全文获取类型
收费全文 | 979篇 |
免费 | 41篇 |
国内免费 | 87篇 |
出版年
2024年 | 2篇 |
2023年 | 10篇 |
2022年 | 14篇 |
2021年 | 9篇 |
2020年 | 14篇 |
2019年 | 23篇 |
2018年 | 17篇 |
2017年 | 21篇 |
2016年 | 30篇 |
2015年 | 32篇 |
2014年 | 39篇 |
2013年 | 42篇 |
2012年 | 26篇 |
2011年 | 38篇 |
2010年 | 37篇 |
2009年 | 45篇 |
2008年 | 72篇 |
2007年 | 60篇 |
2006年 | 44篇 |
2005年 | 59篇 |
2004年 | 49篇 |
2003年 | 45篇 |
2002年 | 26篇 |
2001年 | 22篇 |
2000年 | 26篇 |
1999年 | 16篇 |
1998年 | 27篇 |
1997年 | 20篇 |
1996年 | 26篇 |
1995年 | 25篇 |
1994年 | 23篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 16篇 |
1985年 | 18篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1975年 | 2篇 |
1965年 | 1篇 |
1963年 | 1篇 |
排序方式: 共有1107条查询结果,搜索用时 15 毫秒
81.
湖北兴山红嘴相思鸟繁殖生态初报 总被引:3,自引:1,他引:2
2005年4月~2007年8月,在湖北省兴山县龙门河地区,对红嘴相思鸟(Leiothrix lutea)的繁殖生态进行了初步研究。研究结果显示,红嘴相思鸟在此地区的繁殖时间为4月中旬至7月下旬。巢多营于山坡上、山谷中的乔木林、灌木林遮蔽下的箬竹(Indocalamus wilsoni)和小灌木上,也营巢于农田边、山间小路旁、林间空地的箬竹丛或灌丛中。巢材主要有细树枝、细树根、细藤、箬竹叶、茅草叶、细草茎、青苔等。雌雄鸟共同筑巢,筑巢过程较为迅速,一般5~7d即可完成。雌鸟每天产一枚卵,产卵时间一般在清晨。窝卵数3.41±0.80(n=61)。孵卵期11~13d(n=6)。雏鸟晚成性,雌雄共同育雏,育雏期9~10d。雏鸟发育十分迅速,但离巢时身体发育尚未完善。红嘴相思鸟在此地区的繁殖成功率约为22.95%(n=61)。 相似文献
82.
Maca (Lepidium meyenii Walpers), is an Andean crop that grows between 3,800 and 4,500 m a.s.l. The persistent interest in this plant is based on its assumed effects on fertility of male mammals due to the prevalence of certain, partially specific, secondary compounds. The present study aimed at evaluating the effect of maca supplementation on quality and quantity of semen, mating behavior, and clinical status of peripubertal breeding bulls. The experiment followed a cross-over design lasting for 23 wk with 3 wk of adaptation and baseline measurements, and 2 × 10 wk of treatment feeding thus covering two times the complete 8-wk spermatogenic cycle. Seventy-eight 55 wk to 84 wk old breeding bulls received either no maca (control) or maca (233 mg dried hypocotyls/kg body weight/day) for 10 wk followed by 10 wk without maca (maca early) or maca only in the last 10 wk (maca late). Measurements were always made in the last 2 wk of each period. Apart from standard analyses, ejaculates were analyzed by flow cytometry. Data was evaluated by analysis of variance considering the repeated measurement structure of the data. Significant treatment by measurement period indicated direct or carry-over effects of maca. Maca supplementation had no direct effect on body weight, testes circumference, rectal temperature, mating behavior, and ejaculate volume. However, supplementing maca in the first 10 wk period increased the number of sperms in the second 10 wk period, i.e., when the animals no longer received maca. The DNA fragmentation index and the visually assessed motility of the sperms of bulls, that initially showed a borderline sperm quality, were significantly improved with early maca supplementation, while no such effect was observed in the two other groups. No effects occurred in the proportion of intact sperm plasma membranes or acrosomes or both. In conclusion, maca supplementation seems to improve sperm quantity and quality of bulls to a certain degree, while mating behavior appears unaffected. 相似文献
83.
84.
