Corticosterone levels as indicators of habitat quality: effects of habitat segregation in a migratory bird during the non-breeding season

During the non-breeding season, many species of territorial migratory birds exhibit a non-random pattern of habitat distribution, with males and females occupying different habitats. In this study, we examined possible physiological consequences arising from such habitat segregation in one migrant passerine species, the American redstart (Setophaga ruticilla), on its non-breeding grounds in Jamaica, West Indies. For 2 years, we measured concentrations of corticosterone, at the time of capture (baseline) and 30 min after capture (profile of acute corticosterone secretion), in redstarts in two distinct habitats, one occupied predominately by males and one mostly by females. All redstarts in both habitat types exhibited similar concentrations of baseline corticosterone levels in fall (October), whereas in spring (March–April), redstarts in female-biased habitat exhibited significantly higher baseline levels regardless of age or sex. In fall, all individuals in both habitats exhibited significant increases in corticosterone concentration with capture and handling, but in spring only redstarts (both sexes) in male-biased habitat continued to exhibit acute corticosterone secretion. Redstarts in female-biased habitat had elevated baseline corticosterone levels and reduced acute corticosterone secretion. In spring, baseline corticosterone concentration was negatively correlated with body mass, suggesting muscle catabolism associated with high corticosterone concentrations or possibly that birds are leaner as a result of increased foraging effort. These results indicate that redstarts (primarily females) in female-biased habitats suffered a decline in physiological condition, which could in turn influence their departure schedules, migration patterns and even their condition and arrival schedules on the breeding grounds. Thus, segregation of populations into habitats of different quality during the non-breeding period may have ramifications throughout the annual cycle of such migratory species. Furthermore, these results show the usefulness of plasma corticosterone levels as indicators of physiological condition and thus habitat quality for birds during the non-breeding period. Received: 14 November 1997 / Accepted: 9 March 1998     

Sexual segregation in western grey kangaroos: testing alternative evolutionary hypotheses

In sexually dimorphic ungulates, sexual segregation is hypothesized to have evolved because of sex-specific differences in body size and/or reproductive strategies. We tested these alternative hypotheses in kangaroos, which are ecological analogues of ungulates. Kangaroos exhibit a wide range of body sizes, particularly among mature males, and so the effects of body size and sex can be distinguished. We tested predictions derived from these hypotheses by comparing the distribution of three sex–sex size classes of western grey kangaroos Macropus fuliginosus , in different habitats, and the composition of groups of kangaroos, across seasons. In accordance with the predation risk-reproductive strategy hypothesis, during the non-breeding season, females, which were more susceptible to predation than larger males, and were accompanied by vulnerable young-at-foot, were over-represented in secure habitats. Large males, which were essentially immune to predation, occurred more often than expected in nutrient-rich habitat, and small males, which faced competing demands of predator avoidance and feeding, were intermediate between females and large males in their distribution across habitats. During the breeding season, females continued to be over-represented in secure habitats when their newly emerged pouch young were most vulnerable to predation. All males occupied these same habitats to maximize their chances of securing mates. Consistent with the social hypotheses, groups composed of individuals of the same sex, irrespective of body size, were over-represented in the population during the non-breeding season, while during the breeding season all males sought females so that mixed-sex groups predominated. These results indicate that body size and reproductive strategies are both important, yet independent, factors influencing segregation in western grey kangaroos.     

The Long and the Short of It: No Dietary Specialisation between Male and Female Western Sandpipers Despite Strong Bill Size Dimorphism

