Low Breeding Density Research Articles

Determinants of departure to natal dispersal across an elevational gradient in a long-lived raptor species.

Attributes of natal habitat often affect early stages of natal dispersal. Thus, environmental gradients at mountain slopes are expected to result in gradients of dispersal behavior and to drive elevational differences in dispersal distances and settlement behavior. However, covariation of environmental factors across elevational gradients complicates the identification of mechanisms underlying the elevational patterns in dispersal behavior. Assuming a decreasing food availability with elevation,we conducted a food supplementation experiment of red kite (Milvus milvus) broods across an elevational gradient toward the upper range margin and we GPS-tagged nestlings to assess their start of dispersal. While considering timing of breeding and breeding density across elevation, this allowed disentangling effects of elevational food gradients from co-varying environmental gradients on the age at departure from the natal home range. We found an effect of food supplementation on age at departure, but no elevational gradient in the effect of food supplementation. Similarly, we found an effect of breeding density on departure age without an underlying elevational gradient. Supplementary-fed juveniles and females in high breeding densities departed at younger age than control juveniles and males in low breeding densities. We only found an elevational gradient in the timing of breeding. Late hatched juveniles, and thus individuals at high elevation, departed at earlier age compared to early hatched juveniles. We conclude that favorable natal food conditions, allow for a young departure age of juvenile red kites. We show that the elevational delay in breeding is compensated by premature departure resulting in an elevational gradient in departure age. Thus, elevational differences in dispersal behaviour likely arise due to climatic factors affecting timing of breeding. However, the results also suggest that spatial differences in food availability and breeding density affect dispersal behavior and that their large-scale gradients within the distributional range might result in differential natal dispersal patterns.

Bestands- und Brutplatzerfassung von Mehlschwalben (Delichon urbicum, Linnaeus, 1758) und Rauchschwalben (Hirundo rustica, Linnaeus, 1758) im Südwesten und Westen Luxemburgs

Population size, distribution and nesting site characteristics of breeding house martins and barn swallows were investigated in the mixed rural and urban landscape of southwestern and western Luxembourg. House martins were mapped in 34 municipalities (670 km2) between 2013 and 2020 and barn swallows in 11 municipalities (228 km2) between 2017 and 2020, representing nearly a quarter and a tenth of the national surface, respectively. By means of one visit per season, nests as well as their occupation status were recorded in a comprehensive mapping effort comprising settlements as well as farmsteads. For the house martin, 4,109 nests were mapped, 1,025 of which were occupied and distributed over 368 colonies with an average number of 2.78 breeding pairs per colony and a population density of 1.62 breeding pairs per km2. Nearly one third of the recorded martin pairs were settled in artificial nests, underlining the effectiveness of this conservation measure. Barn swallow nests numbered 1,395, of which 655 housed breeding pairs, distributed over 107 sites with an average colony size of 6.12 breeding pairs and a population density of 2.89 breeding pairs per km2. Between-year occupancy rates as well as abundances varied strongly between different municipalities in the study area in both species. Generally low breeding densities reflect the unfavourable conservation status for both red-listed swallow species.

Growing up in a crowd: social environment shapes the offspring's early exploratory phenotype in a colonial breeding species.

In colonial breeding species, the number of adverse social interactions during early life typically varies with breeding density. Phenotypic plasticity can help deal with this social context, by allowing offspring to adjust their behaviour. Furthermore, offspring may not be unprepared since mothers can allocate resources to their embryos that may pre-adjust them to the post-hatching conditions. Thus, we hypothesize that lesser black-backed gull chicks raised in dense breeding areas, with greater exposure to intra-specific aggression, show higher levels of anxiety and lower levels of exploration compared to chicks in low-density areas, and that this is facilitated by prenatal effects. To test this, we cross-fostered clutches within and across pre-defined high- and low-breeding density areas. We measured chicks' anxiety and exploration activity in an open-field test that included a novel and a familiar object. We found that both pre- and post-natal social environment contributed nearly equally and shaped the offspring's exploratory behaviour, but not its anxiety, in an additive way. Post-natal effects could reflect a learned avoidance of intra-specific aggression, yet identifying the pathways of the prenatal effects will require further study.

