Life history stage and vessel voyage profile can influence shipping-mediated propagule pressure of non-indigenous biofouling species

To control the spread of non-indigenous species it is necessary to understand how early stages of the invasion process, such as age of propagule at time of entrainment and transport, influence the quality and quantity of propagules delivered to recipient environments (i.e., the propagule pressure). Using ship biofouling as a model pathway scenario and the bryozoan Bugula neritina, the effect of two early-stage selective filters—the age of recruits and the pattern of the voyage—on reproductive output post-arrival were examined. Voyage scenarios were created by manipulating food levels in a series of field experiments. Recruit Age had three levels (1-day, 1-week and 1-month), and Voyage Pattern (four levels) represented vessels that undertake frequent short voyages or infrequent long voyages, plus control treatments. Spawning success, number of larvae and larval size were measured over multiple spawning events after each scenario. One-day and 1-week old colonies that had been exposed to short-voyage scenarios had higher reproductive output than those that had been exposed to longer and less frequent voyages. In contrast, 1-month old colonies did not release larvae under any voyage scenario. The survivorship of bryozoans in all treatments was relatively high, but reproductive output was influenced by colony age and voyage type. Our results suggest that vessels undergoing frequent, short intra-coastal or domestic voyages are able to deliver viable propagules to subsequent destinations even if they have prior port residencies as short as 1 day. At least for bryozoans, juvenile recruits may even pose a higher risk of spread compared to reproductively mature recruits.     

Bilge water as a vector for the spread of marine pests: a morphological,metabarcoding and experimental assessment

Vessel movements are considered the primary anthropogenic pathway for the secondary spread of marine non-indigenous species. In comparison to the well-studied mechanisms of hull fouling and ballast water, the importance of bilge water for domestic and cross-regional spread of non-indigenous species is largely unknown and has the potential to compromise the overall effectiveness of biosecurity management actions. In this study, the diversity and abundance of biological material contained in bilge water from 30 small vessels (<20 m) was assessed using traditional and molecular identification tools (metabarcoding of the 18S rRNA gene). Laboratory-based studies were also used to investigate the relationship between voyage duration and propagule success. A large taxonomic diversity in organisms was detected, with 118 and 45 distinct taxa identified through molecular and morphological analyses, respectively. Molecular techniques identified five species recognised as non-indigenous to the study region in 23 of the 30 bilge water samples analysed. Larvae and fragments passed through an experimental bilge pump system relatively unharmed. Time spent in the bilge sump was found to affect discharge success, particularly of short-lived and sensitive larvae, but survival for 3 days was observed. Our findings show that bilge water discharges are likely to pose a non-negligible biosecurity threat and that further research to identify high-risk vessel operating profiles and potential mitigation measures are warranted.     

Sounding out pests: the potential of hydroacoustics as a surveillance and compliance tool in aquatic biosecurity

Shipping is the main method for goods transportation, accounting for approximately 60% of all global trade. Biofouling of these shipping vessels is a critical pathway for the introduction of non-indigenous species (NIS) across the world’s oceans. In order to reduce the likelihood of NIS introductions, appropriate biosecurity mechanism need to be in place, including maintaining high standards in vessel hygiene, such as zero secondary biofouling. Development of methodologies that can accurately quantify vessel biofouling without impeding maritime operations, while simultaneously providing effective biosecurity for the marine environment and resources, and mitigation of introduced marine pests would be highly advantageous. This study tests such a methodology. We conduct a proof-of-concept study that uses hydroacoustics to quantify the biofouling on a vessel’s hull, using surrogates for a vessel hull and biofouling. Based on a simple off-the-shelf single beam echosounder, the method was able to visually detect and quantify various biofouling mimics (ranging in height from 10 to 200 mm in height) at a slow tow speed (0.5 m/s). The efficacy of the hydroacoustic method was influenced by the movement of the echosounder, with the discriminating capability reduced to the detection of only larger mimics as the speed of movement increased. With further development, the use of hydroacoustics could become a viable biosecurity surveillance option for the mitigation of introduced marine pest incursions.     

