The relative influence of prey abundance and co-breeders on the reproductive performance of polyandrous Pale Chanting-goshawks

This study investigates if the reproductive performance of polyandrous Pale Chanting-goshawks, Melierax canorus, is governed by the abundance of dominant rodent-prey species or a co-breeding male participating fully in prey being delivered to the female and young. Polyandrous trios in prey-rich habitat, the only habitat where these trios occurred, failed to produce more offspring than monogamous pairs, but attempted more second broods in years of high prey abundance. The higher the prey-delivering rates at the nesting sites by either one monogamous male or two polyandrous males (combined effort), the higher the proportion of time their females spent in close proximity to the nest. Since there were no significant differences between the body masses of nestlings fledged by polyandrous and monogamous groups, higher delivery rates relieved females from away-from-the-nest hunting duties and permitted them to feed on prey not fed to offspring. Although breeding in polyandrous trios held fitness benefits for females, e.g. to accumulate the necessary body reserves for laying a second clutch in years of high prey abundance, it is still not clear how males benefited. The reproductive performance of each group was strongly linked to the abundance of the dominant, otomyine rodent prey (Otomys unisulcatus and Parotomys brantsii) and it was therefore prey abundance, and not the number of males delivering prey to the female and nestlings at each nesting site, that controlled the reproductive performance of cooperative-breeding Pale Chanting-goshawks.     

Temporal shifts in the pair-bond dynamics of New Zealand robins (

Winter is a challenging time for temperate insectivorous songbirds, due to colder temperatures, reduced prey activity and shorter diurnal foraging times. For species that are non-migratory, territorial and monogamous, winter conditions may result in within-pair competition. However, little is known about how monogamous pairs coexist on their winter territories. We investigated temporal patterns in male?female interactions of the New Zealand robin (Petroica australis to better understand mechanisms of coexistence during winter. Previous work has shown that male robins are physically dominant over females and maintain priority access to food year-round. We quantified female behaviour throughout the 2008 non-breeding season to better understand how females coexist with physically dominant males on winter territories. Results showed that pairs rarely forage in close proximity in autumn and winter, suggesting females avoid males at this time of year. Males and females begin to spend more time foraging together as winter turns to spring. During this winter?spring transitional period, females steal large amounts of food hoarded by males. These results indicate that male and female New Zealand robins use different behavioural mechanisms to coexist on their winter territories. While males are dominant physically, females show a seasonally variable strategy where they avoid males in autumn and winter, and then steal male-made caches from early spring until the onset of inter-pair cooperation and the breeding season.     

Sex roles in the monogamous Purple Sandpiper Calidris maritima in Svalbard

The Purple Sandpiper Calidris maritima, an Arctic shorebird, displays unusual sex roles during breeding. In a 5-year study in Svalbard, in the Norwegian high Arctic, data were collected on the roles of the sexes in this species. Purple Sandpipers were similar to most shorebirds in that males actively courted females, established territories and vigorously defended their territories from intruders. Both sexes shared incubation duties approximately equally over the entire 21-day incubation period. Males incubated very little initially, but increased their effort significantly during the first 11 days of incubation to over 50% of the time from days 11–21. However, Purple Sandpipers contrasted with most other shorebird species in that females discontinued their breeding efforts at hatching. In nearly all cases, broods were attended solely by the male until the chicks reached fledging age (or even longer). Nonetheless, the pair bond must be described as monogamous because neither males nor females were found to re-mate or lay a second clutch during a season. This pattern of parental care, which is found only in very few other shorebird species, is discussed in an evolutionary context.     