M. G. BETTS D. MITCHELL A. W. DIAMOND J. BÉTY 《The Journal of wildlife management》2007,71(7):2266-2273
ABSTRACT Roadside survey data have been used frequently to assess species occurrence and population trends and to establish conservation priorities. However, most studies using such data assume that samples are representative of either the amount of habitat or its rate of change at larger spatial scales. We tested both of these assumptions for the Breeding Bird Survey (BBS) from 1974 to 2001 in New Brunswick, Canada. Our study focused on mature forest—a cover type that we predicted would be characterized by rapid change due to human activities and that is of high ecological importance. We also sought to determine whether land cover changes adjacent to BBS routes were related to bird population trends detected in BBS data. Within all 3 time periods examined (1970s, 1980s, and 1990s), the amount of mature forest adjacent to BBS routes was significantly lower than in surrounding 1° blocks of latitude and longitude. This could be problematic for studies that use roadside data to compare the relative abundance of species. On average, mature forest declined at a rate of-1.5% per year over the 28-year study period. We detected no significant difference in the rate of change between degree blocks and BBS routes over this time span. However, in the 1970s and 1980s, mature forest declined more rapidly in degree blocks (-2.7%/yr) than adjacent to BBS routes (-0.5/yr). We also found that the BBS trend for a mature forest-associated species, blackburnian warbler (Dendroica fusca), was correlated with the trend in mature forest along BBS routes. This, combined with slower rates of mature forest change along routes in the 1970s and 1980s, suggests that BBS data may have underestimated population declines during this period. It is important that research be conducted to test for potential biases in roadside surveys caused by uneven rates of landscape change, particularly in regions characterized by rapid habitat alteration. 相似文献
85.
The formation of colonies and the evolution of coloniality have been suggested to be a by-product of the use of public information
to select breeding habitats. In this study we performed an experimental test to investigate whether the Lesser Kestrel (Falco naumanni) uses breeding success of conspecifics as a source of public information to select a breeding colony. We considered four
potential cues based on reproductive performance of conspecifics in each colony: mean and variance of breeding success measured
in both all pairs (TRS, total reproductive success), and only successful pairs (PRS, partial reproductive success). Both mean
PRS and mean TRS could be predictive cues of the future reproductive output at a given site, because they varied among colonies
and were autocorrelated from one year to the next. To dissociate any relationship between habitat quality and reproductive
success we manipulated the mean brood sizes by transferring chicks among ten colonies. In five colonies, all the broods with
at least two nestlings were reduced in one chick and in other five colonies all broods were enlarged by one chick. In the
following year, we found that the number of adult immigrant kestrels settled at a colony was only explained by mean PRS in
the previous year. We also examined the settlement of yearlings, which lack of information on breeding success in previous
years. Unlike adult immigrant kestrels, yearlings tended to settle more frequently in those colonies with low mean PRS in
the previous year, probably because of the lower pressure for occupation in those colonies. A multiple regression analysis
showed that the growth of colonies was positively dependent on the mean PRS in the experimental year, and negatively on the
number of predated nests. This study provides a solid support to the hypothesis that colonial species can use mean breeding
success of successful pairs as a source of public information to make settlement decisions. 相似文献
86.
Jakob Augustin Donald Blomqvist Tibor Szép Zoltán D. Szabó Richard H. Wagner 《Journal of Ornithology》2007,148(2):189-198
Genetic parentage studies of socially monogamous birds reveal a widespread prevalence of extra-pair paternity. Variation in
extra-pair paternity among individuals may depend on how different individuals benefit from extra-pair fertilisations and
on the opportunity to pursue extra-pair copulations. A long-term study of sand martins (Riparia riparia) in Hungary allowed us to examine patterns of extra-pair fertilisations in a large colony of over 3,000 breeding pairs with
many known age individuals. We used multi-locus DNA fingerprinting to determine whether extra-pair fertilisations occur when
females are paired to (1) presumably low quality mates, or (2) genetically similar or dissimilar mates, and whether extra-pair
fertilisations result in offspring of higher quality. Extra-paternal young were found in 38% of 47 broods and comprised 19%
of 190 offspring. Males that lost paternity did not differ significantly from others in age or body condition. Social mates
of broods containing extra-pair offspring did not differ in genetic similarity from pairs without extra-pair offspring. Furthermore,
there was no significant difference in body condition between extra-pair young and their maternal half-siblings. We were unable
to assign paternity and therefore cannot exclude the possibility that extra-pair males differed from the within-pair males
they cuckolded, in age, body condition or genetic similarity with the female. We found a positive relationship between paternity
losses and breeding density, suggesting that low breeding density may constrain opportunities for seeking extra-pair copulations. 相似文献
87.