The effect of vessel speed on the survivorship of biofouling organisms at different hull locations

This study used a specially designed MAGPLATE system to quantify the en route survivorship and post-voyage recovery of biofouling assemblages subjected to short voyages (<12 h) across a range of vessel speeds (slow, medium, fast; in the range 4.0–21.5 knots). The effect of hull location (bow, amidships and stern) was also examined. While no significant differences were evident in en route survivorship of biofouling organisms amongst hull locations, biofouling cover and richness were markedly reduced on faster vessels relative to slower craft. Therefore, the potential inoculum size of non-indigenous marine species and richness is likely to be reduced for vessels that travel at faster speeds (>14 knots), which is likely to also reduce the chances of successful introductions. Despite this, the magnitude of introductions from biofouling on fast vessels can be considered minor, especially for species richness where 90% of source-port species were recorded at destinations.     

The effect of butenolide on behavioral and morphological changes in two marine fouling species,the barnacle Balanus amphitrite and the bryozoan Bugula neritina

Butenolide [5-octylfuran-2(5H)-one] is a very promising antifouling compound. Here, the effects of butenolide on larval behavior and histology are compared in two major fouling organisms, viz. cypris larvae of Balanus amphitrite and swimming larvae of Bugula neritina. Butenolide diminished the positive phototactic behavior of B. amphitrite (EC₅₀ = 0.82 μg ml^?1) and B. neritina (EC₅₀ = 3 μg ml^?1). Its effect on the attachment of cyprids of B. amphitrite was influenced by temperature, and butenolide increased attachment of larvae of B. neritina to the bottom of the experimental wells. At concentrations of 4 μg ml^?1 and 10 μg ml^?1, butenolide decreased attachment of B. amphitrite and B. neritina, respectively, but the effects were reversible within a certain treatment time. Morphologically, butenolide inhibited the swelling of secretory granules and altered the rough endoplasmic reticulum (RER) in the cement gland of B. amphitrite cyprids. In B. neritina swimming larvae, butenolide reduced the number of secretory granules in the pyriform-glandular complex.     

Exploring the regulatory role of nitric oxide (NO) and the NO-p38MAPK/cGMP pathway in larval settlement of the bryozoan Bugula neritina

The bryozoan Bugula neritina is a cosmopolitan marine fouling species that causes major fouling problems in sub-tropical waters. Settlement of B. neritina larvae can be triggered without an obvious external cue. Here, the negative regulatory role of nitric oxide (NO) during larval settlement of B. neritina was demonstrated to be mediated by cyclic guanosine monophosphate (cGMP). Although the regulatory role of the NO-p38 MAPK signaling axis in larval settlement was not evident, inhibition of nitric oxide synthase (NOS) led to the deactivation of p38 MAPK. Exclusive localization of NO and NO signaling components in sensory-related organs of the larvae is consistent with its signal transduction function in metamorphosis. Overall, this study provides new insights into the regulatory roles of the NO-p38MAPK/cGMP pathway in B. neritina settlement.     

Species Richness and Patterns of Invasion in Plants, Birds, and Fishes in the United States*

