GROWTH RATES OF BIRDS IN THE HUMID NEW WORLD TROPICS

The growth curves of 40 species of lowland neotropical birds were fitted by logistic equation. The birds were mostly from Panama, Trinidad and Surinam. The growth constants of the fitted equations (asymptote A and growth rate K) were compared within and among species, and with previously published data on temperate species. Growth parameters of tropical passerines are about as variable within species as they are within temperate species. In both cases, variation in A and K between broods is greater than it is within broods. Panamanian birds breed during the dry-wet transition and conditions for growth apparently improve as rainfall increases. Asymptotes of growth curves are higher, and mortality within broods lower, as the breeding season progresses. Asynchronous hatching and the reduction of brood-size by selective starvation of young is a prominent phenomenon during the early part of the breeding season. Several instances are reported, however, of young persisting in nests with inadequate feeding and greatly subnormal weights. Slowed development under conditions of poor nutrition may be adaptive in the tropics if periods of low food availability are short and allow the possibility of recovery from undernourishment. As a group, neotropical lowland passerines (30 species) grow 23% more slowly than a sample of 51 temperate passerines. Variation of growth rates among these tropical species is similar to variation among temperate species, and it is related to adult body-size the length of the nestling period. Young of tropical and temperate species attain similar asymptotes, relative to adult body-weight, by the end of the nestling period. Hypotheses are advanced which might explain the slower growth rate of tropical species, and tested to the extent available data permits. (1) Because brood-size can be changed only by adding or removing whole young, changes in growth rate could provide finer adjustment of the energy requirements of the young to the feeding capacity of the parents. This model predicts different means and variances for growth rate within groups of species with different clutch-sizes, predictions not supported by available data. (2) Growth rate is shown to increase the maximum energy requirement of a nestling only if K exceeds some value determined by the energy requirement of the young, growth rate should vary in proportion to the level of basal maintenance metabolism. In a small sample of tropical species, rates of basal metabolism were 25% lower than in a comparable sample of temperate species. These data therefore support the hypothesis, although the cause of the lower metabolic rate of the tropical nestlings is not known. (3) Daily periods of hypothermia could reduce the energy requirement of the young and at the same time reduce their growth rate; but observations of body temperatures of tropical nestlings are contrary to this hypothesis. (4) The short day-length of tropical climates reduces the time during which young can assimilate energy relative to their energy expenditures. This model predicts that tropical nestlings would have less productive energy available, (consistent with their reduced growth rates), but it also predicts that arctic birds should grow faster than temperate species, which is not confirmed by available data. (5) The low nitrogen content of fruits may cause the slow growth of a few strictly frugivorous species (Oilbird and Bearded Bellbird), but among other tropical species growth rate is not correlated with the estimated proportion of fruit in the diet.     

A quantitative analysis of habitat preferences of Woodcock Scolopax rusticola in the breeding season

In Britain, prior to breeding, Woodcock Scolopax rusticola switch from feeding on pastures by night to feeding in woodland by day. Their major food is earthworms (Lumbricidae). In April and May 1985, vegetation structure and composition and soil parameters including earthworm numbers (30 habitat variables) were recorded for quadrats containing 50 feeding locations of six radio-tagged birds, seven nests and 50 random locations in Whitwell Wood, northeast Derbyshire.
Significantly different mean values between feeding and random sites were found for 11 habitat variables. Feeding sites were in younger stands, with a higher percentage ground cover of dog's mercury Mercurialisperennis and consistently high values for pH. Earthworm biomass was on average 82% greater than in random plots. Areas of beech Fagus sylvaticus were avoided. A discriminant function based on six habitat variables correctly classified 85-5% of the feeding and random sites; 70% of the feeding locations were correctly classified by a jack-knife procedure. The important factors determining where feeding occurs are probably safety from avian predators and high availability of earthworms.
Discriminant analysis of the habitats used for feeding by solitary birds, broods and for nesting produced significant functions which correctly classified 84% of sites. Nests were in areas with a high percentage cover of brambles Rubus spp. and more open ground vegetation. Broods and solitary birds used similar areas characterized by denser ground vegetation than nest sites.
The results of this study indicate that both habitat structure and food availability influence the distribution of Woodcock in the breeding season.     

