Habitat use and movement patterns by dependent and independent juvenile Grasshopper Sparrows during the post‐fledging period

The post‐fledging period is a critical life stage for young grassland birds. Habitat selection by recently fledged birds may differ from that of adults and may change as juveniles transition from the care and protection of parents to independence. To describe patterns of habitat selection during these important life stages, we studied habitat use by juvenile Grasshopper Sparrows (Ammodramus savannarum) in a Conservation Reserve Program grassland in Maryland. We used radio‐telemetry to track daily movement patterns of two age classes of Grasshopper Sparrows during the post‐fledging period. Sparrows were classified as either dependent (<32‐d‐old) or independent (≥32‐d‐old). We characterized the vegetation at 780 vegetation plots (390 plots where birds were located and 390 paired random plots). Microhabitats where dependent birds were found had significantly more bare ground, litter, and plant species richness than paired random plots. In addition, dependent birds were found in plots with less bare ground, more warm‐season grass cover, more total vegetation cover, and more forb cover than plots used by independent birds. Plots where independent birds were located also had significantly more bare ground than random plots. Dependent birds are less able to escape from predators because their flight feathers are not fully grown so they may benefit from remaining in areas of greater vegetation cover. However, juveniles transitioning from dependence to independence must forage on their own, possibly explaining their increased use of more open areas where foraging may be easier. To properly manage habitat for grassland birds, management strategies must consider the changing needs of birds during different stages of development. Our results highlight the importance of diverse grassland ecosystems for juvenile grassland birds during the transition to independence.     

Exclosure fences around nests of imperiled Florida Grasshopper Sparrows reduce rates of predation by mammals

Increasing nest survival by excluding predators is a goal of many bird conservation programs. However, new exclosure projects should be carefully evaluated to assess the potential risks of disturbance. We tested the effectiveness of predator exclosure fences (hereafter, fences) for nests of critically endangered Florida Grasshopper Sparrows (Ammodramus savannarum floridanus) at a dry prairie site (Three Lakes; 2015–2018) and a pasture site (the Ranch; 2015–2016) in Osceola County, Florida, USA. We installed fences at nests an average of 8 days after the start of incubation, and nest abandonment after fence installation was rare (2 of 149 installations). Predation was the leading cause of failure for unfenced nests at both sites (48–73%). At Three Lakes, nest cameras revealed that mammals and snakes were responsible for 61.5% and 38.5% of predation events, respectively, at unfenced nests. Fences reduced the daily probability of predation (0.016 for fenced nests vs. 0.074 for unfenced nests). The probability that a fenced nest would survive from discovery to fledging was more than double that of unfenced nests (60.4% vs. 27.7%). However, we found no difference in daily nest survival at the Ranch between the year before nests were fenced (2015; 0.874) and the year when all but one nest were fenced (2016; 0.867) because red imported fire ants (Solenopsis invicta) were responsible for 86% of predation events at fenced nests at the Ranch. The use of cameras at fenced nests revealed that site‐specific differences in nest predators explained variation in fence efficiency between sites. Our fence design may be useful for other species of grassland birds, but site‐specific predator communities and species‐specific response of target bird species to fences should be assessed before installing fences at other sites.     

Effect of floating nest platforms on the breeding performance of Black Terns

ABSTRACT.   In 2003 and 2004, we placed 41 floating nest platforms on Grassy Lake in southeastern Wisconsin (USA) to test the hypothesis that reproductive success of Black Terns ( Chlidonias niger ) is limited by the quality of suitable nesting habitat. Extreme differences in water levels between these 2 yr provided a natural experiment to evaluate the effectiveness of the nest platforms during a drought year (2003) when natural nesting substrate was abundant, and a flood year (2004) when natural substrate was limited during the peak nesting period. Terns nested on 27 of 41 (66%) of the platforms in 2003 and 26 of 41 (63%) in 2004. No difference in the occupancy rate of platforms and natural nests was evident in 2003, but the pattern of clutch initiations early in the season in 2004 indicated that platforms were preferred over natural substrates. In 2003, nest survival rates did not differ between nests placed on platforms and those placed on natural substrates, but platform nests had significantly higher hatching success and nest survival rates in 2004. Both the Kaplan-Meier and Apparent Nest Success methods of calculating nest survival provided similar estimates. In both years, eggs laid on platforms were significantly larger than those laid on natural substrates, suggesting that platforms were occupied by high-quality birds. Our study indicates that floating nest platforms can be an effective management tool to enhance nesting habitat for Black Terns and other aquatic birds that construct floating nests, primarily because platforms provide nest sites when natural sites are not available due to flooding. Nest platforms also may be useful for addressing questions concerning habitat selection and parental quality.     

