Replacement Clutches Research Articles

Evidence of a twofold ecological trap driven by agricultural change causing a priority farmland bird population crash

AbstractExtensive farmland in Europe hosts high biodiversity levels but is threatened by land use changes associated with market and agricultural policy drivers. We show that a nationwide replacement of rainfed cereals in favor of beef production in Portugal has coincided with a nearly 80% decline in the population of a ground‐nesting raptor, the Montagu's harrier Circus pygargus, since 2002. Reduced cereal crop cover led to nesting attempts occurring mostly in fodder crops, the predominant habitat with suitable sward structure at the onset of breeding. This results in a twofold ecological trap: early hay harvesting destroys first nesting attempts, whereas late cereal harvesting destroys replacement clutches. This double phenological mismatch prevents any breeding success and likely explains the country's observed population decline of Montagu's harriers and other key farmland birds. Reforming current agroenvironmental schemes to promote practices compatible with wild population phenology is paramount to revert these changes.

Reproductive morphophysiology of an iteroparous batch spawner fish: from early follicle development to ovulation.

Batch spawner fishes develop successive clutches of oocytes which allows them to participate in many reproductive cycles during their adult life (iteroparous) and spawn in multiple events within each breeding cycle. Here, ovarian follicular development was morpho-functionally analyzed in females of the iteroparous batch spawner fish Gymnocorymbus ternetzi. To obtain better insights into the reproductive morpho-physiology in batch spawners, the objective of this research was to analyze the dynamics of the follicular development, with its hormonal regulation between two active reproduction events. We found that over 16 days, follicles progressed asynchronously to chromatin nucleolus, Primary and Secondary growth stages of oogenesis with progressive secretion of 17β-estradiol (E2). During the end of secondary growth, the increase in 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-p) was measured relative to the maturation process of the ovarian follicles (e.g., nuclear migration and its rupture during the resumption of meiosis). Interestingly, an additional increase in E2 was observed after fish reproduction, probably related to the recruitment of new batch follicles for secondary growth. We also measured the high values of multiple condition factor post-reproduction measurements, reflecting more energy invested during the pre-reproductive process. We also quantified high concentrations of 17,20β-p, probably related to the recruitment of a new batch of oogonia to meiosis, presumably secreted by post-ovulatory follicles, after fish reproduction. We finally found that fish without exposure to reproductive stimulus developed a regression phase at day 24, characterized by massive follicle atresia, that allow them to recycle energy and constitutive materials of the follicles invested during oogenesis for another reproductive cycle.

Species-Specific Seasonal Shifts in Reproductive Allocation in the Southern Grass Lizard, Takydromus sexlineatus (Lacertidae).

We designed a common garden design to collect data on female reproductive traits from three populations of the southern grass lizard Takydromus sexlineatus, testing the hypothesis that a species-specific pattern of seasonal shifts in reproductive allocation should be shared by geographically separated populations. Of the seven examined traits, six differed among populations, with four of the six also differing among successive clutches. Females grew longer during the breeding season and produced more eggs in the first clutch than in the subsequent clutches; egg size was unchanged throughout the breeding season. After removing the influence of female size or postpartum body mass we found the following. First, postpartum body mass, clutch mass, and relative clutch mass were greater in the Wuzhishan population than in the Shaoguan and Zhaoqing populations. Second, egg size was greatest in the Wuzhishan population and smallest in the Zhaoqing population. Third, clutch size was greatest in the Wuzhishan population and smallest in the Shaoguan population. Females did not trade-off egg size against number within each population × clutch combination. Our study validates the hypothesis tested, supports the conventional view that reproductive output is highly linked to maternal body size in lizards, and follows the classic prediction that females with different amounts of resources to invest in reproduction should give priority to adjusting the total number rather than size of their offspring.

Phylogenetic analysis of viviparity, matrotrophy, and other reproductive patterns in chondrichthyan fishes.