Demographic composition, post-release and natal dispersal, and breeding success of the reintroduced Oriental Stork Ciconia boyciana

Summary Species restoration through reintroduction and reinforcement is widely considered as an effective method for the conservation of threatened animals. Although these approaches have been increasingly implemented over the last half-century, many attempts have failed. Dispersal behaviour is one of the key factors determining reintroduction success. We reported the demographic composition and dispersal pattern (i.e., post-release and natal dispersal) of a reintroduced Oriental Stork Ciconia boyciana population in Japan and identified the determinants of breeding success in 2005–2019. We hypothesized that one of the key determinants of breeding success is the density-dependent effect through post-release and natal dispersal. The F1 generation occurrence corresponded to two years, F2 to seven, F3 to 11, and F4 to 14 years after initial reintroduction. Wild-born birds accounted for 74% of the reintroduced population at F4 occurrence. Post-release dispersal distances were shorter than natal dispersal. Inter-nest distance was selected as the key factor determining breeding success. These results may have been due to feeding habitat restoration and conspecific interactions. In conclusion, steady recruitment to the next generation is thought to be due to a moderate natal dispersal and the subsequent low breeding density of wild-born birds.

Wetlands support higher breeding wader densities than farmed habitats within a nature-rich farming system

ABSTRACT Capsule Wetlands with little or no agricultural activity support higher breeding wader densities than more intensively farmed habitats within a nature-rich farmed landscape. Aims To test whether breeding wader densities differ between habitats likely to receive varying agricultural management intensity, within a nature-rich farmed landscape. Methods Using the island of Sanday as a case study for the wader-rich Orkney archipelago, a whole-island breeding wader survey was used to generate population estimates and test whether breeding densities differed between habitats under varying management intensities. Results The island supported nationally high breeding wader densities, which approach those of high-density areas elsewhere in Europe. Densities of total waders and five out of six species tested varied significantly between habitats. Wetlands subject to no agricultural management or livestock grazing in some land units supported higher densities than more intensively farmed habitats for total waders, Northern Lapwing Vanellus vanellus, Common Snipe Gallinago gallinago and Common Redshank Tringa totanus and second-highest densities for Eurasian Curlew Numenius arquata. Agriculturally unimproved grassland supported the highest densities for many species after wetlands. Agriculturally improved grassland supported consistently low relative breeding densities, and other habitats managed using mechanized farming (lower intensity improved grassland and arable) supported generally low relative densities, apart from for Eurasian Oystercatcher Haematopus ostralegus. Conclusions Describing an entire mixed farming system as nature-rich may mask significant variation in the contribution of different habitats to the maintenance of high nature value. In this system, wetlands that were unmanaged or received low average grazing densities supported disproportionately high breeding wader densities and must be protected to maintain the high densities of most species. The further loss of wetlands, and the move towards intensively managed grassland, is a threat to the maintenance of high breeding wader densities on Orkney and in similar farmed landscapes.

Dynamics of habitat selection in birds: adaptive response to nest predation depends on multiple factors.