Many bird species show spatial or habitat segregation of the sexes during the non-breeding season. One potential ecological explanation is that differences in bill morphology favour foraging niche specialisation and segregation. Western sandpipers Calidris mauri have pronounced bill size dimorphism, with female bills averaging 15% longer than those of males. The sexes differ in foraging behaviour and exhibit partial latitudinal segregation during the non-breeding season, with males predominant in the north and females in the south. Niche specialisation at a local scale might account for this broad geographic pattern, and we investigated whether longer-billed females and shorter-billed males occupy different foraging niches at 16 sites across the non-breeding range. We used stable-nitrogen (δ¹⁵N) isotope analysis of whole blood to test for dietary specialisation according to bill length and sex. Stable-nitrogen isotope ratios increase with trophic level. We predicted that δ¹⁵N values would increase with bill length and would be higher for females, which use a greater proportion of foraging behaviour that targets higher-trophic level prey. We used stable-carbon (δ¹³C) isotope analysis to test for habitat segregation according to bill length and sex. Stable-carbon isotope ratios vary between marine- and freshwater-influenced habitats. We predicted that δ¹³C values would differ between males and females if the sexes segregate between habitat types. Using a model selection approach, we found little support for a relationship between δ¹⁵N and either bill length or sex. There was some indication, however, that more marine δ¹³C values occur with shorter bill lengths. Our findings provide little evidence that male and female western sandpipers exhibit dietary specialisation as a function of their bill size, but indicate that the sexes may segregate in different habitats according to bill length at some non-breeding sites. Potential ecological factors underlying habitat segregation between sexes include differences in preferred habitat type and predation risk.     

Seasonal changes in testosterone and corticosterone levels in four social classes of a desert dwelling sociable rodent

Animals have to adjust their physiology to seasonal changes, in response to variation in food availability, social tactics and reproduction. I compared basal corticosterone and testosterone levels in free ranging striped mouse from a desert habitat, comparing between the sexes, breeding and philopatric non-breeding individuals, and between the breeding and the non-breeding season. I expected differences between breeders and non-breeders and between seasons with high and low food availability. Basal serum corticosterone was measured from 132 different individuals and serum testosterone from 176 different individuals of free living striped mice. Corticosterone and testosterone levels were independent of age, body weight and not influenced by carrying a transmitter. The levels of corticosterone and testosterone declined by approximately 50% from the breeding to the non-breeding season in breeding females as well as non-breeding males and females. In contrast, breeding males showed much lower corticosterone levels during the breeding season than all other classes, and were the only class that showed an increase of corticosterone from the breeding to the non-breeding season. As a result, breeding males had similar corticosterone levels as other social classes during the non-breeding season. During the breeding season, breeding males had much higher testosterone levels than other classes, which decreased significantly from the breeding to the non-breeding season. My results support the prediction that corticosterone decreases during periods of low food abundance. Variation in the pattern of hormonal secretion in striped mice might assist them to cope with seasonal changes in energy demand in a desert habitat.     

Winter segregation of migrant European robins Erithacus rubecula in relation to sex, age and size

Birds often show some form of social segregation during winter, both at large geographical scales (a consequence of differential migration) and at the regional or local level, when comparing different habitats or micro-habitats. However, our understanding of the mechanisms underlying such patterns is still poor. These issues have been rarely investigated in migratory Old-World passerines, particularly with respect to differences between the sexes. In this study, we show that female European robins Erithacus rubecula (sexed by molecular techniques) greatly outnumber males in southern Iberia, which confirms that this species is a differential migrant with a strong latitudinal segregation of the sexes. Furthermore, sex, age and body size influence the habitat distribution of robins in winter. Subordinate birds (females, juveniles and small individuals) were generally more common in habitats with a greater shrub development, and comparatively scarce in woodlands with relatively little undergrowth. Birds wintering in woodlands were in better condition (assessed by breast-muscle scoring) than birds wintering in shrubland. These results are consistent with the hypothesis that proposes that social dominance, mediated by differences in size and experience, is important in determining the habitat segregation of sex and age classes. The alternative hypothesis (habitat specialization), although not specifically supported by our findings, cannot be ruled out on the basis of the available evidence.     