We quantified broad-scale patterns of species richness and species density (mean # species/km²) for native and non-indigenous plants, birds, and fishes in the continental USA and Hawaii. We hypothesized that the species density of native and non-indigenous taxa would generally decrease in northern latitudes and higher elevations following declines in potential evapotranspiration, mean temperature, and precipitation. County data on plants (n = 3004 counties) and birds (n=3074 counties), and drainage (6 HUC) data on fishes (n = 328 drainages) showed that the densities of native and non-indigenous species were strongly positively correlated for plant species (r = 0.86, P < 0.0001), bird species (r = 0.93, P<0.0001), and fish species (r = 0.41, P<0.0001). Multiple regression models showed that the densities of native plant and bird species could be strongly predicted (adj. R² = 0.66 in both models) at county levels, but fish species densities were less predictable at drainage levels (adj. R² = 0.31, P<0.0001). Similarly, non-indigenous plant and bird species densities were strongly predictable (adj. R² = 0.84 and 0.91 respectively), but non-indigenous fish species density was less predictable (adj. R² = 0.38). County level hotspots of native and non-indigenous plants, birds, and fishes were located in low elevation areas close to the coast with high precipitation and productivity (vegetation carbon). We show that (1) native species richness can be moderately well predicted with abiotic factors; (2) human populations have tended to settle in areas rich in native species; and (3) the richness and density of non-indigenous plant, bird, and fish species can be accurately predicted from biotic and abiotic factors largely because they are positively correlated to native species densities. We conclude that while humans facilitate the initial establishment, invasions of non-indigenous species, the spread and subsequent distributions of non-indigenous species may be controlled largely by environmental factors. The U.S. Government’s right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.     

Cultivation Fosters Plant Naturalization by Reducing Environmental Stochasticity

A large fraction of the immigrant (or founder) populations of terrestrial plants are small (< 10⁴) and are acutely sensitive to environmental stochasticity. As a result, they undergo radical size fluctuations during a prolonged lag phase that almost always result in their extirpation. Naturalizations are those rare examples in which an immigrant population increases above a threshold size such that the consequences of environmental stochasticity are markedly lower. The likelihood that a non-indigenous population will reach this threshold size would be enhanced substantially through either deliberate or inadvertent cultivation. Cultivation (e.g. protection from predators, parasites, drought, frost) shields small immigrant populations from the extreme expressions of environmental stochasticity. In addition, cultivation can preserve through seed storage a residual non-indigenous population from which new populations can be established. As disseminules are spread locally, and even regionally, immigrant populations sample a wide variety of micro-habitats, thus increasing the likelihood that some plants will survive even without cultivation. Origins of naturalized floras in Australia and the US reveal a strong circumstantial link between cultivation and subsequent naturalization: the single largest group of naturalized species was deliberately introduced as either crops, forage spp., or ornamentals. Another group was introduced inadvertently as contaminants in crop seeds. This correspondence between cultivation and subsequent naturalization provides a common demographic explanation for non-indigenous plant persistence that largely transcends species’ attributes and the commonly ascribed features of communities that are vulnerable to the entry of non-indigenous plants. Humans have played a more profound role in fostering plant naturalizations than by acting simply as plant dispersers; their post-immigration cultivation fosters much naturalization. This revised version was published online in July 2006 with corrections to the Cover Date.     

The role of containerships as transfer mechanisms of marine biofouling species

Fouling of ships is an important historical and enduring transfer mechanism of marine nonindigenous species (NIS). Although containerships have risen to the forefront of global maritime shipping since the 1950s, few studies have directly sampled fouling communities on their submerged surfaces, and little is known about differences in the fouling characteristics among commercial ship types. Twenty-two in-service containerships at the Port of Oakland (San Francisco Bay, California) were sampled to test the hypothesis that the extent and taxonomic richness of fouling would be low on this type of ship, resulting from relatively fast speeds and short port durations. The data showed that the extent of macroorganisms (invertebrates and algae) was indeed low, especially across the large surface areas of the hull. Less than 1% of the exposed hull was colonized for all apart from one vessel. These ships had submerged surface areas of >7000 m², and fouling coverage on this area was estimated to be <l7 m² per vessel, with zero biota detected on the hulls of many vessels. The outlying smaller vessel (4465 m²) had an estimated coverage of 90% on the hull and also differed substantially from the other ships in terms of its recent voyage history, shorter voyage range and slower speeds. Despite the low extent of fouling, taxonomic richness was high among vessels. Consistent with recent studies, a wide range of organisms were concentrated at more protected and heterogeneous (non-hull) niche areas, including rudders, stern tubes and intake gratings. Green algae and barnacles were most frequently sampled among vessels, but hydroids, bryozoans, bivalves and ascidians were also recorded. One vessel had 20 different species in its fouling assemblage, including non-native species (already established in San Francisco Bay) and mobile species that were not detected in visual surveys. In contrast to other studies, dry dock block areas did not support many organisms, despite little antifouling deterrence in some cases. Comparisons with previous studies suggest that the accumulation of fouling on containerships may be lower than on other ship types (eg bulkers and general cargo vessels), but more data are needed to determine the hierarchy of factors contributing to differences in the extent of macrofouling and non-native species vector risks within the commercial fleet.     