The reproductive diversity of extant cartilaginous fishes (class Chondrichthyes) is extraordinarily broad, reflecting more than 400 million years of evolutionary history. Among their many notable reproductive specialisations are viviparity (live-bearing reproduction) and matrotrophy (maternal provision of nutrients during gestation). However, attempts to understand the evolution of these traits have yielded highly discrepant conclusions. Here, we compile and analyse the current knowledge on the evolution of reproductive diversity in Chondrichthyes with particular foci on the frequency, phylogenetic distribution, and directionality of evolutionary changes in their modes of reproduction. To characterise the evolutionary transformations, we amassed the largest empirical data set of reproductive parameters to date covering nearly 800 extant species and analysed it via a comprehensive molecular-based phylogeny. Our phylogenetic reconstructions indicated that the ancestral pattern for Chondrichthyes is 'short single oviparity' (as found in extant holocephalans) in which females lay successive clutches (broods) of one or two eggs. Viviparity has originated at least 12 times, with 10 origins among sharks, one in batoids, and (based on published evidence) another potential origin in a fossil holocephalan. Substantial matrotrophy has evolved at least six times, including one origin of placentotrophy, three separate origins of oophagy (egg ingestion), and two origins of histotrophy (uptake of uterine secretions). In two clades, placentation was replaced by histotrophy. Unlike past reconstructions, our analysis reveals no evidence that viviparity has ever reverted to oviparity in this group. Both viviparity and matrotrophy have arisen by a variety of evolutionary sequences. In addition, the ancestral pattern of oviparity has given rise to three distinct egg-laying patterns that increased clutch (brood) size and/or involved deposition of eggs at advanced stages of development. Geologically, the ancestral oviparous pattern arose in the Paleozoic. Most origins of viviparity and matrotrophy date to the Mesozoic, while a few that are represented at low taxonomic levels are of Cenozoic origin. Coupled with other recent work, this review points the way towards an emerging consensus on reproductive evolution in chondrichthyans while offering a basis for future functional and evolutionary analyses. This review also contributes to conservation efforts by highlighting taxa whose reproductive specialisations reflect distinctive evolutionary trajectories and that deserve special protection and further investigation.

Unpredictability of maternal environment shapes offspring behaviour without affecting stress-induced cortisol in an annual vertebrate

Exposure of females to stressful conditions during pregnancy or oogenesis has a profound effect on the phenotype of their offspring. For example, offspring behavioural phenotype may show altered patterns in terms of the consistency of behavioural patterns and their average level of performance. Maternal stress can also affect the development of the stress axis in offspring leading to alterations in their physiological stress response. However, the majority of evidence comes from studies utilising acute stressors or exogenous glucocorticoids, and little is known about the effect of chronic maternal stress, particularly in the context of stress lasting throughout entire reproductive lifespan. To bridge this knowledge gap, we exposed female sticklebacks to stressful and unpredictable environmental conditions throughout the breeding season. We quantified the activity, sheltering and anxiety-like behaviour of offspring from three successive clutches of these females, and calculated Intra-class Correlation Coefficients for these behaviours in siblings and half-siblings. We also exposed offspring to an acute stressor and measured their peak cortisol levels. An unpredictable maternal environment had no modifying effect on inter-clutch acute stress responsivity, but resulted in diversification of offspring behaviour, indicated by an increased between-individual variability within families. This may represent a bet-hedging strategy, whereby females produce offspring differing in behavioural phenotype, to increase the chance that some of these offspring will be better at coping with the anticipated conditions.

Breeding ecology, population size and nest site preferences of Red-billed Tropicbirds at St Helena, South Atlantic Ocean

ABSTRACT We describe the population size and breeding ecology of the Red-billed Tropicbird, Phaethon aethereus, a poorly studied pantropical seabird, at St Helena, South Atlantic. The population size of 81–246 pairs and 272–564 individuals identifies the study population as the largest colony of Red-billed Tropicbirds in the South Atlantic, but also an internationally important part of the global population. We estimated the survival from laying to fledging of 158 nests between 2004 and 2017 at only 33%, among the lowest values reported globally for the species. Most nest failures occurred during incubation, with predation identified as the predominant cause of fledging failure. Intervals between breeding attempts were longer after successful nesting attempts than failed attempts. Previous breeding interval and nest cavity fidelity further influenced the interval between breeding attempts, while the presence of replacement clutches did not. Multiple nest site and cavity characteristics were important predictors of cavity use, nest site selection and productivity. Management options for reducing mammalian predators to ensure the long-term viability of this important population at St Helena are discussed.

