Reed warblers guard against cuckoos and cuckoldry

Previous studies have shown that reed warblers, Acrocephalus scirpaceus, are more likely to reject a cuckoo, Cuculus canorus, egg if they have seen a cuckoo at their nest. This suggests that they would benefit from watching out for cuckoos. We tested whether presentations of a cuckoo mount near the nest (to simulate nest inspection) led to increased nest attendance by the warblers. Cuckoo presentations at completed nests before laying, when males guarded their females closely, led to desertion at 40% of nests before any eggs were laid (there were no desertions after presentations of a jay,Garrulus glandarius , a nest predator). In the remaining cases, there was no effect of the cuckoo on nest attendance before laying began, but a marked increase in male nest attendance (compared with jay and no-presentation controls) on the days the first and second eggs were laid. Cuckoo presentations at the one-egg stage led to the same increase in male nest attendance as did the prelaying presentations. Increased male nest attendance at the one-two-egg stage was not at the expense of mate guarding, because this declined anyway when laying began, and it did not lead to increased paternity loss compared with controls. Overall, 15% of broods had one or two extrapair young (6% of all young extrapair). We conclude that male reed warblers do increase nest guarding in response to cuckoos, but only after their females have begun egg laying, when there are less likely to be costs in lost paternity. Females did not increase nest guarding, perhaps because they need to spend more time foraging during the egg-laying period. Our results suggest that cuckoos should be secretive not only when they lay but also when they monitor host nests beforehand. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Influence of nest-floor slope on the nest choice of laying hens

Group nests in alternative housing systems for laying hens primarily fulfil the hen's needs for seclusion and protection. Commercial nests used in Switzerland are built according to the provisions of the Swiss Animal Welfare Legislation. However, nest types can differ in aspects, such as floor slope, that could have an impact on egg-laying behaviour. Floor slope has to be designed so that eggs roll away without breaking and so that hens feel comfortable laying their eggs. In commercial nests, the slope is usually between 12% and 18%. The aim of this study was to investigate the effect of floor slope on the hen's nest preference and laying behaviour. We predicted that hens would prefer nests with a lower sloped floor for evolutionary reasons and for reasons related to comfort.Eight pens, each with 17-18 white laying hens (LSL), were equipped with two roll-away nests (0.54 m²) having different floor slopes (12% and 18%). Eggs were collected each day (from approximately 20 weeks of age until 28 weeks of age); the number of eggs in each nest and on the floor of the pens was recorded. Behaviour inside the nest was filmed for two consecutive days during the main egg-laying time from the second hour to the fifth hour (4 h) after lights came on in week 27/28. The following data were recorded: number of hens in each nest, the nest visits/egg number ratio, the number of sitting events, the body alignment of hens sitting in the nest and the number and duration of nest visits. Data were analysed with a repeated-measures ANOVA. There was no difference between the numbers of eggs in the two nests, but more hens were counted in nests with a 12% slope (p = 0.027). The ratio between the number of nest visits and number of eggs did not differ significantly between the nests. However, we counted more sitting events in the nest with 12% slope (p = 0.007). The percentage of body alignment towards the back (p = 0.044) and towards the front (p = 0.028) of the nest differed between the nests. Furthermore, for nest visits lasting between 10 and 90 min, we found significant differences in the total number of nest visits (p = 0.039). For visits in this range of duration, we also found significant differences for nest visits with sitting (p = 0.025) and for the number of nest visits with egg laying (p = 0.049). All of these differences favoured the 12% nest.Both nests were generally accepted by the hens. However, because of the higher number of hens counted in the 12% nest and the higher amounts of nest visits and sitting events found in these nests, we recommend to use nests with a floor slope of 12% rather than 18%.     

