Postfire forest management and Mediterranean birds: the importance of the logging remnants

Fire is a crucial element needed to understand the biodiversity patterns of forest landscapes in most Mediterranean countries. However, little is known about the quantitative responses of bird communities to postfire forest management in this region, in which the logging of burnt trees is a common practice. Several studies have already described the negative effects of felling burnt trees on birds but none has focussed on the remaining wood remnants. We investigated this question in a large burnt area located in north-east Iberian Peninsula. The amount of logging remnants left on the ground had positive linear and negative quadratic relationships with the indices of bird abundance and bird richness. The results obtained at a species level were similar, since 36% of the most abundant species revealed the same type of relations with logging remnants, whereas none showed an opposite pattern. Thus, birds in general seem to be positively influenced by the amount of wood remnants left on the ground to a certain point, from which the relation reverses. The results of this study indicate that a moderate amount of wood remnants left on the ground may be positive for the overall bird community. We suggest that management plays a role in the recovery of the bird community after fire and, therefore, biodiversity criteria should be incorporated in the guidelines driving postfire actions.     

The effects of fire on communities,guilds and species of breeding birds in burnt and control pinewoods in central Italy

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.     

Effects of a short fire‐return interval on resources and assemblage structure of birds in a tropical savanna

Fire frequency is a key land management issue, particularly in tropical savannas where fire is widely used and fire recurrence times are often short. We used an extended Before‐After‐Control‐Impact design to examine the impacts of repeated wet‐season burning for weed control on bird assemblages in a tropical savanna in north Queensland, Australia. Experimentally replicated fire treatments (unburnt, singularly bunt, twice burnt), in two habitats (riparian and adjacent open woodland), were surveyed over 3 years (1 year before the second burn, 1 year post the second burn, 2 years post the second burn) to examine responses of birds to a rapid recurrence of fire. Following the second burn, species richness and overall bird abundance were lower in the twice‐burnt sites than either the unburnt or singularly burnt sites. Feeding group composition varied across year of survey, but within each year, feeding guilds grouped according to fire treatment. In particular, abundance of frugivores and insectivores was lower in twice‐burnt sites, probably because of the decline of a native shrub that produces fleshy fruits, Carissa ovata. Although broader climatic variability may ultimately determine overall bird assemblages, our results show that a short fire‐return interval will substantially influence bird responses at a local scale. Considering that fire is frequently used as a land management tool, our results emphasize the importance of determining appropriate fire‐free intervals.     

Fire frequency and time‐since‐fire effects on the open‐forest and woodland flora of Girraween National Park,south‐east Queensland,Australia

Abstract The effects of recent fire frequency and time‐since‐fire on plant community composition and species abundance in open‐forest and woodland vegetation in Girraween National Park, south‐east Queensland, Australia, were examined. Cover‐abundance data were collected for shrub and vine species in at least 10 400‐m² plots in each of four study areas. Study areas were within one community type and had burnt most recently either 4 or 9 years previously. Variations in fire frequency allowed us to compare areas that had burnt at least three times in the previous 25 years with less frequently burnt areas, and also woodlands that had experienced a 28‐year interfire interval with more frequently burnt areas. Although species richness did not differ significantly with either time‐since‐fire or fire frequency, both these factors affected community composition, fire frequency being the more powerful. Moisture availability also influenced floristics. Of the 67 species found in five or more plots, six were significantly associated with time‐since‐fire, whereas 11 showed a significant difference between more and less frequently burnt plots in each of the two fire‐frequency variables. Most species, however, did not vary in cover‐abundance with the fire regime parameters examined. Even those species that showed a marked drop in cover‐abundance when exposed to a particular fire regime generally maintained some presence in the community. Five species with the capacity to resprout after fire were considered potentially at risk of local extinction under regimes of frequent fire, whereas two species were relatively uncommon in long‐unburnt areas. Variable fire regimes, which include interfire intervals of at least 15 years, could be necessary for the continuity of all species in the community.     