Effect of MHC and inbreeding on disassortative reproduction: A data revisit, extension and inclusion of fertilization in sand lizards

The harmful effects of close inbreeding have been recognized for centuries and, with the rise of Mendelian genetics, was realized to be an effect of homozygosis. This historical background led to great interest in ways to quantify inbreeding, its depression effects on the phenotype and flow-on effects on mate choice and other aspects of behavioral ecology. The mechanisms and cues used to avoid inbreeding are varied and include major histocompatibility complex (MHC) molecules and the peptides they transport as predictors of the degree of genetic relatedness. Here, we revisit and complement data from a Swedish population of sand lizards (Lacerta agilis) showing signs of inbreeding depression to assess the effects of genetic relatedness on pair formation in the wild. Parental pairs were less similar at the MHC than expected under random mating but mated at random with respect to microsatellite relatedness. MHC clustered in groups of RFLP bands but no partner preference was observed with respect to partner MHC cluster genotype. Male MHC band patterns were unrelated to their fertilization success in clutches selected for analysis on the basis of showing mixed paternity. Thus, our data suggest that MHC plays a role in pre-copulatory, but not post-copulatory partner association, suggesting that MHC is not the driver of fertilization bias and gamete recognition in sand lizards.

Replacement Broods in a Cavity Nesting Bird Species under High Predation Pressure in Primeval Forest

Predation is one of the most important factors influencing breeding success. Many bird species suffer from high losses because of predators. I studied breeding success and replacement broods in the Red-breasted Flycatcher Ficedula parva. The Red-breasted Flycatcher is a small bird breeding in natural holes in dense deciduous and mixed forests of Europe. Data were collected during twelve breeding seasons in a primeval forest, in the Bialowieża National Park, Northeast Poland, the best preserved and protected area of the Białowieża Forest with a rich community of birds and mammals including predators. Almost half of the broods of the Red-breasted Flycatcher were lost and most of them because of predation. In cases where the brood was lost in the period of egg laying or incubation, females tended to lay a replacement clutch. First and replacement nest-sites differed insignificantly in characteristics, but new clutches were always laid in a new site. Out of 68 broods, in which a loss was found, replacement broods were recorded in 47.1% cases. Replacement clutches were much smaller and pairs raised significantly fewer fledglings. The proportions of successful and lost broods were similar in the first and replacement attempts (56.1%, and 46.7%, respectively). Red-breasted Flycatchers choose shallow nest-sites that allow the females to observe their surroundings. This allows the female to escape from the nest when a predator approaches, increasing the possibility of a repeat clutch and the attainment of breeding success in the same season.

Effect of Male Body Size on Female Reproduction in Pyrrhocoris apterus (L.) (Heteroptera, Pyrrhocoridae).

Simple SummaryThe benefits of copulation with large males were investigated in the firebug, Pyrrhocoris apterus (L.) (Heteroptera). Sexually immature adults were transferred from the field and kept as female-male pairs with different partner size ratios. We investigated the influence of male body size on the number and weight of eggs and time elapsed from the transfer to their deposition. The expected positive effect of increasing male body size on the characteristics of female reproduction was not detected.Females and males of the abundant heteropteran species Pyrrhocoris apterus (L.) mate with the larger of the available partners. The male benefits from copulation with a large female, which is more productive than a small female. However, the benefit to females from copulation with a large male has not yet been investigated. Overwintered sexually immature adults were transferred from the field to indoors for a long day at 25 °C and subsequently kept as female-male pairs with different partner size ratios. The female lays eggs in several successive clutches. We investigated the influence of male size on the number and weight of eggs in individual clutches and the time elapsed from the transfer to their deposition. These characteristics of reproduction were first regressed on female size, and residuals of this regression were then regressed on male size. The positive effect of increasing male size on the characteristics of female reproduction manifested as a significant value of the latter regressions. The expected positive effect of increasing male body size on female reproductive characteristics was not detected. Several reasons for this deficiency are discussed.