GRASSLAND BIRD USE OF RIPARIAN FILTER STRIPS IN SOUTHEAST IOWA

Abstract: The U.S. Department of Agriculture (USDA) under its Continuous Enrollment Conservation Reserve Program (CRP) has actively promoted establishment of conservation buffers. Although these programs are intended to benefit wildlife in addition to protecting soil and water resources, benefits to grassland birds may be compromised by narrow widths, presence of woody vegetation, and high predation pressure. During 2001 and 2002, we surveyed breeding grassland birds and searched for nests in 33 CRP filter strips that varied in planting mixture (cool-season vs. warm-season grasses) and adjacent edge type (non-wooded vs. wooded). The most abundant species in filter strips were red-winged blackbird (Agelaius phoeniceus), dickcissel (Spiza americana), song sparrow (Melospiza melodia), and common yellowthroat (Geothlypis trichas). Relative abundances of birds and nests were similar between cool-season and warm-season planting mixtures. Dickcissels and red-winged blackbirds and their nests were relatively less abundant at wooded than non-wooded sites. Our nest success estimates generally were low in all treatments, and nest success varied little with the variables we studied. Predation was the major cause of nest failure; 62% of all nests were depredated. Although the most common birds using filter strips are generalists, filter strips also have potential to provide breeding habitat for some species of management concern.     

An Experimental Study of Chick Provisioning in the Cooperatively Breeding Acorn Woodpecker

Acorn woodpeckers (Melanerpes formicivorus) are cooperative breeders in which groups consist of a variable number of cobreeding males, joint‐nesting females, and non‐breeding helpers of both sexes that are offspring from prior nests. We temporarily manipulated brood size of nests to determine the feeding response of birds in relation to their status (breeder or non‐breeding helper) and sex. All categories of birds responded similarly to brood size increases, adjusting their feeding rate upwards so as to maintain approximately the same per‐nestling feeding rate. Breeders, however, exhibited more flexibility with respect to brood size reductions, decreasing their feeding rate while helpers did not. This suggests that the ‘feeding rules’ of helpers are less flexible than those of breeders, a result not previously detected in other cooperative breeders that have been studied to date. Particularly surprising was the finding that helpers maintain their feeding rates when brood demand is decreased rather than when it was increased, suggesting that the flexibility they exhibit is not a result of birds using the opportunity afforded by reduced brood demand to engage in other less cooperative activities.     

Breeding habitat and nest‐site selection by an obligatory “nest‐cleptoparasite”, the Amur Falcon Falco amurensis

The selection of a nest site is crucial for successful reproduction of birds. Animals which re‐use or occupy nest sites constructed by other species often have limited choice. Little is known about the criteria of nest‐stealing species to choose suitable nesting sites and habitats. Here, we analyze breeding‐site selection of an obligatory “nest‐cleptoparasite”, the Amur Falcon Falco amurensis. We collected data on nest sites at Muraviovka Park in the Russian Far East, where the species breeds exclusively in nests of the Eurasian Magpie Pica pica. We sampled 117 Eurasian Magpie nests, 38 of which were occupied by Amur Falcons. Nest‐specific variables were assessed, and a recently developed habitat classification map was used to derive landscape metrics. We found that Amur Falcons chose a wide range of nesting sites, but significantly preferred nests with a domed roof. Breeding pairs of Eurasian Hobby Falco subbuteo and Eurasian Magpie were often found to breed near the nest in about the same distance as neighboring Amur Falcon pairs. Additionally, the occurrence of the species was positively associated with bare soil cover, forest cover, and shrub patches within their home range and negatively with the distance to wetlands. Areas of wetlands and fallow land might be used for foraging since Amur Falcons mostly depend on an insect diet. Additionally, we found that rarely burned habitats were preferred. Overall, the effect of landscape variables on the choice of actual nest sites appeared to be rather small. We used different classification methods to predict the probability of occurrence, of which the Random forest method showed the highest accuracy. The areas determined as suitable habitat showed a high concordance with the actual nest locations. We conclude that Amur Falcons prefer to occupy newly built (domed) nests to ensure high nest quality, as well as nests surrounded by available feeding habitats.     