城市公共绿地常见木本植物组成对鸟类群落的影响

快速城市化背景下, 城市公共绿地已经成为重要的鸟类栖息地, 其中的木本植物群落构成对鸟类群落结构有显著影响, 研究木本植物配置与鸟类多样性的关系对提升城市公共绿地作为鸟类栖息地的生态服务功能有重要的理论和应用价值。我们于2009–2010年间在上海市滨江森林公园就木本植物和鸟类群落的相互关系展开研究。样线法结合样点调查共记录到鸟类10目25科64种5,368只(次), 鸟类多度全年变化显著, 峰值分别出现在4月和11月。丰富度全年也呈现双峰型变化, 峰值出现在4月和12月。全年鸟类多样性(Simpson指数)差异显著, 10月最高, 8月最低。样方调查共记录到77种木本植物, 其中有14种(乔木9种、灌木5种)出现率超过5个样方, 定义为常见种, 其果期主要集中在9月到翌年2月, 其上常见林业致病害虫的发生盛期集中在5–11月。主成分分析显示, 常见木本植物上观察到的鸟类可划分为8个鸟类集团, Spearman秩相关检验显示秋冬季常见木本植物果期与植食性、肉食性、食虫性、杂食性等多个鸟类集团的多度均呈显著正相关。说明在秋冬季鸟类迁徙高峰期不同鸟类集团均能获得丰富的食物资源。就常见木本植物对鸟类群落的重要值进行排序, 结果显示, 大叶杨(Populus lasiocarpa)、枫杨(Pterocarya stenoptera)、香樟(Cinnamomum camphora)等乔木对鸟类重要值较高, 而灌木层对鸟类的重要性整体偏低, 说明滨江森林公园的灌木层作为鸟类栖息地的功能建设尚需加强。根据以上研究结果, 我们提出了提高上海城市公园鸟类多样性的植被配置建议。     

Effects of fire on the bird communities of tropical woodlands and open forests in northern Australia

Birds were censused at 24 plots which had been subjected to 14 years of four experimental fire regimes (fire-exclusion, annual early burn, annual late burn and biennial burn) in Eucalyptus open forest and woodland at Munmarlary, Northern Territory. In addition, short-term responses of birds to fire at a savannah woodland site near Katherine were examined. At this site patches of the study area were burnt in April, June and August 1987, and birds were censused monthly over the period April to October 1987. Many bird species were attracted to areas that had been recently burnt. These were mostly granivorous species, and omnivorous and carnivorous species which fed on the ground. Influxes of these species occurred because fires increased accessibility to food by clearing the ground of its previous dense grass layer. Succession of bird species to long-unburnt areas was relatively limited, although species which fed or nested in the shrubby understorey occurred at greater densities in such areas. Frugivorous birds may become more common in areas protected from fire, thereby driving succession to monsoon vine forests from Eucalyptus forests, though, in terms of current fire regime, this process may be prohibitively slow. The hot fires of the mid to late Dry season may be less beneficial to granivores than the cooler fires typical of the early Dry. Accordingly, recent changes in the fire regime may have contributed to the decline of some bird species.     

Bird community responses to disturbance in a forestry concession in lowland Bolivia

Bird community characteristics of three sites with different levels of disturbance were studied using transect surveys during the dry season in a subtropical humid forest in Bolivia. One area had been unintentionally burned 4 years prior and selectively harvested (DIS) 1 year prior to sampling. A second area had been selectively harvested 1 year prior to sampling and had no recent history of fire (HAR). Species richness, as assessed by species–time curves and rarefaction, was higher in both altered areas than in undisturbed forest (INT). In general, frugivores and omnivores were more abundant in both altered areas compared to intact forest. Canopy frugivores, understory omnivores and multiple-strata omnivores were most abundant in HAR. Canopy frugivores, near-ground insectivores, understory and multiple-strata omnivores were least abundant in INT, although INT had the highest abundances of canopy insectivores and near-ground omnivores. Richness and abundance of widespread species with low habitat specificity was higher in both areas that experienced disturbance compared to intact forest. Differences in bird community structure between disturbed and intact forest at this site are attributed primarily to the addition of widespread species with less narrow habitat requirements, and possibly to changes in the distribution of different food types.     