Habitat use and mating system of the houbara bustard (Chlamydotis undulata undulata) in a semi-desertic area of North Africa: implications for conservation

Studies of the movements and home-ranges of houbara bustards (Chlamydotis undulata undulata) showed sexual and seasonal differences in the use of space, with a polygynous mating system similar to an ‘exploded-lek’ or a ‘resource-defence-polygyny’, that remains undefined. We used the arthropod biomass as an index of the trophic quality of six defined habitats and we radio-tracked 7 females and 13 males to test whether sexual and seasonal variations in habitat use were related to resource availability, and to verify if critical resources for breeding females were monopolised by males. We analysed habitat selection in both sexes separately. We used the habitat type composition of buffer zones around radio-locations to study annual and seasonal habitat selection and to identify preferred habitats, using the chi-square goodness-of-fit test. Habitat use between sexes and between seasons were compared using MANOVA based on log-ratios of habitat proportions. During the year, and in each season, both sexes appeared to be significantly selective for habitats in comparison to their availability. But males avoided esparto grass, while females used all habitats. Habitat use differed between sexes in the breeding season, but not in the non-breeding season. In spring, when food resources were abundant and uniformly distributed in space, males preferred ‘temporarily flooded areas’ and females preferred ‘reg with tall perennials’ that offered both food and cover for brooding. Critical resources were not monopolised by males and the mating system fulfilled the definition of the ‘exploded-lek’. Leks are key sites for reproduction and should be considered as priority areas in further conservation plans.     

The value of abandoned olive groves for blackcaps (<Emphasis Type="Italic">Sylvia atricapilla</Emphasis>) in a Mediterranean agroecosystem: a year-round telemetry study

The European Common Agricultural Policy has promoted the intensification of productive olive orchards and the abandonment of the unproductive ones. This strategy has resulted in a significant loss of biodiversity and ecosystem functionality of this type of agroecosystem. Here, we studied the blackcap Sylvia atricapilla, a common frugivorous bird species, in an olive-dominated agroecosystem of southern Spain to assess the effects of the abandonment of olive groves and undergrowth structure on habitat selection in this species. By means of radiotelemetry, we determined habitat selection of blackcaps during both the breeding (N = 30) and the non-breeding (N = 27) seasons of 2011 and 2012. We found that outside the breeding season, olive groves were widely used by blackcaps, which invariably preferred abandoned olive groves over intensively managed ones. Additionally, deciduous woods and poplar plantation were positively selected, whereas open habitats and Pinus forests were avoided. Generally, woody habitats without undergrowth or with herbaceous undergrowth were avoided. During the breeding season, the species selected mainly riverine habitats and poplar plantations, favoring habitats with well-developed undergrowth. Our results suggest that the blackcap could benefit from maintaining patches of abandoned olive groves freely left to natural succession within intensive olive groves. In such a landscape, ecological services (i.e., seed dispersal provided by frugivorous birds) and functioning could also improve as a consequence of these measures.     

Ontogenetic niche partitioning in southern elephant seals from Argentine Patagonia

Elephant seals, Mirounga spp., are highly dimorphic, having different energetic requirements according to age and sex, and foraging in various ecological and oceanographic contexts. Resource partitioning has been shown for the sub-Antarctic populations of southern elephant seals, M. leonina, where colonies are surrounded by narrow shelves that deepen abruptly. In contrast, seals from Península Valdés (Argentina), in the northernmost extent of the breeding range, face an extended, shallow, temperate, and productive continental shelf. We integrated tracking data from 98 animals (juveniles and adults, males and females) gathered over more than two decades, and found that although all available habitats were used, individuals segregated by age and sex. Juvenile males favored shelf habitats, whereas subadult and adult males also used the shelf break. Juvenile females preferred the shelf and the more distant Argentine Basin used by postbreeding and postmolt adult females. Males showed the highest proportion of area-restricted search locations, suggesting more spatially concentrated feeding activity, and likely reflecting a preference for foraging habitat and prey. Our results are consistent with those from other populations, implying that elephant seals show remarkable similarities in habitat use by age and sex classes, despite broad differences in the offshore habitats between sub-Antarctic and temperate ecosystems.     