Influence of olfactory and visual cover on nest site selection and nest success for grassland‐nesting birds

Habitat selection by animals is influenced by and mitigates the effects of predation and environmental extremes. For birds, nest site selection is crucial to offspring production because nests are exposed to extreme weather and predation pressure. Predators that forage using olfaction often dominate nest predator communities; therefore, factors that influence olfactory detection (e.g., airflow and weather variables, including turbulence and moisture) should influence nest site selection and survival. However, few studies have assessed the importance of olfactory cover for habitat selection and survival. We assessed whether ground‐nesting birds select nest sites based on visual and/or olfactory cover. Additionally, we assessed the importance of visual cover and airflow and weather variables associated with olfactory cover in influencing nest survival. In managed grasslands in Oklahoma, USA, we monitored nests of Northern Bobwhite (Colinus virginianus), Eastern Meadowlark (Sturnella magna), and Grasshopper Sparrow (Ammodramus savannarum) during 2015 and 2016. To assess nest site selection, we compared cover variables between nests and random points. To assess factors influencing nest survival, we used visual cover and olfactory‐related measurements (i.e., airflow and weather variables) to model daily nest survival. For nest site selection, nest sites had greater overhead visual cover than random points, but no other significant differences were found. Weather variables hypothesized to influence olfactory detection, specifically precipitation and relative humidity, were the best predictors of and were positively related to daily nest survival. Selection for overhead cover likely contributed to mitigation of thermal extremes and possibly reduced detectability of nests. For daily nest survival, we hypothesize that major nest predators focused on prey other than the monitored species’ nests during high moisture conditions, thus increasing nest survival on these days. Our study highlights how mechanistic approaches to studying cover informs which dimensions are perceived and selected by animals and which dimensions confer fitness‐related benefits.     

The importance of microclimate variation in determining size,growth and survival of avian offspring: experimental evidence from a cavity nesting passerine

Organisms are expected to balance energy allocation in such a way that fitness is maximized. While much research has focussed on allocation strategies of reproducing parents, in particular birds, relatively little attention has been paid to how nestlings allocate energy while in the nest. Nestling birds are faced with a trade-off between devoting energy to growth or to thermoregulation, and in altricial species it is likely that the thermal environment of the nest site influences the nature of this trade-off. Here, we experimentally investigate how altering the microclimate of nests affects the growth, size and survival, as well as cell-mediated immune (CMI) response, of nestling tree swallows (Tachycineta bicolor) in a temperate environment. We place air-activated heating pads in nests of swallows when young were between 4 days and 16 days of age, and compared performance of offspring to control nests. Our manipulation raised temperatures of heated nests by approximately 5°C compared to control nests. Offspring in heated nests had enhanced survival while in the nest, and we also found that they were heavier and had longer ninth primary feathers at 16 days of age. In addition, heating nest boxes resulted in significantly faster growth of primaries, and there was a trend for growth rates of mass to also be higher in heated nests. There were no significant differences between heated and control nests in growth rate or size of tarsus at age 16 days, and we speculate that this lack of response to elevated nest temperatures may be due to growth of skeletal structures being limited by other factors such as calcium availability. We also found no difference between heated and control nests in CMI response. Nonetheless, our results show overall that increasing temperatures of nests has significant benefits that enhance the fitness of offspring. As provisioning rates to offspring did not differ between heated and control nests, we suspect that the beneficial effects of heating were not the consequence of changes in parental behaviour. Our results provide insight into factors, other than food supply, that have important consequences in determining reproductive success of birds breeding in temperate environments.     