Factors that influence vital rates of Seaside and Saltmarsh sparrows in coastal New Jersey,USA

As saltmarsh habitat continues to disappear, understanding the factors that influence the population dynamics of saltmarsh breeding birds is an important step in the conservation of these declining species. Using 5 yrs (2011–2015) of demographic data, we evaluated and compared apparent adult survival and nest survival of Seaside (Ammodramus maritimus) and Saltmarsh (A. caudacutus) sparrows at the Edwin B. Forsythe National Wildlife Refuge in New Jersey, USA. We determined the effect of site management history (unditched vs. ditched marshes) on adult and nest survival to aid in prioritizing future management or restoration actions. Apparent adult survival (61.6%, 95% CI: 52.5–70.0%) of Seaside Sparrows averaged > 1.5 times greater than that of Saltmarsh Sparrows (39.9%, 95% CI: 34.0–46.2%). Nest survival and predation and flooding rates did not differ between species, and predation was the primary cause of nest failure for both species. Apparent adult survival and nest survival did not differ between unditched and ditched marshes for either species, indicating that marsh ditching history may not affect the quality of breeding habitat for these species. Because predation was the primary cause of nest failure for both species in New Jersey, we suggest that future studies should focus on identifying predator communities in salt marshes and the potential for implementing predator‐control programs to limit population declines.     

Western Snowy Plovers Charadrius alexandrinus nivosus select nesting substrates that enhance egg crypsis and improve nest survival

Predation is an important cause of nest failure for many birds and has shaped the life‐history characteristics of many species, especially ground‐nesting shorebirds. We examined nesting success, causes of clutch failure and nest survival in relation to variation in substrate characteristics in a colour‐marked population of Western Snowy Plovers Charadrius alexandrinus nivosus breeding on riverine gravel bars in coastal northern California. Plovers experienced higher nesting success on gravel bars than on nearby beaches, which were characterized by more homogeneous, sandy substrates. On gravel bars, Plovers nested in habitats characterized by large, heterogeneous substrates, with more egg‐sized stones, compared with random sites. Egg crypsis, as indexed by time required of a naïve observer to detect a nest, increased with number of egg‐sized substrates. Nest survival correlated negatively with heterogeneity of substrates and positively with the number of egg‐sized stones. Consistently high nesting success of Plovers on gravel bars indicates that this high‐quality habitat deserves special management considerations given the species’ threatened status.     

Green plant material versus ectoparasites in nests of Bonelli's eagle

The greenery and arthropod nest composition has been studied in a population of Bonelli's eagle Hieraaetus fasciatus in south-east Spain, relating them to the plant availability within territories and breeding success of pairs. Greenery was invariably from trees and shrubs, with pine and oak species accounting for 78% of the nest composition in weight. All eagle pairs with Pinus pinaster availability in territories actively selected it for nest greenery. This pine species is characterized by a high level of aromatic compounds, particularly β -pinene, highly repellent for insects. The amount of pine greenery in the nest was correlated with a lower presence of ectoparasites in that nest (blow fly larvae, Protocalliphora) , and higher breeding success of pairs. We discuss whether a coevolutionary process between parasites and their hosts has guided a particular nesting strategy of the eagles to improve their breeding success.     

The value of nestboxes in the conservation of Eurasian Rollers Coracias garrulus in southern Spain

The installation of nestboxes is a widely used conservation measure for enhancing habitat suitability for cavity‐nesting birds wherever natural holes are scarce. However, nestboxes may attract birds to unsuitable breeding places or induce a non‐random distribution of individuals in relation to their qualities. We investigated the factors influencing nestbox selection and the quality of breeding territories for the globally near‐threatened Eurasian Roller in a semi‐arid area of southeast of Spain over a 3‐year period. Rollers preferentially used exposed nestboxes that were placed far from highly modified areas. However, breeding success was significantly lower in exposed nestboxes. Early breeders preferred nestboxes oriented to the northwest over any other orientation. Exposure had opposite effects on nestbox use and breeding success, suggesting that nestboxes installed in exposed sites could function as ecological traps for Rollers.     