According to theory, habitat selection by organisms should reflect underlying habitat-specific fitness consequences and, in birds, reproductive success has a strong impact on population growth in many species. Understanding processes affecting habitat selection also is critically important for guiding conservation initiatives. Northern pintails (Anas acuta) are migratory, temperate-nesting birds that breed in greatest concentrations in the prairies of North America and their population remains below conservation goals. Habitat loss and changing land use practices may have decoupled formerly reliable fitness cues with respect to nest habitat choices. We used data from 62 waterfowl nesting study sites across prairie Canada (1997-2009) to examine nest survival, a primary fitness metric, at multiple scales, in combination with estimates of habitat selection (i.e., nests versus random points), to test for evidence of adaptive habitat choices. We used the same habitat covariates in both analyses. Pintail nest survival varied with nest initiation date, nest habitat, pintail breeding pair density, landscape composition and annual moisture. Selection of nesting habitat reflected patterns in nest survival in some cases, indicating adaptive selection, but strength of habitat selection varied seasonally and depended on population density and landscape composition. Adaptive selection was most evident late in the breeding season, at low breeding densities and in cropland-dominated landscapes. Strikingly, at high breeding density, habitat choice appears to become maladaptive relative to nest predation. At larger spatial scales, the relative availability of habitats with low versus high nest survival, and changing land use practices, may limit the reproductive potential of pintails.

Fear creates an Allee effect: experimental evidence from seasonal populations.

Allee effects driven by predation can play a strong role in the decline of small populations but are conventionally thought to occur when generalist predators target specific prey (i.e. type II functional response). However, aside from direct consumption, fear of predators could also increase vigilance and reduce time spent foraging as population size decreases, as has been observed in wild mammals living in social groups. To investigate the role of fear on fitness in relation to population density in a species with limited sociality, we exposed varying densities of Drosophila melanogaster to mantid predators either during an experimental breeding season or non-breeding season. The presence of mantids in either season decreased the reproductive performance of individuals but only at low breeding densities, providing evidence for an Allee effect. We then used our experimental results to parametrize a mathematical model to examine the population consequences of fear at low densities. Fear tended to destabilize population dynamics and increase the risk of extinction up to sevenfold. Our study provides unique experimental evidence that the indirect effects of the presence of predators can cause an Allee effect and has important consequences for our understanding of the dynamics of small populations.

Breeding Distribution and Population Persistence of Loggerhead Shrikes in a Portion of the North Carolina Sandhills

Abstract This study provides the first comparison of historical (1979–1994) and recent (2012–2013) breeding distributions of Lanius ludovicianus (Loggerhead Shrike) at a single location in southeastern North America. I conducted roadside and area searches for Loggerhead Shrikes in the spring (April—early June 2012 and late March—early May 2013) in Richmond County, NC, and small areas of 4 adjacent counties. Most of the study area is within the Sandhills, a subregion of the Coastal Plain. I documented a total of 44–45 breeding territories in 2012–2013, of which 36–38 (82–84%) were in the Sandhills, where I estimated the recent breeding density to be 3.2–3.4 pairs/100 km2. In both years, a smaller percentage of confirmed historic breeding sites were occupied in the Piedmont compared to the Sandhills portion of the study area, and the Piedmont had a much lower breeding density (0.6–0.8 pairs/100 km2). Sixteen of 22 (73%) confirmed historical breeding sites were occupied (n = 10) or retained suitable breeding...

Sequential polyandry in female Tengmalm's owl (Aegolius funereus) during a poor rodent year

In birds of prey, food availability affects the entire breeding process, including reproductive success. Sequential polyandry, sometimes documented in raptors and owls, has been observed occasionally in Tengmalm's owl in years of high food availability. In this study, the effect of food supply on the reproductive strategy of the central European Tengmalm's owl population has been investigated. The availability of the main prey of owls was significantly below average in the study year 2014. This resulted in low breeding density of owls, delayed egg laying, small clutch sizes, and low reproductive success. Nevertheless, successful sequential polyandry of one Tengmalm's owl female was recorded during the breeding season. The polyandrous female laid four eggs in each nests, and 50 % and 75 % of four hatched nestlings left the nest during the first and second breeding, respectively. In both nesting attempts, the two-year old female was of a substandard body mass and she abandoned the fledglings before they left the nests. Prey delivered by males to both nests was comprised mainly of alternative prey (birds and shrews). The results of this study suggest that there are probably some other factors, in addition to food availability, that may play a role in Tengmalm's owl's decisions in matters of parental care.