Meat‐eating enhances larval development of Anthracophora rusticola Burmeister (Coleoptera: Scarabaeidae), which breeds in bird nests

The scarab beetle Anthracophora rusticola Burmeister breeds in the nests of carnivorous birds including the honey buzzard Pernis apivorus (Linnaeus), Chinese goshawk Accipiter soloensis (Horsfield), oriental stork Ciconia boyciana Swinhoe and great cormorant Phalacrocorax carbo (Linnaeus). Although larvae of the beetle probably feed on animal matter (e.g. remnants of birds' food) in the nests of these birds, the effect of eating flesh on the growth of larvae has not been investigated. Laboratory experiments clearly showed that the development was faster, survival rate higher and body heavier just before pupation in larvae fed dried chicken meat with humus soil than in those fed only humus soil. We also investigated feeding behaviors of larvae of the beetle. A piece of chicken meat placed on the surface of humus soil was pulled under the soil by the larva, mostly at night. Video recordings made during the night showed that larvae came to the humus soil surface and ate the dried chicken meat or tried to take the meat under the soil. Our laboratory observations suggest that larvae in nature stay under the humus accumulated in bird nests during the daytime, and then come up to the surface mostly at night to actively seek and eat remnants of the birds' food.     

Provision of supplementary food for wild birds may increase the risk of local nest predation

In countries such as the UK, USA and Australia, approximately half of all households provide supplementary food for wild birds, making this the public's most common form of active engagement with nature. Year‐round supplementary feeding is currently encouraged by major conservation charities in the UK as it is thought to be of benefit to bird conservation. However, little is understood about how the provision of supplementary food affects the behaviour and ecology of target and non‐target species. Given the scale of supplementary feeding, any negative effects may have important implications for conservation. Potential nest predators are abundant in urban areas and some species frequently visit supplementary feeding stations. We assess whether providing supplementary food affects the likelihood of nest predation in the vicinity of the feeder, by acting as a point attractant for potential nest predators. We provided feeding stations (empty, peanut feeder, peanut feeder with guard to exclude potential nest predators) in an area of suburban parkland in the UK and monitored the predation rate of eggs placed in artificial nests located at distances that replicated the size of typical suburban gardens. Nest predators (Magpies Pica pica, Grey Squirrels Sciurus carolinensis) were frequent visitors to filled feeders, and predation caused by Magpies, European Jays Garrulus glandarius and Grey Squirrels was significantly higher when nests were adjacent to filled feeders. The presence of a feeder guard did not significantly reduce nest predation. As supplementary feeding is becoming increasingly common during the breeding season in suburban habitats, we suggest that providing point attractants to nest predators at this time may have previously unconsidered consequences for the breeding success of urban birds.     

Behavioral responses of nesting birds to human disturbance along recreational trails

ABSTRACT The presence of recreational trails can impact breeding birds either indirectly by altering habitat or the movement patterns of predators or directly if the presence of humans disturbs birds. We examined the behavioral responses of nesting female Northern Cardinals (Cardinalis cardinalis) to human disturbance using both experimental and observational approaches. From April to August 2006 and 2007, we monitored Northern Cardinal nests in 18 riparian forests in Ohio, USA. Two experimental trials were conducted at each nest (N= 63), with Flight Initiation Distance (FID, the distance at which a bird flushed from the nest) recorded as we approached nests by walking directly toward them and by walking along trails located variable distances from nests. We also measured flight initiation distance (FID) when nests were approached during routine nest checks (N= 160). Cardinals were six times more likely to flush when nests were approached directly, and females on higher nests were less likely to flush regardless of distance to trail. FID was not significantly influenced by the distance of nests from trails. We found no association between nest survival and the tendency of birds to flush. Rather, nest survival was best explained by nest height. Thus, our findings suggest that the responses of birds to human use of recreational trails have only short‐term effects, with no apparent effects of on nest survival. Because the reaction of birds to humans in our study depended on how nests were approached, studies where FID is used as an indicator of sensitivity to human disturbance and is determined by direct approaches may overestimate the potential impact of trails on nesting birds.     