Intraspecific correlations among morphology, performance and habitat use within a green anole lizard (Anolis carolinensis) population

We examined habitat use, morphology, jumping and clinging ability for 403 juvenile, female and male green anole lizards, Anolis carolinensis, in a population in south‐eastern Louisiana. We sought to answer three questions: (1) Do age/sex classes differ in habitat use, morphology and performance ability? (2) Do habitat use, morphology and performance correlate among all individuals across three age/sex classes (juveniles, females and males)? (3) Do juveniles compensate for their poor absolute performance capacities by being better performers on a relative scale? The three age/sex classes were found to differ significantly in size‐adjusted morphology, habitat use and size‐adjusted performance capacity. Juveniles tended to occupy perches which were closer together than those of adult males and females. The distal elements of the hindlimb (femur, tibia) were significantly longer in males than in females and juveniles, while females were more stocky than males and juveniles. The only significant overall ecomorphological relationship detected was between the lengths of the distal hindlimb elements and maximum jump acceleration. Our hypothesis that juveniles should be better performers (relative to size) compared to adults was disproved, as adult females were always the best performers relative to size. Our analysis of a mainland anole population presents a different view of population structure compared to similar studies involving Caribbean Anolis lizards, which show more ecological differentiation among age/sex classes, and also show that juveniles are relatively good performers. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85 , 211–221.     

Endogenous reserve dynamics of northern common eiders wintering in Greenland

Endogenous reserves influence both survival and reproduction of many waterfowl species, but little is known about reserve levels of most species during the nonbreeding season, particularly those wintering at high latitudes. We investigated whether age, sex, and season were related to carcass composition of northern common eiders (Somateria mollissima borealis) wintering in southwest Greenland during 1999–2002. Adults carried more lipid and protein than juveniles during all winters. Among both age classes, males and females had similar fat levels but males carried slightly more protein. There was no dramatic seasonal variation in lipid or protein content. This suggests that during the period of this study, these eiders did not experience large-scale nutritional shortfalls. As predicted, Greenlandic eiders carried more lipid reserves than eider populations wintering in more temperate environments. Contrary to prediction, there was little relation between reserve levels and photoperiod, ambient temperature, or hunting disturbance intensity. Our results suggest that both sexes are equally capable of dealing with nutritional deficits, and that juvenile birds are more prone to nutritional stress as evidenced by their consistently poorer body condition.     

Sex differences in habitat use by African elephants (Loxodonta africana) in the Okavango Delta,Botswana: is size really the deciding factor?

Many sexually dimorphic mammals show sexual segregation of habitat use. We studied habitat selection and use by male and female elephants in the Okavango Delta, and males of different ages and therefore sizes, to assess whether size was a driving force behind any sex differences in habitat selectivity. There was variation in habitat choices, with males preferring island vegetation and mopane woodland and avoiding grassland/floodplain and Terminalia woodland; females showed no selection for or against most habitats other than selecting for mopane woodland in the rainy season. Male habitat choice changed dramatically during the dry season, when resources were most limited and there was the least sexual difference in habitat selection. Females were more selective in the flood season, when access to resources was restricted. Contrary to other studies and the forage selection hypothesis, males showed greater habitat selectivity than females, but age, and therefore size, did not affect their habitat selection. Whilst our data suggested that males were superior competitors, we could not discount that females excluded males from preferred habitats. In the Okavango Delta, habitat selection by male elephants may be more dependent on social groupings, and hence the decisions of others, than individual size and energetic requirements.     

Sex differences in settlement behaviour and condition of chiffchaffs Phylloscopus collybita at a wintering site in Portugal. Are females doing better?