Nest Defence Behaviour and Testosterone Levels in Female Pied Flycatchers

AbstractNesting holes are a scarce resource for obligated cavity‐nesting birds and an important selective force for the evolution of aggressive female behaviours, which may be mediated by testosterone (T) levels. It is known that during periods of intense intrasexual competition such as initial breeding stages, females are highly aggressive towards intruding females. Here, we studied the implications of T levels for female–female competition by comparing levels of aggressiveness towards simulated female intruders (decoys) in two populations of the pied flycatcher (Ficedula hypoleuca) with a marked difference in breeding density. To this end, we exposed free‐living females to simulated territorial intrusions during 30 min when nest construction was almost complete. T levels of females were measured at the beginning of incubation under the assumption that they are positively associated with T levels during nest building. We also related aggressiveness to T levels in both populations. Furthermore, we aimed at detecting whether variation of T levels may explain female incubation attendance. Females showed higher T levels in the populations where pied flycatchers were exposed to a higher likelihood of conspecific interactions (high breeding density) than in the population with low breeding density. Female territorial presence, vigilance at the nest box and proximity to decoys were negatively related to circulating T levels in the high‐density population, but not in the low‐density population. Differences in T levels between populations did not result in differences in female incubation attendance, but T levels were negatively related to the incubation attendance in females from the population showing high T levels. In our populations, T levels in females prior to laying reflect the need to defend nesting cavities which is higher at high breeding density and in subdominant females. High T levels are costly in terms of incubation attendance.

Extra-pair Paternity in Relation to Age of the Red-breasted FlycatcherFicedula Parvamales

Despite advances in knowledge, in many bird species, the true mating system is unknown, and especially so for birds breeding at low densities under natural conditions. We studied paternity at the population and breeding pair level in the Red-breasted Flycatcher ( Ficedula parva), a small, migrant passerine breeding in old-growth Białowieża Forest. Over seven breeding seasons we genotyped a total of 159 nestlings and 25 adult males as fathers from 33 broods of the Red-breasted Flycatcher. Extra-pair young were present in eight broods (24.2%). Twelve (7.5%) chicks mismatched with the ‘social’ male in several loci. Both low breeding density and synchrony in breeding phenology can explain such findings. Also of interest was the share of extra-pair paternity at the individual level, in relation to the age/ornamentation of the extra-pair male. Male Red-breasted Flycatchers exhibit delayed plumage maturation. Young males (second year) are without an orange badge on the throat and breast, while older males (after-second year) are ornamented and have an orange badge. The extra-pair nestlings were found in eight broods, four with young males as a fathers (36.4%) and four in broods of old males (18.2%). However, in all seven cases when the age of the extra-pair partner was known, irrespective of the age of the social father, the extra-pair partner was a male with an orange patch on the breast. This indicated female preference for ornamented males as partners for extra-pair copulation.