Egg Eviction Imposes a Recoverable Cost of Virulence in Chicks of a Brood Parasite

Background

Chicks of virulent brood parasitic birds eliminate their nestmates and avoid costly competition for foster parental care. Yet, efforts to evict nest contents by the blind and naked common cuckoo Cuculus canorus hatchling are counterintuitive as both adult parasites and large older cuckoo chicks appear to be better suited to tossing the eggs and young of the foster parents.

Methodology/Principal Findings

Here we show experimentally that egg tossing imposed a recoverable growth cost of mass gain in common cuckoo chicks during the nestling period in nests of great reed warbler Acrocephalus arundinaceus hosts. Growth rates of skeletal traits and morphological variables involved in the solicitation of foster parental care remained similar between evictor and non-evictor chicks throughout development. We also detected no increase in predation rates for evicting nests, suggesting that egg tossing behavior by common cuckoo hatchlings does not increase the conspicuousness of nests.

Conclusion

The temporary growth cost of egg eviction by common cuckoo hatchlings is the result of constraints imposed by rejecter host adults and competitive nestmates on the timing and mechanism of parasite virulence.     

Arthropods in the nests of lesser spotted eagle (Aquila pomarina)

In 2001–2007, altogether 57 nests of lesser spotted eagle were collected in the Orava region in northwestern Slovakia and four groups of arthropods were extracted from them. Richest in number of species and individuals were mites (23 species, 17,500 ind.), followed by beetles (12 species, 725 ind.), whereas pseudoscorpions were represented only by Pselaphochernes scorpioides (39 ind.) and fleas by Ceratophyllus garei (3 ind.). Unlike nests of other birds, free-living mites predominated in the nests fauna (83% of individuals), followed by nidicolous species with more or less free relationship to the nests, while parasitic species represented only a negligible part of the fauna. For the first time we observed phoresy of Nenteria pandioni, a specific and abundant mite in the eagles’ nests, on the nidicolous staphylinid Haploglossa puncticollis. The beetle fauna in the nests was much poorer than in nests of other birds. The predatory H. puncticollis was dominant in the nests (83%) and occurred continuously during the whole investigation period. Other beetles, even the widely distributed nidicols such as the histerid Gnathoncus buyssoni, were found rarely in nests. Predators were also the only abundant trophic group of beetles in the nests, while other trophic groups of beetles abundantly co-occur in nests of majority of other birds. The occurrence of all beetles was very unevenly distributed during the investigation period, but was positively correlated with occurrence of mites. The relatively low number of species and individuals of mites and beetles in the lesser spotted eagle nests resulted from their position on tree tops, at a height of 20–30 m and their quick drying out by sun and wind. It was also indicated by an enormously low number of species and individuals of mycetophagous beetles, which represent a significant component of the fauna in nests of other birds.     