In passerine birds, males are generally larger and dominant over females. In line with the dominance theory, in all known differential migrant passerines, females migrate further than males. However, there are alternative explanations to the dominance hypothesis, including the specialisation hypothesis, predicting that females should do better than males in habitats and/or regions to which they are specially adapted, and where they predominate in numbers. Few studies have aimed at comparing the behaviour and condition of males and females wintering in locations largely dominated by female birds. We studied a partly nomadic species, the common chiffchaff Phylloscopus collybita, at a wintering site in Portugal. Most individuals caught at this site were transients. The only consistent and relatively powerful predictor of settlement behaviour was sex. Almost all the birds that settled in the study area were female, while there was an almost even sex ratio in the transient sample. Females had higher muscle and post-juvenile moult scores when first captured. In addition, there was a significant sex-based difference in mass trajectories, with females maintaining body mass and males losing body mass when settled at the study site. Results from this study suggest that the poor performance of males was either due to inferior male competitive ability in a region/habitat where females predominate and/or it resulted from the fact that males that reach these wintering grounds are of inferior individual quality.     

Consequences of the settlement of migrant European Robins Erithacus rubecula in wintering habitats occupied by conspecific residents

In wintering areas where migrant birds meet sedentary conspecifics, early settlement of local residents in the best habitat patches might reduce the availability of suitable sites for arriving migrants. We studied how sympatric migratory and sedentary European Robins Erithacus rubecula occupy two wintering habitats of different quality (forests and shrublands) in southern Spain, and how such a distribution affects individuals of each population sector. In September, before migrants arrived, Robins were only found in forests, and they had already saturated these habitats, so that rather than increasing Robin abundance in these habitats, the arrival of migrants caused a massive occupation of the previously vacant shrublands. During the winter, we captured Robins and identified them as migrants or residents using a discriminant function based on morphological traits. Residents always predominated in forests, and migrants in shrublands, but through the winter around 35% of residents (mainly juveniles) moved to shrublands, having been replaced by some migrants in forests. Although food was more abundant in shrublands, Robins had better body condition in forests, suggesting that other factors determined habitat preferences (e.g. shelter availability or food diversity, which were higher in forests). In addition, we observed a greater variance in body mass relative to body size in forests, suggesting that energy management was less constrained in this habitat (for example owing to a lower exposure to predators or a higher food predictability). Our results suggest that sedentary Robins benefit from an early occupation of the best habitats in the wintering grounds, forcing migrants to colonize apparently less suitable sites. This would explain the persistence of these small southern populations despite the yearly flooding of the area by huge numbers of migrant conspecifics.     

Climate change and the decline of a once common bird

Climate change is predicted to negatively impact wildlife through a variety of mechanisms including retraction of range. We used data from the North American Breeding Bird Survey and regional and global climate indices to examine the effects of climate change on the breeding distribution of the Rusty Blackbird (Euphagus carolinus), a formerly common species that is rapidly declining. We found that the range of the Rusty Blackbird retracted northward by 143 km since the 1960s and that the probability of local extinction was highest at the southern range margin. Furthermore, we found that the mean breeding latitude of the Rusty Blackbird was significant and positively correlated with the Pacific Decadal Oscillation with a lag of six years. Because the annual distribution of the Rusty Blackbird is affected by annual weather patterns produced by the Pacific Decadal Oscillation, our results support the hypothesis that directional climate change over the past 40 years is contributing to the decline of the Rusty Blackbird. Our study is the first to implicate climate change, acting through range retraction, in a major decline of a formerly common bird species.     

The importance of traditional farmland areas for steppe birds: a case study of migrant female Great Bustards Otis tarda in Spain

A detailed knowledge of the habitat requirements of steppe birds living in farmland habitats is necessary to identify agricultural practices compatible with their conservation. The globally threatened Great Bustard Otis tarda is a partial migrant in central Iberia, but factors affecting its winter habitat use have not been identified. We assessed habitat differences between breeding and wintering areas and winter habitat selection of radiotagged migrant female Great Bustards in central Spain. Of 68 tagged females, 35% moved to wintering areas located 64.3 ± 24.0 km south of their breeding areas, and 80% wintered in a single area of c. 236 km². A census of the population in this area identified it as one of the most important wintering areas of this species in the world, holding c. 1500 individuals. There were significant differences between breeding and wintering habitats of individually marked migrant females. Compared with breeding areas, wintering areas of migrant females were located further from roads and urban nuclei, had lower human population densities and area of urban developments, and a higher diversity of land‐use types, with less cover of cereals and more vineyards and olive groves. Within this area, radiotracked migrant females preferred sites with more vineyards and a lower land‐use diversity. Our results highlight the importance of traditional Mediterranean dry farmland mosaics, and suggest that different conservation strategies are needed for migrant and resident populations in winter to secure the conservation of suitable wintering habitat for Great Bustards in the Iberian Peninsula.     