Adrenoceptor Compounds Prevent the Settlement of Marine Invertebrate Larvae: Balanus amphitrite (Cirripedia), Bugula neritina (Bryozoa) and Hydroides elegans (Polychaeta)

The effects of the neurotransmitter blockers idazoxan and phentolamine on the larval settlement of three marine invertebrate species belonging to three different phyla were investigated by using in vitro concentration-response bioassays. Since neurotransmitters are known to influence metamorphic transitions in invertebrate larvae, neurotransmitter blockers were tested to evaluate their sublethal effects on larvae. The α-adrenergic antagonists idazoxan and phentolamine inhibited settlement of Balanus amphitrite (Cirripedia), Bugula neritina (Bryozoa) larvae, and larvae of the polychaete Hydroides elegans (Polychaeta) in a concentration- and taxon-dependent manner. At concentrations of 10^?3 M of both agents, larvae of all three species became immobile and subsequently died within 24 h. While cumulative settlement rates were observed after 48 h for B. amphitrite and H. elegans, and after 5 h for B. neritina, > 90% of the larvae that settled did so within 24 h for the first two species and within 1 h for B. neritina. The tendency of the hydrophobic idazoxan and phentolamine to accumulate at solid surfaces most probably contributes to their successful inhibition of larval settlement. This ability makes them particularly attractive as candidates for the development of slow-release carriers in antifouling paints.     

Vessel generator noise as a settlement cue for marine biofouling species

Underwater noise is increasing globally, largely due to increased vessel numbers and international ocean trade. Vessels are also a major vector for translocation of non-indigenous marine species which can have serious implications for biosecurity. The possibility that underwater noise from fishing vessels may promote settlement of biofouling on hulls was investigated for the ascidian Ciona intestinalis. Spatial differences in biofouling appear to be correlated with spatial differences in the intensity and frequency of the noise emitted by the vessel’s generator. This correlation was confirmed in laboratory experiments where C. intestinalis larvae showed significantly faster settlement and metamorphosis when exposed to the underwater noise produced by the vessel generator. Larval survival rates were also significantly higher in treatments exposed to vessel generator noise. Enhanced settlement attributable to vessel generator noise may indicate that vessels not only provide a suitable fouling substratum, but vessels running generators may be attracting larvae and enhancing their survival and growth.     

Fish species diversity in the lakes of Alma-Gol,Adji-Gol,and Ala-Gol,Golestan province,northern Iran

The study was conducted to explore fish species diversity and understand some ecological aspects of the lakes of Golestan province, which represent a group of the small and rather isolated water bodies, from September 2000 to August 2002. With respect to environmental properties, there were no significant differences among wetlands. The results revealed that non-indigenous fish species are found to be dominant in the lakes. Of ten sampled species, four, including sharp belly Hemiculter leucisculus, silver crucian carp Carassius auratus, top mouth gudgeon Pseudorasbora parva, and mosquito fish Gambusia holbrooki, were non-indigenous. Observation of sharp belly and top mouth gudgeon represent actually the first record of these species in these lakes. Endemicism was highest in Adji-Gol (52.37% native) and much lower in Alma-Gol (10.20%) and Ala-Gol (20.43%). Species richness, diversity, and evenness were highest in Adji-Gol and lowest in Ala-Gol. Dominance of non-indigenous species could be explained by their wider ecological and physiological tolerance; therefore, if greater emphasis is not placed on the conservation of native fishes, these species could face continued declines or might be quietly lost in the lakes. The text was submitted by the authors in English.     