GRASSLAND BIRDS NESTING IN HAYLANDS OF SOUTHERN SASKATCHEWAN: LANDSCAPE INFLUENCES AND CONSERVATION PRIORITIES

Abstract: To determine the benefits to grassland birds of converting cropland to hayland in southern Saskatchewan, Canada, we quantified the relative nest abundance and success of grassland nesting birds in haylands and the influence landscape variables have on these parameters. We found nests of 26 species of grassland nesting birds, primarily waterfowl and vesper sparrow (Pooecetes gramineus). With the exception of the northern pintail (Anas acuta), few nesting attempts were recorded for species of high priority in the Prairie Pothole Bird Conservation Region. Mayfield nest success for all waterfowl (20 and 13% in 1999 and 2000, respectively) was high relative to previously reported nest success estimates in other habitat types—especially spring-seeded cropland—and was near levels thought to be required to sustain populations (15–20%). Vesper sparrow nest success (39 and 33% in 1999 and 2000, respectively) also was high relative to that reported in other studies. Haying destroyed few nests as wet weather delayed operations in 1999 and 2000. More nests may be destroyed by haying in other years as approximately 25% of nests in this study were still active on the long-term average haying date for southern Saskatchewan. Among models we developed to explain waterfowl relative nest abundance, amount of cropland in the surrounding landscape and field area were the most informative. Evidence that a specific set of landscape variables was important to models of waterfowl nest success was equivocal. Landscape variables did not explain variation in vesper sparrow relative nest abundance or nest success. Within our study area, conversion of cropland to hayland appears to provide significant benefits to a variety of grassland species, including some species of high conservation priority (e.g., northern pintail). Grassland species of conservation concern nested less frequently in hayland than in native grassland.     

Mesostigmatic mites (Acari: Mesostigmata) in nests of the Eurasian griffon vulture (<Emphasis Type="Italic">Gyps fulvus</Emphasis>) in Croatia

We examined the species composition and community structure of mites of the order Mesostigmata (Acari) in nests of the Eurasian griffon vulture (Gyps fulvus Hablizl, 1783) in Croatia. Material collected from 18 nests included 565 mites belonging to seven species. The most abundant species were Leiodinychus orbicularis (C.L. Koch, 1839) (Trematuridae) and Androlaelaps casalis (Berlese, 1887) (Laelapidae). The results were compared with the community structure and frequency of dominant species of Mesostigmata in nests of 32 other bird species. Leiodinychus orbicularis occurred in the nests of 13 species of birds. It is a typical nidicolous species which occurs most frequently in the perennial nests of birds of prey. In contrast, A. casalis rarely occurs in the nests of birds of prey.     

震旦鸦雀在镶嵌型芦苇收割生境中的巢址选择

大规模的芦苇收割会对在该生境中繁殖的鸟类产生严重影响。为降低芦苇收割对鸟类繁殖的负面影响,许多地区的湿地管理者逐渐采用了镶嵌型的芦苇收割方式。但是,这种新的芦苇管理方式对鸟类繁殖期生境选择的作用机理尚不清楚。震旦鸦雀是一种主要分布在我国东部、完全依赖芦苇生境而生存的珍稀鸟类,已被IUCN定为全球性近危物种。大规模的芦苇收割导致其适宜生境丧失,被认为是该物种面临的一个主要威胁。选择山东黄河三角洲国家级自然保护区为研究地区,采用AIC信息准则法,从巢址微生境、巢域斑块和全模型3个水平上对可能影响震旦鸦雀巢址选择的生境因子进行了逻辑斯蒂回归分析,并重点探讨芦苇镶嵌收割对其巢址选择的影响以及不同水平的相互效应。结果发现,震旦鸦雀的巢址选择受巢址微生境和巢域斑块尺度双重水平的影响,但巢址微生境因子的影响要大于巢域斑块因子。在巢址微生境水平上,影响震旦鸦雀巢址选择的因素有:1.5—2.5 m旧芦苇的密度、2.5 m以上新芦苇的密度、香蒲密度和巢的可见度;在巢域斑块水平上,影响因素包括芦苇生境类型、旧芦苇斑块面积的比例和植被盖度;在综合模型中,影响因素有1.5—2.5 m旧芦苇的密度、2.5 m以上新芦苇的密度、香蒲密度、巢的可见度和植被盖度。综上,震旦鸦雀倾向于在植被盖度较高、具有一定比例(18.3%)旧芦苇面积、1.5—2.5 m旧芦苇密度较高、可见度较低的生境斑块中筑巢。研究中,78.7%(n=141)的震旦鸦雀将巢筑在新旧芦苇混合的微生境中,其中36.9%的巢址附近旧芦苇的比例在一半以上;53.2%(n=111)的巢筑于新、旧两种芦苇茎秆之上,22.3%的巢的支撑芦苇中旧芦苇所占比例在一半以上。研究结果表明,旧芦苇在震旦鸦雀巢址选择和筑巢过程中发挥了重要作用。由于大范围的芦苇收割在微生境水平上可降低1.5—2.5 m旧芦苇的密度,在巢域斑块尺度上可降低旧芦苇斑块面积的比例,因此建议,在震旦鸦雀的繁殖地,未来的芦苇收割应采用镶嵌收割的形式,并至少应保留面积在20%以上的结构较好的旧芦苇斑块,以供震旦鸦雀选择巢址和繁衍后代。     