High within-clutch repeatability of eggshell phenotype in Barn Swallows despite less maculated last-laid eggs

Abstract Ecological and life-history variation and both interspecific and intraspecific brood parasitism contribute to diversity in egg phenotype within the same species. In this study, Barn Swallows (Hirundo rustica erythrogaster) laid eggs with high intraclutch repeatability in egg size, shape, and maculation. Despite this high intraclutch repeatability, last-laid eggs had consistently less of the eggshell covered in spots and fewer spots than earlier-laid eggs in the clutch. We examined sources of interclutch and intraclutch variation using both direct measurements and custom software (SpotEgg, NaturePatternMatch) that provide detailed information on egg characteristics, especially maculation measures. In addition to our main findings, maculation on different sides of the egg was highly repeatable; however, only shape, proportion of the eggshell maculated, and average spot size were repeatable between first and replacement clutches. Low intraclutch variation in maculation could allow females to recognize their clutch and this may be adaptive for colonial nesting species, such as the Barn Swallow. Characterizing intraspecific variation in egg size, shape, and maculation is the first step in understanding whether intraclutch variation is low enough—and interclutch variation high enough—such that eggs could serve as identity signals.

Records of multiple clutching in captive mountain chicken frogs Leptodactylus fallax

Multiple clutching, with two or three successful clutches raised in a single breeding season, is reported from two females of Leptodactylus fallax in two European zoos. Previously, only single clutches were known to be raised by this species. Multiple clutching is perhaps unexpected in this species due to its resource-heavy parental care behaviour. Potential drivers of multiple clutching, including food resourcing and timing and size of initial clutches, are discussed.

Helpers during replacement clutches buffer the impacts of late breeding on a cooperative bird

Renesting is an important strategy employed by many bird species to compensate for nest failure. However, although replacement clutches may increase an individual's annual reproductive success, they impose substantial energetic and fitness costs. In some cooperatively breeding species, helpers lighten breeders' workloads thereby facilitating a second clutch within a season or increasing their survival prospects. Alternatively, renesting may increase the chances of having help in replacement clutches as some individuals that have lost their own clutch could redirect their parental effort to another nest. Here we used a 20-year dataset of a facultative cooperative breeder, the Iberian magpie, Cyanopica cooki , to investigate how helping and renesting behaviours combine to affect reproductive output. After controlling for clutch size and laying date, we show that helpers had a positive effect on the reproductive success of pairs, particularly in replacement clutches. Indeed, our study revealed that replacement clutches were much more common than previously thought (observed in three-quarters of the females that lost the first clutch), and that the probability of receiving help was higher in replacement clutches. As helpers increased in number over the breeding season, they could contribute to buffering the negative effects of breeding late, when environmental conditions can be more adverse, and the probability of successful independent reproduction is lower. This study indicates that species with cooperative breeding and a short lifespan can prioritize current over future reproduction by renesting at the end of the breeding season, when the benefits of having help outweigh the costs associated with deteriorating environmental conditions. We propose that renesting can be a context-dependent strategy (mediated by social and environmental factors) that may increase reproductive success in avian cooperative breeding systems. • Renesting is an important context-dependent strategy in Iberian magpies. • Probability of receiving help is higher in replacement than in first clutches. • Helpers increase reproductive success, but more so in replacement clutches. • Helpers thus contribute to buffer the negative effects of late breeding.

Massive genome inversion drives coexistence of divergent morphs in common quails

The presence of population-specific phenotypes often reflects local adaptation or barriers to gene flow. The co-occurrence of phenotypic polymorphisms that are restricted within the range of a highly mobile species is more difficult to explain. An example of such polymorphisms is in the common quail Coturnix coturnix, a small migratory bird that moves widely during the breeding season in search of new mating opportunities, following ephemeral habitats,1,2 and whose females may lay successive clutches at different locations while migrating.3 In spite of this vagility, previous studies reported a higher frequency of heavier males with darker throat coloration in the southwest of the distribution (I. Jiménez-Blasco etal., 2015, Int. Union Game Biol., conference). We used population genomics and cytogenetics to explore the basis of this polymorphism and discovered a large inversion in the genome of the common quail. This inversion extends 115 Mbp in length and encompasses more than 7,000 genes (about 12% of the genome), producing two very different forms. Birds with the inversion are larger, have darker throat coloration and rounder wings, are inferred to have poorer flight efficiency, and are geographically restricted despite the high mobility of the species. Stable isotope analyses confirmed that birds carrying the inversion have shorter migratory distances or do not migrate. However, we found no evidence of pre- or post-zygotic isolation, indicating the two forms commonly interbreed and that the polymorphism remains locally restricted because of the effect on behavior. This illustrates a genomic mechanism underlying maintenance of geographically structured polymorphisms despite interbreeding with a lineage with high mobility.