Rainfall and habitat interact to affect the condition of a wintering migratory songbird in The Bahamas

On the subtropical and tropical wintering grounds of migratory birds, variation in moisture levels and habitat can influence the availability of food resources and subsequently impact overwintering birds. Using stable carbon isotopes in blood samples as a measure of moisture, we assessed the interactive effects of rainfall, vegetation, and moisture on the demographics and condition of Prairie Warblers (Setophaga discolor) wintering in The Bahamas. Carbon isotopes in Prairie Warbler blood were more depleted in taller, wetter habitats; we additionally detected novel temporal effects of rainfall on isotope values. During a winter with more rainfall, most birds maintained mass and pectoral muscle regardless of the habitat type occupied. In a winter with less rainfall, birds lost mass and pectoral muscle, and this effect was more pronounced in birds with enriched isotope values and birds that occupied drier, shorter habitat. Prairie Warblers exhibited strong patterns of sexual habitat segregation with males disproportionately observed in areas with taller vegetation and females in shorter vegetation. During the drier winter, older males had better maintenance of pectoral muscle compared to females and younger individuals. Also in the drier winter, daily rainfall patterns explained more of the variation in body condition compared to the date of capture; pectoral muscle was best explained by recent precipitation (during the previous 30 days), while size‐corrected mass was more a function of longer‐term (90‐day) rainfall and habitat moisture. Our findings along with other studies suggest that Prairie Warblers and other migratory birds are sensitive to interactions between annual variation in winter rainfall, within‐season daily rainfall patterns, and habitat quality. Increasing drought and habitat loss in the Caribbean may be having a negative impact on wintering bird populations. To best conserve Nearctic–Neotropical migratory passerines in the region, we recommend prioritizing the protection of the least drought‐prone wintering areas.     

Territorial aggression and hormones during the non-breeding season in a tropical bird

The hormonal control of territorial aggression in male and female vertebrates outside the breeding season is still unresolved. Most vertebrates have regressed gonads when not breeding and do not secrete high levels of sex steroids. However, recent studies implicate estrogens in the regulation of non-breeding territoriality in some bird species. One possible source of steroids during the non-breeding season could be the adrenal glands that are known to produce sex steroid precursors such as dehydroepiandrosterone (DHEA). We studied tropical, year-round territorial spotted antbirds (Hylophylax n. naevioides) and asked (1). whether both males and females are aggressive in the non-breeding season and (2). whether DHEA is detectable in the plasma at that time. We conducted simulated territorial intrusions (STIs) with live decoys to male and female free-living spotted antbirds in central Panama. Non-breeding males and females displayed robust aggressive responses to STIs, and responded more intensely to decoys of their own sex. In both sexes, plasma DHEA concentrations were detectable and higher than levels of testosterone (T) and 17beta-estradiol (E(2)). In males, plasma DHEA concentrations were positively correlated with STI duration. Next, we conducted STIs in captive non-breeding birds. Captive males and females displayed robust aggressive behavior. Plasma DHEA concentrations were detectable in both sexes, whereas T was non-detectable (E(2) was not measured). Plasma DHEA concentrations of males were positively correlated with aggressive vocalizations and appeared to increase with longer STI durations. We conclude that male and female spotted antbirds can produce DHEA during the non-breeding season and DHEA may serve as a precursor of sex steroids for the regulation of year-round territorial behavior in both sexes.     