Fire studies in mallee (Eucalyptus spp.) communities of western New South Wales: The effects of fires applied in different seasons on herbage productivity and their implications for management

The post-fire development of herbaceous understoreys of Tnodia/mallee burnt in different seasons was studied over a 3 year period in south-western New South Wales, Australia. Near Pooncarie, NSW, a 3 year sequence of above-average rainfall immediately following burning in either the spring, autumn or winter resulted in a substantial increase in species richness and herbage dry matter production. Plots burnt in the spring produced significantly more dry matter than either the autumn or the winter-burnt plots but only in the first growing season after burning. Some species such as Haloragis odontocarpa behaved as ephemerals, being abundant in the first post-fire season, particularly after spring fire, before becoming inconspicuous. Short-lived grasses, such as Stipa species, then became relatively abundant except during drought. Other experiments confirmed the potential of fire to increase herbage diversity and productivity when there was adequate rainfall. During drought, post-fire herbage production was low and the dominant perennials, viz. mallee Eucalyptus and Triodia irritans, regenerated at the expense of the herbs. There was a strong negative correlation between combined mallee and Triodia cover and total herbage cover over time. Seedling recruitment of Triodia irritans was significantly higher on plots burnt in the spring at Pooncarie.     

Immediate and short‐term responses of bird and mammal assemblages to a subalpine wildfire in the Snowy Mountains,Australia

Abstract Over 35 days in January–February 2003, wildfires burnt across much of the subalpine/alpine landscape of south‐eastern Australia, including about 70% of the land above 1500 m in the Snowy Mountains. At the time of the fire, studies of two subalpine faunal assemblages were being undertaken. The opportunity to resurvey the assemblages was taken in order to examine the immediate impact of fire in an environment where it is uncommon but predicted to occur increasingly with global warming. A study area in the Whites River Valley, where the number of bird species was counted monthly from 1996 to 2003, and weekly in late winter–spring from 2000 to 2003, was burnt in one fire. As well as the expected decrease in the number of individual birds, the fire resulted in an immediate decrease in the number of bird species, unlike in previously studied montane forest, with only the regularly wintering species plus the olive whistler and the ground‐feeding flame robin remaining. During the post‐winter avian immigration, few regular spring migrants appeared on burnt sites despite their nearby presence on the unburnt side of the valley. Five of six small mammal trapping grids were burnt. As with fires at lower altitudes, there was an immediate reduction in mammal numbers on burnt grids following the fire, but in addition, one species, Mastacomys fuscus, declined further in the ensuing 2 months both on burnt and unburnt sites. Numbers of Antechinus swainsonii and Rattus fuscipes stabilized until autumn/winter when there was a further decline due to the unavailability of subnivean space to allow winter foraging, allied with a concentration of fox predation on areas still carrying populations of small mammals.     

Bird Community Changes in Response to Single and Repeated Fires in a Lowland Tropical Rainforest of Eastern Borneo

Our current understanding of bird community responses to tropical forest fires is limited and strongly geographically biased towards South America. Here we used the circular plot method to carry out complete bird inventories in undisturbed, once burned (1998) and twice burned forests (1983 and 1998) in East Kalimantan (Indonesia). Additionally, environmental variables were measured within a 25 m radius of each plot. Three years after fire the number of birds and bird species were similar for undisturbed and burned forests, but species diversity and turnover were significantly lower in the burned forests. The bird species composition also differed significantly between undisturbed and burned forests, with a strong decline of closed forest preferring bird species accompanied by a strong increase in degraded forest preferring species in burned forests. These differences were strongly related to differences in environmental conditions such as shifts in vegetation cover and layering and differences in ground and understorey vegetation structure. We also found significant shifts in body mass distribution, foraging height and feeding guilds between the bird communities in unburned and burned forests. Surprisingly, repeated burning did not lead to increasing impoverishment of the avifauna, and both once and twice burned forests still contained most of the bird species that were also present in undisturbed forest, even though their densities were considerably lowered.     

Fire effects on the composition of a bird community in an Amazonian savanna (Brazil).