放养密度对鲢鱼淀粉酶和脂肪酶活性的影响

本实验就高密度养殖与低密度养殖下对鱼体内淀粉酶和脂肪酶活性的影响进行了探究。2013年4月~10月，在山西省永济市温流水良种繁育场设置了两组池塘，每组三个，一组为低密度鲢鱼养殖(21.2 g•m−3)，一组为高密度鲢鱼养殖(42.4 g•m−3)。对此两种密度养殖下鲢鱼肠道淀粉酶和脂肪酶活性进行了测定，结果表明：高密度养殖池塘中鲢鱼肠道的淀粉酶活力和脂肪酶活力均高于低密度养殖池塘，且鲢鱼前肠此两种消化酶活性高于后肠；高密度鲢鱼养殖池塘与低密度相比，蓝藻门的细胞丰度明显下降，绿藻门细胞丰度所占百分比明显增加；淀粉酶活力与蓝藻的细胞丰度呈负相关，与其余门类浮游植物细胞丰度呈正相关，脂肪酶活力与蓝藻和裸藻的细胞丰度呈负相关，与其余门类浮游植物细胞丰度呈正相关。 This experiment has carried on exploration including the influence on amylase and lipase activities in fish body under high and low breeding density. From April to October in 2013, the influence of different breeding densities on amylase activity and lipase activity in H. molitrix intestinal tract was investigated in Wenliushui excellent fish breeding grounds in Yongji, Shanxi province, North China. Two experimental groups (each including three ponds) were set, which represent low and high breeding of H. molitrix density of 21.2 g•m−3 and 42.4 g•m−3 respectively. The results showed that amylase activity of high density breeding ponds was higher than that of low density breeding ponds, and amylase activity in foregut is higher than that in hindgut, as is the same with the lipase activity. The cell abundance of Cyanophyta in high breeding density ponds was obviously lower, while the proportion of the cell abundance of Chlorophyta was higher than that in low breeding density ponds. The amylase activity had a negative correlation with the cell abundance of Cyanophyta but a positive correlation with the cell abundance of the other divisions. The lipase activity had a negative correlation with the cell abundance of Cyanophyta and Euglenophyta, but a positive correlation with the cell abundance of the other divisions.

鲢鱼放养密度对水体中浮游藻类的影响

2013年4~9月调查了山西省永济市温流水良种繁育场鲢鱼放养密度对水体浮游藻类，特别是蓝藻细胞丰度以及溶氧量的影响。两组鱼塘，一组为低密度鲢鱼养殖(21.2 g/m3)，一组为高密度鲢鱼养殖(42.4 g/m3)。调查结果显示：1) 高密度鲢鱼放养鱼塘溶氧高于低密度鲢鱼放养鱼塘；2) 初期水体的浮游藻类总细胞丰度差异不大，但后期鲢鱼高密度放养鱼塘浮游藻类总细胞丰度低于低密度放养鱼塘；3) 初期两种密度鲢鱼养殖池塘中的蓝藻细胞丰度差异不大，后期高密度鲢鱼养殖池塘比低密度鲢鱼养殖池塘中的蓝藻细胞丰度明显降低，在浮游藻类总细胞丰度中所占比例也有同样的变化趋势；4) 两种养殖密度下，后期高密度养殖池塘中优势种有由蓝藻门向其他门类变化的趋势。总体结论，高密度鲢鱼养殖较低密度鲢鱼养殖可以更有效地控制和减少水体中的浮游藻类细胞丰度，特别是蓝藻的细胞丰度，更有利于水质净化，同时也提高了渔产潜力。 An investigation was carried out that the breeding density of Hypophthalmichthys molitrix influenced the cell abundance of phytoplankton, especially Cyanobacteria in Wenliushui Excellent Fish Breeding Grounds, Yongji, Shanxi Province, North China, from April to September of 2013. Two experimental groups were set, which represented low and high breeding density of H. molitrix, and were 21.2 g/m3 and 42.4 g/m3 respectively. The results showed that: 1) the dissolved oxygen of high density breeding ponds was higher than that of low breeding density ponds; 2) the cell abundance of phytoplankton had little difference at first but distinct difference later, and it was obvious lower in high breeding density ponds of H. molitrix than that in low breeding density ponds; 3) the same trend was reflected in Cyanobacteria; 4) compared with the low breeding density ponds, the dominant species had the trend from Cyanobacteria to the other divisions in high density breeding ponds of H. molitrix. In conclusion, the high density breeding of H. molitrix can control the cell abundance of phytoplankton especially Cyanobacteria, and purify water body, and meanwhile, the aquatic productivities are increased.