Nest-site preference and maternal effects on offspring growth

Maternal preferences for oviposition sites are assumed to beadaptive, but offspring fitness is not always higher at preferredsites and, thus, further study of the selection pressures thatinfluence oviposition behavior is warranted. Among birds, predationis regarded as the primary agent of selection on nest-site microhabitatpreferences, but alternatives are rarely considered. We testedthe hypothesis that avian nest-site preferences are an adaptiveresponse to fitness costs imposed by variation in nest-sitemicroclimate. We documented that Chestnut-collared Longspurs(Calcarius ornatus) strongly preferred to orient nests towardsthe southeast and showed that this preference influenced microclimate:nests facing southeast had the highest midday temperatures.Yet, preferences were not adaptive because nestlings in nestswith the preferred orientation gained mass at a slower rate,had retarded skeletal growth, and reached a smaller final size.We experimentally tested this result by altering orientationof nests and confirmed, for the first time, that variation innestling growth was causally linked to variation in nest microclimatearising from nest-orientation preferences. Adults respondedto the high temperatures at preferred southeast-facing nestsby spending more time shading young from the sun, apparentlyattempting to ameliorate heat costs. This response, however,resulted in parents spending less time feeding young, potentiallyexplaining slower growth in these nests. Direct effects of highertemperatures may also play a role in slower growth. Althoughwe lack an explanation for this apparently maladaptive preference,these results demonstrate that nest-site choices of birds canyield fitness costs imposed by variation in nest microclimate.     

Brood reduction in temperate and sub-tropical ospreys

Summary In an effort to understand patterns and causes of nestling loss in Ospreys (Pandion haliaetus), I studied brood reduction in three eastern U.S. Osprey colonies during 1978 and 1979. The colonies, located in Florida Bay (1) and on coastal Long Island, N.Y. (2), differed in the average daily amount of food delivered to nestlings; Florida nests received 43% and 11% less fish per day than nests in the two N.Y. colonies, largely because latitude and season restricted day length and thus foraging time for the winter-breeding Florida Ospreys. Increased distance from stable food sources accounted for the lower rate of feeding at one of the N.Y. colonies. Variation in clutch size in the three colonies reflected differences in latitude more than in food availability; average clutch sizes in Long Island were larger than Florida clutches by 0.5 of an egg, but were similar to each other and to those in other northeastern U.S. Osprey populations.Increased nestling loss coincided with reduced food delivery rates and, in food stressed colonies, this loss was 2–3 times greater than any recorded for Ospreys. Starvation was the primary cause of nestling death, with mortality concentrated on third chicks, which hatched on average 3.9 d later and from eggs 5.6% smaller than chicks hatching first. Sibling aggression accounted for the preferential feeding of older nestmates,but only in colonies or nests where food was limited. Aggressive chicks nearly always stopped fighting after being fed. This behavior provided a reversible mechanism for controling brood reduction that was based on nutrition. Growth rates of young measured during the first half of the growth period were more variable between colonies than within nests. This is interpreted as reflecting both the differences in colony food delivery rates as well as the evolutionary pressures of sibling competition to equalize the growth of nestmates.     