The effects of fire on the composition of a bird community were investigated in an Amazonian savanna near Alter-do-Ch?o, Pará (Brazil). Mist-net captures and visual counts were used to assess species richness and bird abundance pre- and post-fire in an approximately 20 ha area. Visual counts along transects were used to survey birds in an approximately 2000 ha area in a nearby area. Results using the same method of ordination analysis (multidimensional scaling) showed significant effects of fire in the 20 ha and 2000 ha areas and strongly suggest direct effects on bird community composition. However, the effects were different at different spatial scales and/or in different years, indicating that the effects of fire vary spatially and/or temporally. Bird community composition pre-fire was significantly different from that found post-fire. Using multiple regression analysis it was found that the numbers of burned and unburned trees were not significantly related to either bird species richness or bird abundance. Two months after the fire, neither bird species richness nor bird abundance was significantly related to the number of flowering trees (Lafoensia pacari) or fruiting trees (Byrsonima crassifolia). Since fire is an annual event in Alter-do-Ch?o and is becoming frequent in the entire Amazon, bird community composition in affected areas could be constantly changing in time and space.     

Which forest bird species are the main hosts of the tick,Ixodes ricinus,the vector of Borrelia burgdorferi sensu lato,during the breeding season?

Wild birds are important hosts for vector-borne pathogens, especially those borne by ticks. However, few studies have been conducted on the role of different bird species within a community as hosts of vector-borne pathogens. This study addressed individual and species factors that could explain the burden of Ixodes ricinus on forest birds during the reproductive periods of both vectors and hosts. The goal was to identify which bird species contribute the most to the tick population at the community level. Birds were mist-netted on four plots in 2008 and on seven plots in 2009 in two forests (Sénart and Notre Dame, near Paris, France). The dependence of the tick load per bird upon environmental conditions (questing nymph density, year and plot) and on host species traits (species, age, sex, body size, vertical space use, level of innate and acquired immunity) was analysed. Finally, the relative contribution of each bird species to the local dynamics of ticks was estimated, while accounting for their respective abundance. Tick burden differed markedly between bird species and varied according to questing nymph density. Bird species with a high body mass, those that forage low in the vegetation, and those that had a high innate immune response and a high spleen mass were more likely to have a high tick burden. Four species (the Common Blackbird, Turdus merula, the European Robin, Erithacus rubecula, the Song Thrush, Turdus philomelos, and the Winter Wren, Troglodytes troglodytes) hosted more than 90% of the ticks in the local bird community. These species, and particularly T. merula which was host to a high proportion of the nymphs, are likely to contribute significantly to the circulation of pathogens for which they are competent, such as the agent of Lyme borreliosis.     

Using citizen science to measure recolonisation of birds after the Australian 2019–2020 mega-fires

Large and severe fires (‘mega-fires’) are increasing in frequency across the globe, often pushing into ecosystems that have previously had very long fire return intervals. The 2019–2020 Australian bushfire season was one of the most catastrophic fire events on record. Almost 19 million hectares were burnt across the continent displacing and killing unprecedented numbers of native fauna, including bird species. Some bird species are known to thrive in post-fire environments, while others may be absent for an extended period from the firegrounds until there is sufficient ecosystem recovery. To test for systematic patterns in species use of the post-fire environment, we combined citizen science data from eBird with data on sedentism, body size, range size and the specialisation of diet and habitat. Using generalised additive models, we modelled the responses of 76 bird species to the 2019–2020 Australian mega-fires. Twenty-two species decreased in occurrence after the fire; 30 species increased; and no significant effect was found for the remaining 24 species. Furthermore, diet specialists, and birds with smaller body sizes and range sizes were less likely to be found in burnt areas after the fire event compared to before, a result which generates testable hypotheses for recovery from other mega-fires across the globe. Being displaced from the firegrounds for an event of this geographic magnitude may have severe consequences for population dynamics and thus warrant considerable conservation attention in pre-fire planning and in the post-fire aftermath.     