The timing of breeding may not only affect breeding patterns such as the overlap of chick rearing period with the peak in
food availability but also the opportunity for extra-pair mating. A negative relationship has been predicted between extra-pair
paternity and breeding synchrony, assuming that male extra-pair activity is traded against mate guarding and parenting duties.
In contrast, if female ability to assess male quality is temporally constrained, sperm competition might be a positive function
of breeding synchrony. Here we manipulated the progress of nesting by nest material exchange within nesting aggregations to
see whether the timing of breeding affects extra-pair paternity in house sparrows. We found that late broods within nesting
clusters contained extra-pair young more often than early broods, but breeding synchrony did not turn out to be a significant
predictor of extra-pair paternity. Our study indicates that temporal constraints of male extra-pair activity may account for
extra-pair paternity levels, but it is also possible that late-breeding females may accept extra-pair copulations to ensure
egg fertilization. 相似文献
88.
Emiliano Ukmar Corrado Battisti Luca Luiselli Marco A. Bologna 《Biodiversity and Conservation》2007,16(12):3287-3300
Breeding bird communities in burnt and unburnt residual pinewoods were studied over 3 years by line-transect method, following
a catastrophic fire event in Castelfusano (Rome, Central Italy; July 2000). We applied bootstrap procedures to evaluate whether
the observed data were true or just produced by chance, and then examined the emerging patterns at three levels: community,
guild and species levels. At the community level, fire acted on breeding bird communities by altering especially the total
abundance patterns: the species abundance decreased in the burnt pinewood compared to the residual one, but other parameters
were not significantly affected by fire. As a consequence of fire, the destruction and structural simplification of the canopy
and shrubby component, as well as the increase of edge habitat and patchiness at landscape scale, induced a turnover in species
between pinewoods. Species turnover was higher at the burnt than at the residual pinewoods, during all the 3 years of study.
At the guild level, the forest species decreased strongly in terms of richness and abundance in the burnt pinewoods, contrary
to the edge and open habitat species which increased in terms of richness, abundance and evenness. Edge species showed the
highest turnover in burnt pinewood during the whole period of study. At species level, after an a priori subdivision (based
on bibliographic search) of the various species in two ecological guilds (forest versus edge species), it was found that an
a posteriori statistical analysis confirmed the expected trend, i.e. that the species which decreased significantly in burnt
pinewood were essentially the forest species, whereas the species which increased were essentially the edge/open habitat ones.
Overall, in order to investigate the effects of fire catastrophes on birds, the guild approach seems more exhaustive than
the taxonomic community approach, where intrinsic confounding trends are present. 相似文献
89.
A large variety of reproductive patterns is present among alpine plants to ensure the persistence of populations in such harsh environments. In the present study, the role of spontaneous selfing and its contribution to the actual reproductive success of an alpine pioneer plant was investigated. The results showed that Saxifraga aizoides is clearly self-compatible. Open-pollinated flowers exhibited higher seed numbers per capsules than selfing flowers, albeit the difference was not significant. Although seed weight seemed to be independent from the kind of pollination, open-pollinated flowers had a significantly higher proportion of germinated seeds than selfed ones. Furthermore, the ability of fast germination found in S. aizoides enables the seeds to take advantage of all possible opportunities for germination. In summary, S. aizoides exhibits a successful recruitment strategy for an alpine pioneer species. 相似文献
90.
Jesper Madsen Mikkel Tamstorf Marcel Klaassen Nina Eide Christian Glahder Frank Rigét Helene Nyegaard Fred Cottaar 《Polar Biology》2007,30(11):1363-1372
During four breeding seasons, 2003–2006, we studied the relationship between snow cover and nesting performance in pink-footed
geese (Anser brachyrhynchus) in a key breeding site on Svalbard. Snow cover in late May, i.e., at the time of egg laying of geese, was derived from MODIS
satellite images. Snow cover had a profound cascading effect on reproductive output via the number of nesting pairs and timing
of nesting, which affected nest success, while there was only a tendency for a negative effect on clutch size. Hence, we estimated
a five-fold difference in the number of young produced (to post-hatching) between years with little snow and years with high
snow cover. The results from the study area correlated with whole-population productivity estimates recorded in autumn. Thus,
snow cover derived from MODIS satellite images appears to provide a useful indicator of the breeding conditions in the Arctic. 相似文献