Ecological impacts of non-indigenous invasive fungi as forest pathogens

Non-indigenous pathogenic fungi increasingly threaten North American tree species. Ecosystems may be fundamentally changed when abundant tree species are functionally eliminated, as occurred with American chestnut (Castanea dentata (Marsh.) Borkh.). Conversely, changes may be more subtle but still significant and long lasting when populations are lost, or all trees in the larger size classes are killed. Proposed approaches for characterizing the magnitude of ecological impacts use characteristics of both the non-indigenous pathogen and the host species. Impacts are most significant when highly successful invading pathogens attack foundation species, setting in motion a long-lasting cascade of effects on the host and associated species. Such impacts have generally not been well documented at the ecosystem level. Several North American forest tree species have been functionally eliminated or severely reduced by non-indigenous pathogens. Historical invasions, such as that of chestnut blight (Cryphonectria parasitica (Murr.) Bar), caused very significant ecological impacts that will never be completely understood because of lack of quantitative data. Beech bark disease, caused by a combination of an introduced scale insect (Cryptococcus fagisuga Lindinger) and a fungus (Neonectria faginata (Lohman et al.) Castl. & Rossman), is still advancing and provides opportunities for studying ecosystem-level impacts when a major tree species is removed or markedly reduced in abundance from the overstorey. Butternut canker, caused by the fungus, Sirococcus clavigignenti-juglandacearum N.B. Nair, Kostichka & Kuntz, has spread throughout the host range, endangering species’ survival. Other non-indigenous invasive fungi such as Cronartium ribicola J.C. Fisch and Phytophthora lateralis Tucker & Milbrath continue to move into new populations, causing high mortality and associated restructuring of these ecosystems. Global trade and environmental change trends will ensure new challenges by non-indigenous fungal pathogens, presenting an urgent need for improved understanding of long-term impacts across ecological systems.     

Recruitment patterns of scleractinian corals in an isolated sub-tropical reef system

The density of recruits of scleractinian corals on settlement plates at Lord Howe Island, a small isolated sub-tropical island 630 km off the Australian coastline, was within the range of values reported for comparable studies on the Great Barrier Reef. However, there was a difference in the relative abundance of taxonomic groups, with recruitment at Lord Howe Island during the summer of 1990/91 dominated by corals from the Family Pocilloporidae, Family Poritidae, and sub-genus Acropora (Isopora) (in order of abundance). By contrast, on the Great Barrier Reef, recruits are generally predominantly species from the Family Acroporidae (other than the Acropora (Isopora) group). Both the recruits and the established coral communities at Lord Howe Island are dominanted by corals which release brooded planulae, as opposed to the pattern of mass-spawning with external fertilisation more typical of Great Barrier Reef corals. I hypothesise that the release of brooded planulae would be advantageous in an isolated reef community because (a) brooded larvae can travel large distances and survive the journey to the isolated reef and/or (b) brooded larvae have a shorter period before they are competent to settle and are therefore more likely to be retained on the parental reef once a population has been established.     