Species‐specific and temporal scale‐dependent responses of birds to drought

Global climate change is increasing the frequency and intensity of weather extremes, including severe droughts in many regions. Drought can impact organisms by inhibiting reproduction, reducing survival and abundance, and forcing range shifts. For birds, considering temporal scale by averaging drought‐related variables over different time lengths (i.e., temporal grains) captures different hydrologic attributes which may uniquely influence food supplies, vegetation greenness/structure, and other factors affecting populations. However, studies examining drought impacts on birds often assess a single temporal grain without considering that different species have different life histories that likely determine the temporal grain of their drought response. Furthermore, while drought is known to influence bird abundance and drive between‐year range shifts, less understood is whether it causes within‐range changes in species distributions. Our objectives were to (a) determine which temporal grain of drought (if any) is most related to bird presence/absence and whether this response is species specific; and (b) assess whether drought alters bird distributions by quantifying probability of local colonization and extinction as a function of drought intensity. We used North American Breeding Bird Survey data collected over 16 years, generalized linear mixed models, and dynamic occupancy models to meet these objectives. Different bird species responded to drought at different temporal grains, with most showing the strongest signal at annual or near‐annual grains. For all drought‐responsive species, increased drought intensity at any temporal grain always correlated with decreased occupancy. Additionally, colonization/extinction analyses indicated that one species, the dickcissel (Spiza americana), is more likely to colonize novel areas within the southern/core portion of its range during drought. Considering drought at different temporal grains, along with hydrologic attributes captured by each grain, may better reveal mechanisms behind drought impacts on birds and other organisms, and therefore improve understanding of how global climate change impacts species and the landscapes they inhabit.     

人类休闲活动对森林鸟类群落无有害影响

我们对西班牙马德里森林深处(未受到人类干扰)、森林小路沿线(间或有徒步旅行者经过)以及森林野餐区(有较持久干扰)的鸟类进行了调查,以查明人类休闲活动对该地区鸟类群落的分布和多度的影响.结合样方法对以上3类区域鸟类群落的总体特征(如:鸟类的多度、物种的丰富度和多样性)、不同鸟类群体的特征(如:鸦科鸟类的多度,营巢或者取食集团的特征)、各鸟种的多度等参数进行了比较.结果表明:小路沿线和野餐区样方内鸟类的多度和物种丰富度比森林深处更高,已查明14种鸟类的多度在人为干扰区域比森林深处高,只有5种鸟类的多度在森林深处更高一些;在树冠层筑巢的鸟类、在树干和地面觅食的鸟类及鸦科鸟类在休闲活动区有更高的多度;地巢性鸟类在休闲区的多度较低,这种差别尤以阔叶落叶林中加明显.而森林小路的鸟类群落结构与森林内部非常相似,说明人类休闲活动对这些林区鸟类群落的影响更多缘自野餐.原因是野餐区常常具有以下特点:具有高大的乔木、中层植被稀疏、有较多的洞和缝隙以及野餐后会遗留下一些垃圾.以上结论与研究的预期目标正好相反,人类的这些休闲活动可能会降低地巢性鸟类的适宜度,但是对大多数鸟的类栖息地没有不利影响.     

A SUMMARY OF THE BIRDS OF WESTMINSTER O.F.S. AND SURROUNDINGS (1953-1955)

Results are presented from point-counts at six sites in the Kalahari in Botswana. Counts were repeated three times: during a dry season following good rains (1991), during the next wet season when rains were far below average, and the following dry season (1992) when the area became drought-stricken. Compared to the wet season, bird numbers decreased during the drought by 37–81% and species by 8–52%; compared to the previous dry season, birds decreased by 5–71% and species by 2–47%. Bird diversity (relative to numbers) tended to increase during the wet season but was little affected by drought, except in the northern Kalahari, where a greater proportion of birds moved out in response to drought. This gave the northern Kalahari the most distinct bird community during a wet cycle, but it became again typically Kalahari during the drought. Thus, the typical Kalahari bird communities expanded their range during drought into the moister periphery. Changes in numbers, most probably resulting from (local) movements were found in many species. Most confirmed earlier reports on their nomadic nature but some, like Red-crested Lophotis ruficrista and Northern Black Afrotis afraoides Korhaan, Chestnut-vented Tit-Babbler Parisoma subcaeruleum, Ant-eating Chat Myrmecocichla formicivora, Tinkling Cisticola Cisticola rufilatus, Black-chested Prinia Prinia flavicans, Marico Flycatcher Bradornis mariquensis and Brown-crowned Tchagra Tchagra australis have not been or are not widely recognised as mobile species.     