Abundance,territory sizes,and pairing success of male Henslow's Sparrows in restored warm‐ and cool‐season grasslands

Henslow's Sparrows (Ammodramus henslowii) are known to breed in restored grasslands consisting of either warm‐ or cool‐season grasses, but additional information is needed concerning their breeding biology in these two types of grasslands. We compared the abundance, territory sizes, and pairing success of male Henslow's Sparrows in grasslands in west‐central Missouri in 2010 and 2011 using a paired‐treatment design, where warm‐ and cool‐season grasslands were located in close proximity. Selection indices indicated no apparent preference by male Henslow's Sparrows for either type of grassland, and the territory sizes and pairing success of males in cool‐ and warm‐season grasslands did not differ. In addition, we found no significant differences in habitat structure between these grassland types. Thus, our results suggest that the warm‐ and cool‐season grasslands in our study provided suitable habitat for Henslow's Sparrows. Other investigators have drawn similar conclusions for Henslow's Sparrows and for grassland birds in general, with the structural characteristics and, for some species, the size of grasslands being more important for grassland birds than plant species composition.     

American Black Duck Nesting Ecology in North Carolina

North Carolina, USA, is the southernmost extent of the American black duck's (Anas rubripes) breeding range; however, little is known about their nesting ecology in this region. We located and monitored 140 nesting black ducks over 2 years (2017–2018) to quantify preferred nesting habitat and assess nesting productivity within coastal North Carolina. We located nests in brackish marshes (75%) and man-made dredge spoil islands (25%) at a density of 1 nest/22 ha. Black ducks selected high marsh and nested an average of 21.81 m from open water at a mean elevation of 1.36 m. In preferred nesting habitat, visual obstruction readings were 0.50 m with a maximum mean vegetation height of 0.81 m and land cover consisted largely of grasses (84.6%). Apparent nest success rates varied from 31% (2017) to 63% (2018) across years. The majority (72.2%) of variability in nest success was best predicted by nest location (mainland marsh, natural island, or spoil island), vegetation density, maximum vegetation height, and year. Management for breeding black ducks in coastal North Carolina should focus on promoting selected nesting habitat and reducing nest predators. Prescribed burns, used to set back succession on spoil islands and in brackish marshes should be conducted in the winter or in the early growing season not to exceed the twenty-fifth quantile date of black duck nest initiation (2 Apr). © 2021 The Wildlife Society.     

The design and function of birds' nests

All birds construct nests in which to lay eggs and/or raise offspring. Traditionally, it was thought that natural selection and the requirement to minimize the risk of predation determined the design of completed nests. However, it is becoming increasingly apparent that sexual selection also influences nest design. This is an important development as while species such as bowerbirds build structures that are extended phenotypic signals whose sole purpose is to attract a mate, nests contain eggs and/or offspring, thereby suggesting a direct trade‐off between the conflicting requirements of natural and sexual selection. Nest design also varies adaptively in order to both minimize the detrimental effects of parasites and to create a suitable microclimate for parents and developing offspring in relation to predictable variation in environmental conditions. Our understanding of the design and function of birds' nests has increased considerably in recent years, and the evidence suggests that nests have four nonmutually exclusive functions. Consequently, we conclude that the design of birds' nests is far more sophisticated than previously realized and that nests are multifunctional structures that have important fitness consequences for the builder/s.     

Validating the use of temperature data loggers to measure survival of songbird nests

ABSTRACT.   Accurate determination of nest fates and nest predators is possible through continuous video monitoring, but such monitoring is relatively expensive and labor intensive. If documenting of the timing of nest termination events is sufficient, then data loggers (DL) may allow more extensive sampling and may represent a viable alternative. I validated temperature DL records of nest survival time by simultaneous videotaping and compared results derived from DL records with those obtained by regular nest visits by an observer. I estimated the fate of 937 nests of nine species of open cup-nesting songbirds, including 673 nests monitored using DL, 165 monitored using video cameras, 33 validation nests monitored simultaneously using both DL and video cameras, and 132 control nests monitored only by observer visits. Deployment of DL did not negatively influence nest survival rate. DL reliably recorded survival time and allowed classification of nest fates based on the potential fledging age, regardless of the frequency of nest visits by an observer. The true fate of nests that survived beyond the potential fledging age can not be safely determined from time of failure, except for nocturnal events that suggest partial predation. Video revealed frequent partial or complete predation on nests with old nestlings that would have been categorized as successful by other methods. I conclude that temperature DL are efficient, reliable, and relatively inexpensive tools for recording exact nest survival times and classification of nest fates, with implications for nest survival modeling and discriminating between diurnal and nocturnal predation.     