Integration of non-indigenous species within the interspecific abundance–occupancy relationship

There is a broad consensus that habitat disturbance and introduction of non-indigenous species may dramatically modify community structure, particularly in insular ecosystems. However, it is less clear whether emergent macroecological patterns are similarly affected. The positive interspecific abundance–occupancy relationship (IAOR) is one of the most pervasive macroecological patterns, yet has rarely been analyzed for oceanic island assemblages. We use an extensive dataset for arthropods from six islands within the Azorean archipelago to test: (1) whether indigenous and non-indigenous species are distributed differently within the IAOR; and (2) to the extent that they are, can differences can be attributed to two indices of disturbance. We implemented modeling averaged methods using five of the most common IAOR models to derive an averaged IAOR fit for each island. After testing if species colonization status (indigenous versus non-indigenous) may explain the residuals of the IAOR, we identified true negative and positive outliers and tested the effect of colonization status on the likelihood of a species being a positive or negative outlier. We found that the indigenous and non-indigenous species are randomly distributed on both sides of the overall IAOR. Only for Flores Island, were non-indigenous species more aggregated than indigenous species. We were unable to detect a meaningful relationship between deviation from the IAOR and disturbance, despite the undoubted impact of both severe habitat loss and non-indigenous species on these oceanic islands. Our results show that the non-indigenous species have been integrated alongside indigenous species in the contemporary Azorean arthropod communities such that they are mostly undetectable by the study of the IAOR.     

Do alternate escape tactics provide a means of compensation for impaired performance ability?

Whole‐animal performance abilities can facilitate the avoidance of predation and consequently influence fitness, but determining the functional significance of antipredation tactics is difficult without understanding how alternate predator escape strategies are related. We measured maximal sprint speed and dive duration in the semi‐aquatic skink Oligosoma suteri to determine how morphology and behaviour influence these alternate predator escape techniques and the relationship between the two measures. Gravid females and juveniles ran significantly slower, but had equivalent or longer dive durations than males and nongravid females. The two performance measures were not influenced by the same morphological and behavioural traits, and were not correlated among individuals. Thus, individuals that are poor sprinters because of their state (e.g. gravid or tail‐less individuals) would have a greater likelihood of successful escape by adopting an alternate escape strategy. For species that use multiple strategies for the same function, quantifying selection on whole‐animal performance will be difficult. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 241–249.     

Natal recruitment and adult retention in a population of nine-banded armadillos

Nine-banded armadillosDasypus novemcinctus Linnaeus, 1758 are interesting in part because (a) they give birth to litters of genetically identical quadruplets, and (b) the species’ range has expanded rapidly throughout the southern United States during the last 150 years, suggesting substantial dispersal of individuals. Using data from 7 field seasons between 1992 and 1999, we examined the extent of juvenile recruitment and retention of adults in a population of armadillos from northern Florida. There were no sex differences in the likelihood of recruitment or most attributes of male and female recruits at any age. In the few cases where more than one littermate was recruited into the population, siblings were significantly more widely dispersed as adults than they were as juveniles, thus limiting opportunities for interaction among clonal siblings. There was some evidence that recruits ranged more widely than other individuals, suggesting recruits may have been searching for suitable sites to establish themselves. Recruits were heavier than non-recruits as both juveniles and yearlings, which may have aided in establishing a home range, but recruits were lighter than other animals as adults. Overall, slightly less than 50% of armadillos first captured as adults were never seen in a subsequent year, suggesting these individuals may have been transients. However, some adults remained in the population for multiple years, moving very little from the area where they were first sighted. As with recruits, there were no sex differences in the likelihood of adults being retained in the population nor in the attributes of retained males and females. Retained animals exhibited more extensive anatomical damage and moved farther between successive sightings within years than did non-residents. Adults were more likely to be retained in the population than juveniles were to be recruited, and retained adults were older, heavier, and exhibited more extensive anatomical damage than did recruits. Our data seem to indicate a population characterized by limited recruitment of juveniles (particularly of clonemates) and an adult population exhibiting considerable turnover from year to year, but with a core of individuals who are long-term residents.     