北京小龙门森林鸟类群落划分与生态分析

森林鸟类是森林陆栖脊椎动物中种类和数量最丰富的动物类群 ,它们对于促进森林天然更新、传播植物种子及花粉、控制森林害虫和害兽等方面都起着重要作用。关于小龙门林区鸟类群落结构的研究已有报道[1 ] ,但是对于小龙门林区鸟类群落的划分未见有专门报道 ,我们于 1 992年 1 0月～1 993年 2月 ,1 993年 4～ 1 2月 ,1 994年 2～ 8月对北京小龙门林区所有 8条自然沟和 1条公路进行了调查 ,目的在于了解小龙门林区不同林型、海拔等区域的鸟类种类组成和数量状况 ,探讨不同区域的鸟类群落的划分及其影响因素。1　研究地点及方法小龙门林区位于北…     

辽宁老铁山雀形目鸟类秋季迁徙初探

2000~2004年秋在辽宁旅顺老铁山自然保护区,通过鸟类环志和直接观察的方法,对该地区雀形目(Passeriformes)鸟类秋季迁徙规律进行了研究。5年共环志11 040只雀形目鸟类,发现8种保护区新记录种。结果表明,鸟类迁徙高峰大都集中在10月中下旬;气候条件与鸟类迁徙关系密切;不同年份优势种及种群数量均不同;鸟类的迁徙具有一定的顺序性和集群现象,但不同种类迁徙的种群大小又有差别;2004年雀形目鸟类的种类和数量都明显少于前4年,略有下降趋势。     

Wildfire refugia in forests: Severe fire weather and drought mute the influence of topography and fuel age

Wildfire refugia (unburnt patches within large wildfires) are important for the persistence of fire‐sensitive species across forested landscapes globally. A key challenge is to identify the factors that determine the distribution of fire refugia across space and time. In particular, determining the relative influence of climatic and landscape factors is important in order to understand likely changes in the distribution of wildfire refugia under future climates. Here, we examine the relative effect of weather (i.e. fire weather, drought severity) and landscape features (i.e. topography, fuel age, vegetation type) on the occurrence of fire refugia across 26 large wildfires in south‐eastern Australia. Fire weather and drought severity were the primary drivers of the occurrence of fire refugia, moderating the effect of landscape attributes. Unburnt patches rarely occurred under ‘severe’ fire weather, irrespective of drought severity, topography, fuels or vegetation community. The influence of drought severity and landscape factors played out most strongly under ‘moderate’ fire weather. In mesic forests, fire refugia were linked to variables that affect fuel moisture, whereby the occurrence of unburnt patches decreased with increasing drought conditions and were associated with more mesic topographic locations (i.e. gullies, pole‐facing aspects) and vegetation communities (i.e. closed‐forest). In dry forest, the occurrence of refugia was responsive to fuel age, being associated with recently burnt areas (<5 years since fire). Overall, these results show that increased severity of fire weather and increased drought conditions, both predicted under future climate scenarios, are likely to lead to a reduction of wildfire refugia across forests of southern Australia. Protection of topographic areas able to provide long‐term fire refugia will be an important step towards maintaining the ecological integrity of forests under future climate change.     

Heterospecific attraction and food resources in migrants' breeding patch selection in northern boreal forest

We studied experimentally how heterospecific attraction may affect habitat selection of migrant passerine birds in Finnish Lapland. We manipulated the densities of resident tit species (Parus spp.). In four study plots residents were removed before the arrival of the migrants in the first study year, and in four other plots their densities were increased by releasing caught individuals. In the second year the treatments of the areas were reversed, allowing paired comparisons within each plot. We also investigated the relative abundance of arthropods in the study plots by the sweep-net method. This allowed us to estimate the effect of food resources on the abundance of birds. The heterospecific attraction hypothesis predicts that densities of migrant species (especially habitat generalists) would be higher during increased resident density. Results supported this prediction. Densities and number of the most abundant migrant species were significantly higher when resident density was increased than when they were removed. On the species level the redwing (Turdus iliacus) showed the strongest positive response to the increased abundance of tits. Migrant bird abundances seemed not to vary in parallel with relative arthropod abundance, with the exception of the pied flycatcher (Ficedula hypoleuca) which showed a strongly positive correlation with many arthropod groups. The results of the experiment indicate that migrants can use resident tit species as a cue to a profitable breeding patch. The relationship between the abundance of the birds and arthropods suggests that annual changes in food resources during the breeding season probably do not have a very important effect on bird populations in these areas. The results stress the importance of positive interspecific interactions in structuring northern breeding bird communities. Received: 1 September 1997 / Accepted: 22 January 1998