Adult Lesser Cuckoo Cuculus poliocephalus Killing Japanese Bush Warbler Cettia diphone Nestlings

We documented an adult Lesser Cuckoo Cuculus poliocephalus killing nine-day-old Japanese Bush Warbler Cettia diphone nestlings by ejecting them from their nest. Among the available hypotheses concerning brood-parasitic birds destroying host nest contents, only the ‘farming hypothesis’ helps to explain this case, although the attack did not force the warbler to immediately re-nesting because one nestling survived. Considering the long breeding season and frequent re-nesting habit of the Japanese Bush Warbler, as well as the delayed arrival of the Lesser Cuckoo into the breeding area, such predatory behavior seems to be effective in creating replacement clutches for future parasitism.

Life-history trade-offs, density, lay date—not personality—explain multibroodedness in great tits

AbstractIn various taxa, multibroodedness is a common breeding strategy. Life-history theory predicts that individuals can increase fitness by producing multiple broods within a season. Despite the apparent increase in the number of offspring parents might produce per season, not all individuals are multibrooded, suggesting a trade-off. We studied ecological and behavioral factors influencing the initiation of second clutches in great tits (Parus major), an optionally multibrooded bird species, by distinguishing two types of clutches: replacement versus true second clutches, produced after failure versus successful first breeding attempts, respectively. We predicted which lay date, density, and investment in first clutches would decrease the probability of initiating a second clutch, but which faster exploring behavioral types with a faster pace-of-life would be more likely to be multibrooded. The probability of initiating true second clutches varied negatively within-individuals with lay date and breeding density. The initiation of replacement clutches instead varied negatively among-individuals with lay date and density, suggesting nonrandom settlement of behavioral types across environments. Individuals were less likely to be multibrooded when producing many offspring from their first clutch, suggesting within-year reproductive trade-offs, similar to previous studies. No previous research has linked personality to multibroodedness; here we show which neither the repeatable nor the plastic part of an individual’s exploratory behavior predicted multibroodedness. We confirmed our prediction which the resolution of trade-offs may occur either at the within- or among-individual level. Our research contributes to the understanding of life-history evolution in the wild by studying the mechanisms shaping multibroodedness within seasons.

Hatchery efficiency as a conservation tool in threatened sea turtle rookeries with high embryonic mortality

Sea turtles are globally endangered and facing anthropogenic threats. To mitigate the negative impacts on sea turtle populations, different conservation strategies have been developed and implemented. For instance, when an endangered population suffers a high embryonic mortality from predation, human poaching, beach flooding or erosion, the relocation of clutches to safer and controlled areas can lower these threats. However, the use of hatcheries is controversially discussed, with some previous studies reporting negative results such as low hatching success, skewed sex ratio or phenotypic alterations of hatchlings. In this study, we evaluated the efficiency of beach hatcheries compared to in-situ incubation in Cabo Verde using data from six nesting seasons (2013–2018). During this study, several thousand high-risk clutches from loggerhead turtle (Caretta caretta Linnaeus, 1758) were relocated to a hatchery constructed within the Sea Turtle Natural Reserve (STNR, Boa Vista Island). Our findings confirmed that the in-situ nests within the STNR have extremely high egg mortality that is usually over 70%. Mean hatching success of clutches relocated to hatcheries was significantly higher than in-situ clutches with mean values between 70 and 85%. No significant differences were observed in the incubation period, the size and the behavioural response of hatchlings between relocated and in-situ clutches. As long as a sea turtle population is endangered and hatching success is low, clutch relocation programs which are designed to have an impact at the population level, are considered an efficient tool that contributes to its recuperation. Nevertheless, strict recommendations that increase hatching success while maintaining natural hatchling phenotype must be implemented.