A physiological marker for quantifying differential reproductive investment between the sexes in Yellow-legged gulls (Larus michahellis)

Asymmetry between males and females in the energy they invest initially in reproduction has resulted in the evolution of differing reproductive strategies (caring females vs. competitive males). However, parental care in many birds is shared by both sexes suggesting that male energy expenditure in agonistic behaviors and courtship feeding might compensate female costs of clutch production. Here, we tested the hypothesis that initial investment in reproduction by both sexes in the Yellow-legged Gull (Larus michahellis), a species with biparental care, is similar from a physiological perspective. In this income breeder, female and male reproductive investment during early breeding can be ultimately related to muscular activity (local foraging effort required for clutch production in females and courtship feeding and agonistic behaviors in the case of males). Thus, we evaluated sex-specific patterns of creatine kinase (CK, IU/L) levels in plasma, an indicator of physical effort associated with muscular activity dependent behaviors, through incubation as a reflection of the physiological response of both sexes to the reproductive investment they made up to clutch completion. Raw levels of CK were related to plasma levels of total proteins (TP, g/dL) to account for the differential physiological state of individuals when sampled (i.e. differential dehydratation). Female costs of clutch production were associated with post-laying levels of CK/TP. We grouped females according to their relative investment in clutch production: < 15.8%, 15.8 to 17.3% and > 17.3% of field metabolic rate; which showed increasing values of CK/TP (24.6, 53.1 and 66.0 IU/g, respectively). Moreover, we found similar CK/TP trends throughout incubation for both sexes (CK/TP = 50.2− [3.3 × days from laying]) suggesting similar physiological responses to reproductive effort and, therefore, analogous sex-specific initial investment. Thus, male investment in agonistic behaviors and courtship feeding apparently equaled female investment in clutch production. The use of CK measurements is revealed as a useful approach to investigating overall investment in reproduction for both sexes, providing further insights into our comprehension of reproductive strategies in seabirds.     

Females produce larger eggs for large males in a paternal mouthbrooding fish.

When individuals receive different returns from their reproductive investment dependent on mate quality, they are expected to invest more when breeding with higher quality mates. A number of studies over the past decade have shown that females may alter their reproductive effort depending on the quality/attractiveness of their mate. However, to date, despite extensive work on parental investment, such a differential allocation has not been demonstrated in fish. Indeed, so far only two studies from any taxon have suggested that females alter the quality of individual offspring according to the quality/attractiveness of their mate. The banggai cardinal fish is an obligate paternal mouth brooder where females lay few large eggs. It has previously been shown that male size determines clutch weight irrespective of female size in this species. In this study, I investigated whether females perform more courtship displays towards larger males and whether females allocate their reproductive effort depending on the size of their mate by experimentally assigning females to either large or small males. I found that females displayed more towards larger males, thereby suggesting a female preference for larger males. Further, females produced heavier eggs and heavier clutches but not more eggs when paired with large males. My experiments show that females in this species adjust their offspring weight and, thus, presumably offspring quality according to the size of their mate.     

More colourful eggs induce a higher relative paternal investment in the pied flycatcher Ficedula hypoleuca: a cross-fostering experiment

The signalling hypothesis of eggshell colouration in birds posits that females of species with blue-green eggs signal their phenotypic quality or the quality of their eggs to their mates through deposition of the antioxidant biliverdin as pigment. Males respond by investing more in the offspring. Through a cross-fostering experiment where we have exchanged whole clutches between pairs of pied flycatcher Ficedula hypoleuca nests, we managed to break potential associations between female quality and clutch chromaticity. We show that males respond to incubated clutches with more variable and higher peak values in blue-green chroma through a higher proportional investment in nestling provisioning on day 4 of the nestling period, when males invest more heavily than females in provisioning. More variable clutches show higher peak chroma values. We also show that egg colour during the two-week incubation period has a significant effect, which is not found for the colour of eggs during the laying period. Finally, the proportion of male provisioning visits affects negatively female brooding effort and nestling mortality, and thereby has positive effects on female fitness. Blue-green chroma in the pied flycatcher functions as a signal of female or clutch quality to males which respond by adjusting their relative investment with respect to total pair effort.     