Does sex matter? Gender‐specific responses to forest fragmentation in Neotropical bats

Understanding the consequences of habitat modification on wildlife communities is central to the development of conservation strategies. However, albeit male and female individuals of numerous species are known to exhibit differences in habitat use, sex‐specific responses to habitat modification remain little explored. Here, we used a landscape‐scale fragmentation experiment to assess, separately for males and females, the effects of fragmentation on the abundance of Carollia perspicillata and Rhinophylla pumilio, two widespread Neotropical frugivorous bats. We predicted that sex‐specific responses would arise from higher energetic requirements from pregnancy and lactation in females. Analyses were conducted independently for each season, and we further investigated the joint responses to local and landscape‐scale metrics of habitat quality, composition, and configuration. Although males and females responded similarly to a fragmentation gradient composed by continuous forest, fragment interiors, edges, and matrix habitats, we found marked differences between sexes in habitat use for at least one of the seasons. Whereas the sex ratio varied little in continuous forest and fragment interiors, females were found to be more abundant than males in edge and matrix habitats. This difference was more prominent in the dry season, the reproductive season of both species. For both species, abundance responses to local‐ and landscape‐scale predictors differed between sexes and again, differences were more pronounced in the dry season. The results suggest considerable sex‐mediated responses to forest disruption and degradation in tropical bats and complement our understanding of the impacts of fragmentation on tropical forest vertebrate communities.     

To breed or not to breed: causes and implications of non-breeding habit in the willow tit Parus montanus

Causes and consequences of non-breeding in willow tits were studied in northern Finland during 1986–1992. The breeding status was sex and age biased; males and yearling birds were in excess among the non-reproducers. Due to sex bias in the population it appeared detrimental for males to lose a mate, especially shortly before breeding. Lack of a mate was a important factor for males not reproducing (37% of non-breeding males) than for females (14%). Most of the non-breeding birds maintained a pair bond which only rarely broke up for the next breeding season (divorce rate 5.5%). This implies that parental incompatibility is not a possible explanation for pairs not reproducing. Males that did not breed tended to survive better than reproducing ones, whereas such a relationship was not found for females. It is possible that this sex-related difference in survival cost is attributable to quality differences among non-breeding individuals. It was especially low-quality yearling females, with low survival prospects, that were responsible for the discrepancy. The proportion of non-breeding females in the population correlated highly with clutch size and subsequent juvenile survival. It is therefore suggested that for most of these females non-breeding is a phenotypic response to low offspring value in the prevailing circumstances (inter-generational tradeoff). However, it is uncertain whether willow tits in a northern population can use breeding density as an indicator of changing survival prospects of their descendants, as suggested by Ekman and Askenmo (1986) for southern Sweden.     

Mangrove habitat partitioning by Ucides cordatus (Ucididae): effects of the degree of tidal flooding and tree-species composition during its life cycle

Environmental factors strongly affect mangrove crabs, and some factors modulate population structure and habitat partitioning during the crabs’ life cycle. However, the effect of these environmental factors on habitat selection by mangrove crabs is still unknown. We evaluated habitat selection by the mangrove crab Ucides cordatus in mangrove forests with different degrees of predominance of Rhizophora mangle, Laguncularia racemosa or Avicennia schaueriana, two tidal flooding levels (less- and more-flooded), and two biological periods (breeding and non-breeding seasons). Sampling was conducted in four mangrove forests with different influences of these biotic and abiotic parameters. We used the data for sex ratio to explain environmental partitioning by this species. Females predominated in R. mangle mangroves, independently of the biological period (breeding or non-breeding seasons), and males predominated only in the less-flooded L. racemosa mangroves. The flooding level affected the sex ratio of U. cordatus, with a predominance of males in less-flooded mangroves, independently of the biological period; and a gender balance in the more-flooded mangroves only during the breeding season. Outside the breeding season, the largest specimens were recorded in the R. mangle mangroves, but in the breeding season, the largest crabs were recorded in the L. racemosa mangroves with a higher level of flooding. These results suggest that tree-species composition and tidal flooding level can have a significant effect on the habitat partitioning of sexes and sizes of the mangrove crab U. cordatus both during and outside the breeding season.