Plant choice in the construction of night nests by gorillas in the Bwindi Impenetrable National Park, Uganda

We investigated the choice of plants in nest sites and individual night nests of a group of gorillas (Gorilla beringei) in Bwindi Impenetrable National Park, Uganda. Most of the nests were built on the ground in secondary forest or canopy gaps. The gorillas used 62 plant genera in their nests out of a possible 108 plant genera available in the immediate environment. This group of Bwindi gorillas chose nest sites nonrandomly with respect to habitat type and selected nest sites in which Pteridium spp. or Mimulopsis spp. were the dominant plants. The Bwindi gorillas selected Pteridium spp. and Ipomea spp. to construct their individual night nests in greater quantities compared to their proportional availability in the immediate environment.     

Strong selection on mandible and nest features in a carpenter bee that nests in two sympatric host plants

Host plants are used by herbivorous insects as feeding or nesting resources. In wood‐boring insects, host plants features may impose selective forces leading to phenotypic differentiation on traits related to nest construction. Carpenter bees build their nests in dead stems or dry twigs of shrubs and trees; thus, mandibles are essential for the nesting process, and the nest is required for egg laying and offspring survival. We explored the shape and intensity of natural selection on phenotypic variation on three size measures of the bees (intertegular width, wing length, and mandible area) and two nest architecture measures (tunnel length and diameter) on bees using the native species Chusquea quila (Poaceae), and the alloctonous species Rubus ulmifolius (Rosaceae), in central Chile. Our results showed significant and positive linear selection gradients for tunnel length on both hosts, indicating that bees building long nests have more offspring. Bees with broader mandibles show greater fitness on C. quila but not on R. ulmifolius. Considering that C. quila represents a selective force on mandible area, we hypothesized a high adaptive value of this trait, resulting in higher fitness values when nesting on this host, despite its wood is denser and hence more difficult to be bored.     

Nest sites as limiting resources for cavity‐nesting birds in mature forest ecosystems: a review of the evidence

ABSTRACT Assumptions that populations of cavity‐nesting birds are limited by access to nest sites have largely been based on anecdotal reports or correlative data. Nest‐box‐addition experiments or tree‐cavity‐blocking experiments are potentially rigorous ways to investigate how densities of breeding birds are affected by access to nest cavities. Experimental evidence indicates that natural tree holes are limited in human‐altered landscapes, but the possibility that cavity nests are limited in old growth (unmanaged) forests is less clear. I reviewed 31 nest‐cavity‐removal or addition experiments conducted with 20 species of cavity‐nesting birds in mature forests. Of these 31 experiments conducted with a variety of different species of birds, only 19% reported statistically significant changes in breeding densities. However, none of these studies included data about the reproductive history of individuals colonizing the boxes (i.e., whether birds using the boxes would have otherwise been floaters or that birds excluded from blocked cavities on the plots did not simply move elsewhere), so they provided no strong evidence that the number of breeding pairs was limited by availability of nest sites at the population scale. Although some studies indicate that nest sites are limited at local (plot) scales in old growth forests, there is still little empirical evidence for nest‐site limitation at the population‐ and landscape‐level in mature, unmanaged forests. I review the challenges in designing and interpreting box‐addition experiments and highlight the main gaps in knowledge that should be targeted in the future.     

Comparative nest survival of three sympatric loon species breeding in the Arctic

Identifying factors influencing nest survival among sympatric species is important for understanding and managing sources of variation in population dynamics of individual species. Three species of loons nest sympatrically in northern Alaska and differ in body size, life history characteristics, and population trends. We tested the effects of competition, nest site selection, and water level variations on nest survival of Pacific Gavia pacifica, yellow‐billed G. adamsii, and red‐throated loons G. stellata on the Arctic Coastal Plain in Alaska. Although overall nest survival rates did not differ between species, the factors influencing nest survival varied. Nest site selection influenced nest survival for Pacific and yellow‐billed loons, with both species having high nest survival when nesting on islands and peninsulas, likely due to a reduction in access by terrestrial predators. However, on mainland shorelines, Pacific loons had lower nest survival than yellow‐billed loons, and used a higher proportion of vegetation mats for nest sites suggesting that their smaller body size makes them less adept at nest defense. Nest site selection did not influence nest survival of red‐throated loons corresponding to our result of no nest site preferences by this species. Initiation date had a strong influence on nest survival for Pacific and yellow‐billed loons with nests laid earlier having higher survival. Pacific and yellow‐billed loon nests were susceptible to flooding due to precipitation, which contrasted with red‐throated loons that nest on smaller lakes with lower water level variations. Competition did not affect nest survival for any of the species likely due to most territorial encounters occurring prior to incubation. The only influence we found on red‐throated loon nest survival was differences among years. Our results indicate that loons chose nest sites based on predation risk and that factors influencing breeding success of closely related species may differ under similar breeding conditions.