Assessing the diversity of New Zealand freshwater harpacticoid copepods (Crustacea: Copepoda) using mitochondrial DNA (COI) barcodes

Taxonomic and ecological studies of freshwater harpacticoid copepods are limited globally by the ability to easily and accurately identify specimens. Here, we test the use of the mitochondrial cytochrome c oxidase subunit I (COI) gene locus as a tool for assessing the diversity of freshwater Harpacticoida. We obtained sequences from New Zealand harpacticoid copepods, representing two families, five genera and nine species, including the non-indigenous Elaphoidella sewelli. All species were delineated by the COI gene. However, high intraspecific diversity was evident among populations of Elaphoidella bidens (>12%), and between North and South Island populations of Bryocamptus pygmaeus (>18%), potentially indicating the presence of morphologically cryptic taxa. We suggest that mitochondrial DNA (COI) sequences can provide a useful tool for the routine identification of freshwater harpacticoid copepods. Applications of these data will include assessing species diversity and biogeography as well as assisting with the detection of non-indigenous species.     

The potential role of an unregulated coastal anthropogenic activity in facilitating the spread of a non-indigenous biofoulant

Despite an exponential rise in anthropogenically-mediated transfers of non-indigenous species during the last 150 years, several coastal anthropogenic activities remain unregulated under current legislation frameworks. This study investigates the potential role of commercial periwinkle (Littorina littorea) harvesting as an unregulated facilitator of both small- and large-scale geographic range expansion of an invasive oyster epibiont (Ostrea chilensis) within the Menai Strait (North Wales, UK) and beyond. The frequency of oyster-fouled periwinkles was greatest in areas of high adult oyster abundance and restricted to large, market-sized periwinkles (>20 mm) inhabiting the low shore. Active efforts by commercial collectors to reject oyster-fouled periwinkles were found to be inadequate, with oysters of all sizes observed within collected hauls. Whilst the survival of fouled and unfouled periwinkles was comparable under post-collection refrigerated conditions, a significant decrease in both mobility and flesh content was associated with the presence of oyster epibionts. Survival of all but the smallest oyster epibionts under post-collection refrigerated conditions enhances the possibility of accidental non-indigenous oyster transfers. Better interventions during both initial visual inspection and post-griddling stages are recommended, as well as the development of techniques that kill off all non-indigenous epibionts, whilst leaving the freshness and marketability of the periwinkles uncompromised.     

Yearly variation in factors associated with local recruitment of Tree Swallows

Identifying factors that influence the chances of fledglings becoming local recruits can inform us about the ecology of population dynamics and factors influencing parental fitness. We studied Tree Swallows (Tachycineta bicolor) nesting in boxes in western Michigan from 1994 to 2005 and found that 145 of 2405 (6.0%) nestlings that fledged from 840 nests became local recruits. Most local recruits returned to breed at the study site the year after they fledged. The sex ratio of local recruits (76 males and 69 females) did not differ from 1:1. Analysis of data from all years combined revealed that the likelihood of fledglings becoming local recruits was not significantly influenced by the year they fledged, clutch initiation date, clutch size, brood size, age class and physical characteristics of female parents, or the physical characteristics of putative male parents. However, analysis of data from each year revealed that some variables had a significant effect on the likelihood of fledglings becoming local recruits in some years, including female age class in two years, female parent right tail fork length in one year, female parent right wing length in one year, clutch initiation date in two years, clutch size in two years, and brood size in one year. The proportion of fledglings produced each year that became local recruits was not correlated with either mean air temperature or total rainfall during June and July, the immediate post-fledgling period. Taken together, these analyses suggest that chance or yearly variation in some other factor(s) with the potential to affect post-fledging survival that we did not measure (e.g., predation post-fledging or climatic conditions during migration or in migratory stopover sites and wintering areas) had an important effect on local recruitment of fledgling Tree Swallows in our study. We did find that local recruits were more likely to be from nests where egg-laying began earlier rather than later in the season, with 137 of 145 (94%) of local recruits coming from clutches initiated between 5 and 30 May. Because swallows that fledged earlier in the season had more time to prepare for migration than those that fledged later, we hypothesize that the “relative age effect” had an important effect on local recruitment of Tree Swallows at our study site.