Evolution of reversed sex roles, sexual size dimorphism, and mating system in coucals (Centropodidae, Aves)

The evolution of greater male than female parental care remains poorly understood. In birds it is thought to be related to precocial chicks and small clutch size. This review shows, however, that such role reversal has also evolved in a family with altricial young and relatively large clutch size: coucals (Centropodidae, Cuculiformes). Males perform most nest building, incubation, and feeding of young. As predicted by sexual selection theory, coucals have also reversed sexual size dimorphism, females being larger than males in all 12 species for which size data are available. Most coucals that have been studied are monogamous, but the black coucal Centropus grillii appears to be polyandrous, and males perform almost all parental care, whereas females show more active advertisement behaviour. In this species, females are about 50% heavier than males. Polyandry in the black coucal seems to be associated with a shift to a habitat with seasonally rich food resources. Difficulties for female coucals of gathering enough resources for producing several clutches of relatively large eggs may favour mainly male parental care. Female sexual competition and resource storage, and male foraging economy, may explain why females are larger. Additional field studies are needed to test these hypotheses; the coucals are of great interest to sexual selection and mating systems theory.     

Sex-specific costs of reproduction in Eastern Bluebirds Sialia sialis

In species with bi-parental care, individuals must partition energy between parental effort and mating effort. Typically, female songbirds invest more than males in reproductive activities such as egg-laying and incubation, but males invest more in secondary sexual traits used in attracting mates. Animals that breed more than once within a season must also allocate time and energy between first and subsequent breeding attempts and between current and future breeding seasons. To investigate strategies of reproductive investment by males and females and the consequences of such strategies, we manipulated the size of broods of Eastern Bluebirds Sialia sialis . Pairs with enlarged first broods were less likely to produce a second clutch or took longer to initiate one than pairs with reduced broods. After rearing enlarged broods, females were less likely than males to survive to the following year. Although plumage coloration is a sexually selected trait in Eastern Bluebirds that is influenced by nutritional stress, we did not detect an effect of brood-size manipulation on female coloration. Past research, however, demonstrates that, in males, plumage colour is negatively affected by increasing brood size. We suggest that there are sex-specific strategies of reproductive investment in Eastern Bluebirds, and that researchers should incorporate measures of residual reproductive value in studies of life-history evolution.     

Paternal care and male mate-attraction effort in the European starling is adjusted to clutch size

In facultative polygynous birds with biparental care, a trade-off may occur between male parental care and attraction of additional mates. If there is a cost associated with reduced male parental care, the relative benefit of mate attraction may be predicted to decrease as the size of a male's clutch or brood increases. We tested this prediction in monogamous pairs of facultatively polygynous European starlings (Sturnus vulgaris). The larger the clutch, the more time the male spent incubating and the less time he spent attracting an additional female (i.e. singing near and carrying green nesting material into adjacent empty nest-boxes). Reduced paternal incubation resulted in lower overall incubation (the female did not compensate) and lower hatching success. Immediately after experimental reduction of clutches, males spent significantly less time incubating and more time singing and carrying greenery, and vice versa for experimentally enlarged clutches. Males with experimentally reduced clutches attracted a second female more often than males with experimentally enlarged clutches. This is the first study, to our knowledge, to provide experimental evidence for an adjustment of paternal care and male mate-attraction effort to clutch size. However, a trade-off between paternal nestling provisioning and mate attraction was not revealed, probably due to the absence of unpaired females by that time in the breeding season. Experiments showed that the relative contribution of the male and female to nestling provisioning was unrelated to brood size.     