Low extrapair paternity observed in a tropical bird despite ample opportunities for extrapair mating

Abstract. The occurrence of extrapair paternity (EPP) in socially monogamous species varies greatly among species and across latitudes. Recent work suggests that birds residing in less seasonal, tropical environments will have lower levels of EPP than seasonal tropical and temperate species. Less seasonal tropical birds are predicted to have the lowest rates of EPP because their presumed low breeding densities and low breeding synchrony may reduce the opportunities for extrapair mating, and their potentially higher adult survival and longer-term pair bonds may increase the costs of retaliation by males, resulting in reduced paternal care. Nevertheless, the limited number of studies in tropical regions impedes our understanding of whether EPP varies across environments and why. We report on rates of EPP and the potential underlying mechanisms for the observed rate in the Black-crowned Antshrike (Thamnophilus atrinucha, previously known as the Western Slaty Antshrike), a Neotropical and socially monogamous ...

Breeding synchronization facilitates extrapair mating for inbreeding avoidance

Extrapair (EP) mating can enable females to reduce the negative effects of inbreeding. However, opportunities for EP mating are often ecologically or demographically constrained, and it is unclear whether and how females can overcome these constraints. Here, we show that fitness costs from inbreeding in purple-crowned fairy-wrens (Malurus coronatus) are severe (≥30% reduction in hatchability of eggs in incestuous pairs). Although EP paternity was low for unrelated pairs (3% of broods), and negatively affected by low breeding density, 46% of broods produced by incestuously paired females contained EP young that were almost always sired by a direct neighbor. Moreover, females in incestuous pairs initiated egg laying more synchronously with direct neighbors than females in an unrelated pair, and more synchronous broods were more likely to contain EP young. Because male sperm stores are highest around their own partner’s fertile period, our results imply that incestuously paired females achieve high levels of EP paternity by adjusting the timing of egg laying to match peak fertility of neighboring males. This example of a dramatic increase in EP paternity due to subtle behavioral adjustments illustrates that negative effects of inbreeding exert strong selection for female strategies to facilitate EP mating, and this may contribute to unexplained variation in EP paternity.

Nestling growth of Great Tits Parus major with comparison among altitudes

Fledgling mass can have great influence on individual’s life history and reflects the availability and quality of food in the breeding territory. Thus growth curve is used to compare the difference in the quality between different ecological conditions. The aim of the study was to fill the knowledge gap on nestling growth in Great Tit in Slovenia, to determine the influence of altitude on it and to present a tool for estimating age of nestlings and key dates in breeding phenology of Great Tit. Great Tits young were monitored using nest-boxes at three locations between years 2010 and 2012. At two locations nest-boxes were placed on three separate altitudinal belts. Weight growth curve was compared with the curves from other parts of Europe. Growth curve from Slovenia differs in growth parameters from other European populations but falls within their range. Difference in growth parameters betweenseparate populations probably comes from the difference in ecological conditions. When comparing three altitudes weight parameters of hatchlings were similar at lower and middle but different at upper altitudes indicating that weigh growth changes with the altitude. Lower food abundance that comes with rising altitude may be offset by lower competition through lower breeding density (hence similar growth parameters in lower and middle altitude), but not past certain altitude. Weight parameters are usefulwhen comparing different populations, but wing-length is better in determining the age of young in the nest.

House sparrows selectively eject parasitic conspecific eggs and incur very low rejection costs

Most host species of obligate interspecific brood parasites are under strong selection because such parasitism, e.g., that involving evictor nestmates, is highly costly. Egg rejection is one of the most efficient host defences against avian brood parasites. Many hosts have thus evolved egg-recognition ability and rejection behaviour. However, this defensive mechanism has not evolved in most species where only intraspecific brood parasitism occurs, probably because (1) the eggs of conspecific females are very similar in appearance, making egg rejection less likely to emerge, and (2) such parasitism is frequently less costly than interspecific parasitism. Using a captive population of house sparrows (Passer domesticus) with a low breeding density, we here provide new evidence showing that this species actually has a fine capacity to discriminate conspecific eggs and to eject them (44.2% of foreign eggs ejected) while incurring very low rejection costs (4.2% of own eggs ejected). This result contradicts those previously found in high-density house sparrow populations in which very high rejection costs and very high clutch desertion rates were reported, probably as a consequence of intraspecific competition and infanticide provoked by the high breeding density. The house sparrow has only rarely been reported as the host of an interspecific brood parasite, which implies that it is a newly described example of an altricial species in which egg ejection has evolved and is maintained in response to intraspecific brood parasitism.