Ectoparasitism in marsh tits: costs and functional explanations

Among hole-nesting birds, the blood-sucking hen flea is a commonparasite affecting both nestlings and parents. By adding fleasto marsh tit (Parus palustris) nests, I aimed to investigatethe effect of fleas on nestling growth rate and parental effortas well as evaluating a potential mechanism by which fleas affectnestlings (i.e., resting metabolic rate; RMR). Nestlings fromflea-infested broods were lighter than nestlings from controlbroods. This reduced growth rate was evident as soon as 3 daysafter adding extra fleas to the nest, but the reduction didnot increase after this initial drop in mass. Parents did notalter their feeding frequency in response to the manipulation;thus the small size of nestlings in manipulated nests seemsto be directly caused by the fleas. Mass-specific RMR was significantlyhigher in nestlings from flea-infested nests compared to nestlingsfrom control nests. I used the results to evaluate the suggestedmechanisms for parasite-related decrease in host growth rate.The increase in RMR and the very rapid reduction in nestlinggrowth rate after experimental addition of fleas can be explainedby an immune reaction, mainly by the innate immune system, tosubstances in the saliva of the fleas.     

Effects of Edge Habitat and Nest Characteristics on Depredation of Artificial Nests in Fragmented Australian Tropical Rainforest

Variation in nest predation levels associated with rainforest fragmentation (edge effects) was assessed in Australia's Wet Tropics bioregion. Artificial nests were placed in the forest understorey at seven edge sites where continuous forest adjoined pasture, seven interiors (about 1 km from the edge), and six linear riparian forest remnants (50–100 m wide) that were connected to continuous forest. Four nest types were also compared, representing different combinations of two factors; height (ground, shrub) and shape (open, domed). At each site, four nests of each type, containing one quail egg and two model plasticine eggs, were interspersed about 15 m apart within a 160 m transect during September–October 2001. Predators were identified from marks on the plasticine eggs. The overall depredation rate was 66.5% of 320 nests' contents damaged over a three-day period. Large rodents, especially the rat Uromys caudimaculatus, and birds, especially the spotted catbird Ailuroedus melanotis, were the main predators. Mammals comprised 56.5% and birds 31.0% of predators, with 12.5% of unknown identity. The depredation rate did not vary among site-types, or between open and domed nests, and there were no statistically significant interactions. Nest height strongly affected depredation rates by particular types of predator; depredation rates by mammals were highest at ground nests, whereas attacks by birds were most frequent at shrub nests. These effects counterbalanced so that overall there was little net effect of nest height. Mammals accounted for 78.4% of depredated ground nests and birds for at least 47.4% of shrub nests (and possibly up to 70.1%). The main predators were species characteristic of rainforest, rather than habitat generalists, open-country or edge specialists. For birds that nest in the tropical rainforest understorey of the study region, it is unlikely that edges and linear remnants presently function as ecological population sinks due to mortality associated with increased nest predation.     

A SOCIO-ECOLOGY OF MAGPIES PICA PICA

The behaviour and ecology of unmarked Magpies Pica pica living in the Cotswolds were investigated over 18 months. The spacing of nests was significantly more regular than random and breeding pairs defended territories throughout the year. In winter pairs occasionally left territories to feed with non-breeding birds which flocked throughout the year.
Magpies in flocks had a higher averiage feeding rate than birds alone or in pairs. Aggression rates in flocks increased as birds fed closer together and as food patches became richer.
Agonistic interactions between Crows Corvus corone and Magpies were common; Crows frequently dominated Magpies in disputes over food, and Crows also ate Magpie eggs and young. Single Magpies were most often chased by Crows while Magpies in flocks were able to feed longer in the presence of Crows.
Differences in fledgling success among breeding pairs were related to the location of nests; Magpie pairs breeding near Crow nests suffered higher rates of agonistic encounters with Crows and also produced fewer fledglings than did Magpies breeding at greater distances from Crow nests. Magpie nests were located nearer human habitations than were Crow nests.