Experimental manipulation of female tail length did not cause differential allocation by males in the barn swallow

The evolution and maintenance of female ornamentation has attracted increasing attention, because the previous explanation, that is a non‐functional copy of functional male ornamentation, seems insufficient to explain female ornamentation. A post‐mating sexual selection, differential allocation, may be more common than pre‐mating sexual selection, but few studies have investigated differential allocation by males. Here, we studied differential allocation of incubation investment by male barn swallows Hirundo rustica, a model species for the study of sexual selection, because our previous correlative study demonstrated a positive relationship between female tail length and male incubation investment. We manipulated the length of the outermost tail feathers in females after clutch completion and examined whether males adjust incubation investment according to female ornamentation. Because extra‐pair paternity is virtually absent in the study population, we were able to study differential allocation based on the tradeoff between current and future reproductive investments, rather than the tradeoff between current paternal investment and additional mating effort. The experimental treatment had no significant effect on male nest attentiveness, whereas female tail length before manipulation predicted male nest attentiveness. The observed pattern is consistent with differential access; that is, well‐ornamented individuals have greater access to mates with high reproductive (parental) ability, rather than differential allocation during incubation. Alternatively, males can directly assess eggs in their nests, and thus, as seen in other species, males might adjust their incubation investment based on the egg characteristics of long‐tailed females.     

Cryptic female choice: frogs reduce clutch size when amplexed by undesired males

In species with internal fertilization, females can 'cryptically' choose (e.g. through sperm selection) which individuals sire their offspring, even when their overt preferences for copulatory partners are overrun by male-male competition and sexual coercion. The experiment presented here reveals that control of paternity after copulation has begun is also possible in species with external fertilization. Females of the hybridogenetic Rana essonae-Rana esculenta (LL-LR) waterfrog complex adjust their clutch size in response to mate type: they release fewer eggs when amplexed by hybrid LR males who--jeopardize successful reproduction--than when amplexed by parental LL males. This reduction in the number of eggs laid can increase a female's residual reproductive value through a second mating in the same breeding season or a larger clutch size in the next year. We argue that cryptic female choice through clutch size adjustment (i) may have evolved more often than previously assumed, and (ii) can arise even where females mate only once during a reproductive period.     

Parental behaviour in the Lapwing Vanellus vanellus

Parental behaviour of monogamous and polygynous Lapwings was studied during incubation and brood care. Both parents attended the nest in 86% of monogamous pairs ( n = 29 pairs). In 14% of pairs, only the male parent continued incubation until the eggs hatched, whereas the female deserted the clutch before or at the end of incubation. There was a clear division of parental roles during incubation. Females spent more time incubating (64% of time) than their mates (27%), whereas males spent more time defending the nest (3%) than females (>1%). Time spent incubating did not differ between monogamous and polygynous males. However, polygynous females spent more time incubating (primary females: 95%; secondary females: 97%) than monogamous females. Biparental care was the most common pattern of post-hatching care, although in some broods either the male or the female parent deserted before the chicks fledged. Division of sex roles was less pronounced in brood care than during incubation. Females spent more time brooding (21%) than males (3%), and females attended their chicks more closely than males. Nevertheless, males and females spent similar amounts of time defending the brood from predators and conspecifics. We suggest that the apparent division of parental roles may be explained by sexual selection, i.e. the remating opportunities for male Lapwings might be reduced if they increase their share in incubation. However, the different efficiency of care provision, for example in ability to defend offspring, may also influence the roles of the sexes in parental care.     

Plumage redness predicts breeding onset and reproductive success in the House Finch: a validation of Darwin's theory

Darwin (1871) and later Fisher (1958) suggested that sexual selection can drive the evolution of ornamental traits in monogamous species when female preferences for these traits allow well-ornamented males to begin breeding earlier in a season and, as a result, gain reproductive advantages over poorly ornamented males. However, few studies have been conducted to test this fundamental concept upon which much of the sexual selection theory for monogamous species has been based. In this study, we examined the relationship between breeding onset, reproductive success, and male ornamentation in the House Finch Carpodacus mexicanus , a species in which males display bright carotenoid-based plumage pigmentation. In previous work, it has been shown that bright male House Finches are preferred as social mates by females and, as a result, begin nesting earlier in the season than do drab orange and yellow males. Here we show that, by initiating breeding earlier in the season, brightly colored males fledge more offspring in a season than do drab males. Thus, differential timing of breeding generates considerable variance in reproductive success among male House Finches and contributes to sexual selection for male plumage ornamentation in this species.     