Variation in the extra-pair mating systems of Acadian Flycatchers and Wood Thrushes in forest fragments in southern Ontario

The application of molecular tools to studies of avian mating systems has revealed that most songbird species engage in extra-pair matings. However, little is known about the possible effects of habitat fragmentation on extra-pair mating systems. During the breeding seasons of 2002 and 2003, we quantified the frequency of extra-pair matings in Acadian Flycatchers (Empidonax virescens) and Wood Thrushes (Hylocichla mustelina) nesting in forest fragments in southern Ontario. Acadian Flycatchers are at the limit of their northern range in southern Ontario and occurred at low densities (0.005-0.015 males/ha). Across forest fragments, 14% of young Acadian Flycatchers were the result of extra-pair fertilizations. In contrast, Wood Thrushes were common in forest fragments, with breeding densities ranging from 0.37 to 1 males/ha. Extra-pair mating was common among Wood Thrushes, with 40% of young the result of extra-pair matings. Compared to populations studied in less fragmented habitats in Pennsylvania, rates of extra-pair paternity in Ontario were lower for Acadian Flycatchers and higher for Wood Thrushes. Our results demonstrate that rates of extra-pair mating can vary across landscapes. However, the extra-pair mating systems of Acadian Flycatchers and Wood Thrushes appear to respond differently to fragmentation. We suggest that low breeding densities on fragments may be mediating the low rates of extra-pair mating observed in Acadian Flycatchers in southern Ontario, whereas changes to the behavioral tactics of male and female Wood Thrushes in southern Ontario may explain the high levels of extra-pair mating.

Age at First Return and Breeding of Razorbills (Alca torda) on the Gannet Islands, Labrador and Machias Seal Island, New Brunswick

Resightings of Razorbills (Alca torda) banded as chicks on the Gannet Islands, Labrador (N = 1,873 resightings of 638 individuals) and Machias Seal Island, New Brunswick (N = 870 resightings of 291 individuals) were used to estimate the timing and age at first return to the natal colony and age at first breeding. In total, 14 one-year-old and 110 two-year-old birds were observed on land. All one- and two-year-old birds were seen for the first time on the islands late in the breeding season after laying had occurred, with two-year-olds returning to the colony on average three weeks earlier than one-year-old birds. Age at first breeding was significantly lower on Machias Seal Island than the Gannet Islands, 3.88 ± 0.13 SE and 4.40 ± 0.13 SE years, respectively (range two to six years, N = 123). Four two-year-old birds were observed engaging in courtship and breeding behaviors, including copulation. Overall, age at first return and breeding was lower than for known European Razorbill colonies and is likely due to low breeding densities and fidelity at the two North American colonies studied.

WOOD THRUSH NESTS IN DENSE UNDERSTORY MAY BE VULNERABLE TO PREDATORS

Abstract We used Mayfield logistic regression and an information-theoretic approach to examine habitat characteristics associated with nesting success of Wood Thrushes (Hylocichla mustelina) across an urban to forested gradient in southwestern Pennsylvania in 2003 and 2004. Both nest placement and number of understory stems provided equally plausible models. Mayfield success was 20% higher for nests >3 m above ground level while dense understory was associated with low nest height. Wood Thrush nests in the forest interior averaged 2 m higher with a third less understory than edge nests. Urbanization and distance to the forest edge were not useful predictors of Wood Thrush nest success. The analysis was confounded by low breeding density at the most urbanized sites, but we found moderate success (42%, n = 63) across a fragmented landscape with minimal core forest area. Interior nests in a large contiguous forest were twice as successful (60%, n = 31) compared to edge nests (25%, n = 33) adjacent to a small...