIBIS, Issue 4 2002André A. Dhondt The seasonal pattern of clutch size variation in birds varies among species. In single-brooded bird species clutch sizes decline continuously with date from an early season maximum. In resident multi-brooded species, clutch sizes first increase to a mid-season maximum and then decrease again. Limited data for multi-brooded migratory birds suggest that clutches in these species also show a continuous decline throughout the season, but it remains to be resolved whether this applies generally and whether migration adds a constraint to the system. We here report on a study of the Eastern Bluebird Sialia sialis conducted using data collected by volunteers across most of its range. In the southern part of its range, where Eastern Bluebirds are largely year-round residents, clutches initially increase, reach a mid-season maximum and gradually decline later in the season. In the northern part of the range, where the majority of Eastern Bluebirds are migratory, clutch sizes decline continuously throughout the season. To determine whether seasonal changes in the clutch size of multi-brooded species are determined by migratory behaviour of the population we compared our results with published data on the Dunnock Prunella modularis and conclude that in multi-brooded species migratory behaviour is not a sufficient condition for a continuous seasonal decline in clutch size, but it could be a necessary one. We propose two mechanisms for the pattern observed among multi-brooded migrants: (i) a time or energy cost of migration, and (ii) a more abrupt increase in seasonal resources in spring at more northerly latitudes. [source] No experimental evidence for local competition in the nestling phase as a driving force for density-dependent avian clutch sizeJOURNAL OF ANIMAL ECOLOGY, Issue 4 2009Marion Nicolaus Summary 1In birds, local competition for food between pairs during the nestling phase may affect nestling growth and survival. A decrease in clutch size with an increase in breeding density could be an adaptive response to this competition. To investigate whether breeding density causally affected the clutch size of great tits (Parus major), we manipulated breeding density in three out of eight study plots by increasing nest-box densities. We expected clutch size in these plots to be reduced compared to that in control plots. 2We analysed both the effects of variation in annual mean density (between-year comparisons) and experimental density (within-year comparison between plots) on clutch size variation, the occurrence of second broods and nestling growth. We examined within-female variation in clutch size to determine whether individual responses explain the variation over years. 3Over the 11 years, population breeding density increased (from 0·33 to 0·50 pairs ha,1) while clutch size and the occurrence of second broods decreased (respectively from 10·0 to 8·5 eggs and from 0·39 to 0·05), consistent with a negative density-dependent effect for the whole population. Nestling growth showed a declining but nonsignificant trend over years. 4The decline in population clutch size over years was primarily explained by changes occurring within individuals rather than selective disappearance of individuals laying large clutches. 5Within years, breeding density differed significantly between manipulated plots (0·16 pairs ha,1 vs. 0·77 pairs ha,1) but clutch size, occurrence of second broods and nestling growth were not affected by the experimental treatment, resulting in a discrepancy between the effects of experimental and annual variation in density on reproduction. 6We discuss two hypotheses that could explain this discrepancy: (i) the decline in breeding performance over time was not due to density, but resulted from other, unknown factors. (ii) Density did cause the decline in breeding performance, but this was not due to local competition in the nestling phase. Instead, we suggest that competition acting in a different phase (e.g. before egg laying or after fledgling) was responsible for the density effect on clutch size among years. [source] Food limitation explains most clutch size variation in the Nazca boobyJOURNAL OF ANIMAL ECOLOGY, Issue 4 2001L. D. Clifford Summary 1,Natural selection is expected to optimize clutch size, but intrapopulation variation is maintained in many bird species. The Nazca booby provides a simple system in which to investigate clutch size evolution because clutch size and brood size are decoupled due to obligate siblicide. The indirect effect of brood size on clutch size evolution can therefore be eliminated. 2,In Nazca boobies, second eggs provide insurance against the failure of the first egg or early death of the first hatchling, but approximately half of all females lay only one egg. We tested the hypothesis that one-egg clutches result from food limitation by providing female Nazca boobies with supplemental food. 3,A higher proportion of supplemented females produced two-egg clutches than did control females. Supplemented females produced larger second-laid eggs than did control females, but not first-laid eggs. Laying date and laying interval were not affected. 4,Comparisons of clutch size and egg volume between years indicated that the supplemental feeding experiment was not conducted in a year with a poor natural food supply. Thus supplemented females produced larger clutch sizes despite apparently normal natural food levels. 5,This experiment nearly completes our understanding of clutch size variation in the Nazca booby, and indicates that food limitation and the costs of egg-laying should be considered carefully in studies of clutch size evolution. [source] | ||||||||||