Sex roles, parental effort and offspring desertion in the monogamous Eurasian Curlew Numenius arquata

The reasons for female desertion of offspring and the evolution of predominantly male care among monogamous bird species are not clearly understood. We studied parental effort during the incubation and chick rearing periods in the Eurasian Curlew Numenius arquata in western Finland, and compared timing of brood desertion with other populations in Europe. Males and females contributed equally to incubation and showed no differences in the intensity of mobbing behaviour towards a potential nest predator (stuffed crow) shortly after hatching. However, females deserted their offspring approximately halfway through the brooding period ( c. 16 d after hatching), while males remained with chicks until independence ( c. 35 d). Females with late-laid clutches deserted their offspring sooner after hatching than those with clutches produced earlier in the season. Curlew females deserted younger chicks in northeast Europe, where laying dates were later, breeding seasons shorter and migration distances were longer, than in western and central Europe. We suggest that the most likely reasons for offspring desertion by females may be associated with increased female survivorship and maintenance of pairbond between years.     

Effects of maternal quality and mating status on female reproductive success in the polygynous spotless starling

Female mating status may affect reproductive success in avian polygynous mating systems through a combination of differences in female parental quality and status-dependent male assistance in parental care. Traditionally the literature has emphasized male assistance, neglecting evidence for consistency in female parental quality independent of mating status or repeatability in status. We studied the effects of male assistance on breeding success and its association with female mating status in a population of the polygynous spotless starling, Sturnus unicolor, during 3 years. Nestling provisioning by males improved the fledging success of late (mostly second) but not early (mostly first) broods. Reproductive success of females was affected mainly by female maternal quality: (1) primary and secondary females did not differ in output for early broods and in seasonal output despite a greater male assistance at primary nests; (2) monogamous females were more successful despite receiving no more help than other females; and (3) primary and secondary females had different clutch sizes before male assistance in parental care could operate. Female mating status was not significantly repeatable within seasons or between years. Females who changed status between years were as successful when rearing broods as secondary females as they were as primary or monogamous females. Breeding success (proportion of eggs resulting in fledglings) and clutch size were significantly repeatable between years for late but not for early clutches. Performance showed no strong association with female age or condition. Female breeding success seems to be weakly affected by male assistance in this sedentary, colonial species, and more dependent on inherent differences between females. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Reproductive success and survivorship of the Galapagos Hawk Buteo galapagoensis: potential costs and benefits of cooperative polyandry

The Galapagos Hawk Buteo galapagoensis is one of few species showing cooperative polyandry. As many as four males may mate with one female and share a territory and nest, Such groups, on average, produce more young than monogamous pairs, but on a per male basis (assuming that each male of a group has an equal chance to fertilize each young) polyandrous males produce fewer young per year than monogamous males. Territorial breeding birds show higher yearly survivorship (90%) than non-territorial, non-breeding birds (50% or less). All territorial birds seem to remain on their territories for life, and monogamous pairs actually constitute the remnants of polyandrous groups. Some potential factors producing such an unusual system are discussed. Limited breeding space and high mortality among non-territorial birds seem to work in favour of group breeding in this and other cooperative species. Polyandry may be favoured because of the reduced value of extra females in hawk breeding behaviour. Equal-status males may occur because of the lack of kinship among cooperating males and the high potential genetic cost of being a helper in a long-lived, low fecundity species. Other possible factors favouring group breeding are territory acquisition and the variable nature of the Galapagos climate.     