Consequences of spring arrival dates for the breeding phenology of migratory warblers

ABSTRACT Capsule Early male arrival on the breeding grounds results in early pairing but not early nesting in Chiffchaffs Phylloscopus collybita, and Chiffchaffs can nest, fail, and re-nest before Willow Warblers Phylloscopus trochilus begin nesting. Aims To quantify the consequences of timing of arrival for the subsequent timing of pairing, nesting, and re-nesting of short-distance (Chiffchaff) and long-distance (Willow Warbler) migrants. Methods The arrival dates of 118 Chiffchaffs and 20 Willow Warblers were measured from March to June over 10 weeks in Foxley Wood nature reserve, Norfolk. Colour-ringing of 56 Chiffchaffs (55 males, 1 female) and 11 Willow Warblers (10 males, 1 female) was used to relate individual arrival dates to timing of male pairing, clutch initiation, and re-nesting. Results Male Chiffchaffs started to arrive in early March and increased rapidly in number until early April, while the arrival of male Willow Warblers began in early April. Early-arriving male Chiffchaffs paired earlier than later-arriving individuals, but timing of clutch initiation was unrelated to male arrival dates. Early nesting by Chiffchaffs allowed replacement clutches following nest loss, the earliest of which occurred 12 days after the first Willow Warbler male had paired. Conclusions Although early arrival in male Chiffchaffs does not translate into earlier nesting, timing of nesting in Chiffchaffs was sufficiently early to allow time for replacement clutches following nest loss. The later arrival of Willow Warblers is likely to mean fewer opportunities for replacement clutches following nest loss. This difference in breeding phenology could therefore contribute to differences in productivity between the species, especially if nest failure rates are high.

Hatchery efficiency for turtle conservation in Cabo Verde

This paper evaluated the efficiency of beach hatcheries as a conservation tool for threatened sea turtle clutches. During six nesting seasons (2013 to 2018), several thousand high-risk clutches from loggerhead turtles (Caretta caretta) were relocated to a hatchery constructed on the same beach, within the Sea Turtle Natural Reserve (STNR, Boa Vista Island, Cabo Verde). Some parameters like hatching success; incubation period, hatchlings’ morphology and their behavioral response were compared to in-situ clutches.•Our findings confirmed that the in-situ nests within the STNR had extremely high egg mortality that was usually over 70 %. Mean hatching success of clutches relocated to hatcheries was significantly higher than in-situ clutches with mean values between 70 to 85 % (p < 0.0001).•No significant differences were observed in the incubation period (p = 0.786) and morphology of hatchlings (all p > 0.05) between relocated and in-situ clutches.•This study provided a detailed method and recommendations for sea turtle clutches relocation to the hatchery, that can be beneficial for endangered sea turtle population specially where hatching success is very low.

Comparison of reproductive success between parent-reared and hand-reared northern bald ibis Geronticus eremita in captivity during Proyecto Eremita

Modern zoos actively collaborate in the conservation of many endangered species by captive breeding for reintroduction. This paper presents the reproductive success of a captive colony of northern bald ibis Geronticus eremita (NBI) at ZooBotanico de Jerez between 1993 and 2013 (21 years). Between 2004 and 2011 was “Proyecto Eremita”, a study of the best releasing techniques for hand-reared NBI in Cadiz in order to establish a free-range, self-sustained population in the wild. During this period, first-clutch eggs were artificially incubated and chicks were hand reared until fledgling stage, allowing the pair to produce a second or replacement clutch that was parent reared. This paper compares the colony’s reproductive success between the years where one single clutch was reared by the pairs (n=13) and the years of Proyecto Eremita (n=8), with two clutches, the first hand reared and the second parent reared. The reproductive success rate was measured in 300 nests. A total of 268 fledglings reached 2 months old, the age considered here as the reproductive success. Two reproductive variables were significantly higher during the Proyecto Eremita years: mean number of fledglings per nest (1.8 versus 0.3) and overall number of fledglings recorded per year (26.5 versus 4.3). The reproductive success of hand-reared clutches was similar to parent-reared clutches. There was a significant and negative effect of colony size on the percentage of birds paired and on reproductive success. Parent-reared clutches during the Proyecto Eremita showed a higher reproductive success compared to parent-reared clutches outside this period. The data show that a remarkably high percentage of mating pairs outside of the Proyecto Eremita period failed at reproduction, probably due to density-dependent effects. The combination of hand-rearing and parent-rearing methods used in this study was a very effective tool to significantly increase the number of fledglings produced for the reintroduction programme.

Offspring Sex Ratio in First and Replacement Clutches of Mew Gull (Larus canus): Breeding Tactics in the Riverine Population of a Sexually Size-Dimorphic Bird

Offspring Sex Ratio in First and Replacement Clutches of Mew Gull (Larus canus): Breeding Tactics in the Riverine Population of a Sexually Size-Dimorphic Bird