Polygyny and its fitness consequences for primary and secondary female pied flycatchers

In polygynous species with biparental care, the amount of paternal support often varies considerably. In the pied flycatcher (Ficedula hypoleuca), females mated with monogamous males receive more male assistance during the nestling phase than females mated with bigynous males, as the latter have to share their mates with another female. Bigynous males, however, give more support to their primary broods than to their secondary broods. Using a long-term dataset (31 years), the present study revealed that direct reproductive success, i.e. number of fledglings, was lower in females that mated with bigynous males, especially in secondary broods without male assistance, than in females that mated with monogamous males. Secondary broods with male assistance were more affected than primary broods. Female survival was independent of mating status. In primary broods, a delayed compensation for inferior direct reproductive success was found in terms of the number of grandoffspring, a phenomenon that did not occur in secondary broods. Delayed compensation in primary broods refers to indirect effects, i.e. good genes. According to the sexy son hypothesis, genetically superior (i.e. sexy) males may have sons with a higher number of broods belonging to a polygynous breeding status than do sons from broods with a monogamous father. This was indeed the case for sons descending from primary broods, but not for sons descending from secondary broods.     

Life History Trade-Offs and Behavioral Sensitivity to Testosterone: An Experimental Test When Female Aggression and Maternal Care Co-Occur

Research on male animals suggests that the hormone testosterone plays a central role in mediating the trade-off between mating effort and parental effort. However, the direct links between testosterone, intrasexual aggression and parental care are remarkably mixed across species. Previous attempts to reconcile these patterns suggest that selection favors behavioral insensitivity to testosterone when paternal care is essential to reproductive success and when breeding seasons are especially short. Females also secrete testosterone, though the degree to which similar testosterone-mediated trade-offs occur in females is much less clear. Here, I ask whether testosterone mediates trade-offs between aggression and incubation in females, and whether patterns of female sensitivity to testosterone relate to female life history, as is often the case in males. I experimentally elevated testosterone in free-living, incubating female tree swallows (Tachycineta bicolor), a songbird with a short breeding season during which female incubation and intrasexual aggression are both essential to female reproductive success. Testosterone-treated females showed significantly elevated aggression, reduced incubation temperatures, and reduced hatching success, relative to controls. Thus, prolonged testosterone elevation during incubation was detrimental to reproductive success, but females nonetheless showed behavioral sensitivity to testosterone. These findings suggest that the relative importance of both mating effort and parental effort may be central to understanding patterns of behavioral sensitivity in both sexes.     

Reproductive biology and ecology of white‐winged trumpeters (Psophia leucoptera) and recommendations for the breeding of captive trumpeters

The reproductive biology and ecology of a wild population of white‐winged trumpeters (Psophia leucoptera) were studied in southeastern Peru from 1983 to 1987. Because little information is available about any of the trumpeter species and because trumpeters have proven difficult to breed in captivity, information relevant to breeding and management of captive trumpeters is reported in this paper. White‐winged trumpeters lived in territorial social groups that ranged in size from four to 13 individuals. A typical territorial group contained three adult males, two adult females, and several sexually immature offspring, but smaller temporary groups sometimes formed for the duration of the breeding season. Only the dominant female contributed eggs to the clutch, and all adult males in the group competed to obtain copulations with her. Eggs were laid in elevated nesting cavities and no nest was constructed. The average clutch size was three eggs and incubation was not begun until the final egg was laid. The dominant male and female shared most of the incubation duties, but subordinate males covered approximately 15% of the incubation shifts. Eggs hatched approximately 27 days after incubation was begun and chicks left the nesting cavity the day after they hatched. Chicks were completely dependent on older birds to feed them for their first 3 weeks and then gradually began to feed themselves more and more food. The subordinate adult males fed chicks the most food, the dominant male and female and older offspring fed chicks an intermediate amount, and the subordinate adult female fed chicks the least. Young chicks behaved aggressively toward each other but were separated by adults before they injured each other. If at least one chick from the clutch survived, trumpeters did not breed again until the beginning of the next breeding season the following year. Zoo Biol 19:65–84, 2000. © 2000 Wiley‐Liss, Inc.