Cuckoos versus reed warblers: Adaptations and counteradaptations

At study sites in Cambridgeshire, England, the percentage of reed warbler, Acrocephalus scirpaceus, nests parasitized by cuckoos, Cuculus canorus, in 2 years was 22·5% and 9·1%. The warblers rejected cuckoo eggs at 19% of parasitized nests. Parasitized clutches suffered less predation than unparasitized clutches, suggesting that the cuckoo itself was the major predator, plundering nests too advanced for parasitism so that the hosts would re-lay. The cuckoos laid a mimetic egg, parasitized nests in the afternoons during the host laying period, usually removed one host egg, laid a remarkably small egg and laid very quickly. Nests were experimentally parasitized with model eggs to study the significance of this procedure. Experiments showed that host discrimination selects for: (1) egg mimicry by cuckoos (poorer matching model eggs were more likely to be rejected); (2) parasitism during the laying period (mimetic eggs put in nests before host laying began were rejected); (3) afternoon laying (mimetic eggs were less likely to be accepted in the early morning than in the afternoon, when hosts were more often absent from the nest); (4) a small egg (large eggs, typical of non-parasitic cuckoos, were more likely to be rejected); (5) rapid laying (a stuffed cuckoo on the nest stimulated increased rejection of model eggs), and (6) sets a limit to host egg removal by cuckoos (if more than one or two are removed desertion may occur). Mimicry may also be selected for because it reduced the chance that second cuckoos can discriminate the first cuckoo's egg from the host's clutch. Predation did not select for mimicry; nests with a non-mimetic egg did not suffer greater predation than those with a mimetic egg. Host rejection of model eggs did not depend on: (1) stage of parasitism once host egg laying had begun (nevertheless cuckoos were more likely to lay early in the host laying period probably to increase the chance the cuckoo chick hatched); (2) removal of a host egg (however, this reduced the incidence of unhatched eggs so cuckoos may remove a host egg so as not to exceed the host incubation limit). There were two costs of rejection, an ‘ejection’ cost (own eggs ejected as well as the cuckoo egg) and, with mimetic eggs, a ‘recognition’ cost (own eggs ejected instead of the cuckoo egg). Reed warblers did not discriminate against unlike chicks (another species) and did not favour either a cuckoo chick or their own chicks when these were placed in two nests side by side. Possible reasons why the hosts discriminate against unlike eggs but not unlike chicks are discussed.     

Ablenkungsman?ver bei der Eiablage des H?herkuckucks(Clamator glandarius)

Summary A pair of the Great Spotted Cuckoo exhibited reproductive behaviour during the years 1968–72. Until 1971 the female laid the eggs in nests of Magpies or Blackbirds which were placed in the aviary. Birds which could serve as hosts at these times were not in the aviary. The female Cuckoo visited the nests before laying an egg quite often and stayed at or in the nests for longer periods.1972 two Rollers were kept together with the Cuckoos and one Roller laid four eggs into a cave of a beech-stump. Now the female Cuckoo tried to deposit its eggs into the Roller's nest and was successful doing so several times. As soon as the female Cuckoo was ready to lay, the male Cuckoo diverted the Roller and so made it possible for the female to get into the Rollers nest. The laying of the egg took only a few seconds.     

Experiments and observations of prothonotary warblers indicate a lack of adaptive responses to brood parasitism

It has been suggested that prothonotary warblers, Protonotaria citrea, respond adaptively to brood parasitism by brown-headed cowbirds, Molothrus ater, even though they lack historical habitat and range overlap with cowbirds. I studied behaviours functioning as potential defences against brood parasitism in the prothonotary warbler, a cavity-nesting host species. Opening sizes preferred by prothonotary warblers were not small enough to exclude cowbirds, and warblers were parasitized heavily in nests with larger openings. Male and female prothonotary warblers were always away from their nests before sunrise when cowbirds laid eggs in their nests. Prothonotary warblers infrequently (∼6% of 560 nests) deserted nests that were parasitized during the egg-laying period, but frequently (56% of 151 nests) deserted nests that were parasitized before a female warbler laid her first egg. Prothonotary warblers also deserted 60-70% of nests where a cowbird egg, warbler egg or die were experimentally added before egg laying. However, the experimental addition of one of these three objects during the egg-laying period did not elicit desertion. The desertion of parasitized nests was not affected by nest site availability as has been reported elsewhere in the literature. This lack of a response to brood parasitism by prothonotary warblers may be an example of evolutionary lag, because it is likely that they have only recently been exposed to widespread parasitism, and they accept parasitism at a high cost to their own reproductive success. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Common Cuckoo Cuculus canorus and host behaviour at Reed Warbler Acrocephalus scirpaceus nests

This study is based on continuous video recordings made at 53 Reed Warbler Acrocephalus scirpaceus nests each day during the laying period. Egg-laying by the Common Cuckoo Cuculus canorus was recorded in 14 (26.4%) of these nests. By analysing the activity of the host birds around and at the nest, we found that this is probably not the only cue used by the Common Cuckoo when locating suitable nests to parasitize. Furthermore, in most cases there was no significant difference between the length of time the host birds spent at the nest in the morning and afternoon, thus providing little support for the hypothesis that the Common Cuckoo lays in the afternoon because it is less likely to be seen by the nest owners then. Parasitized Reed Warblers rejected the Common Cuckoo egg more frequently when they observed the parasite at their nests. However, contrary to what should be expected, most Common Cuckoos laid their eggs in the presence of the host(s), and in general their egg-laying behaviour (for example duration of stay at the nest) was less secretive than described earlier. When partially depredating host clutches, Cuckoos showed the same behavioural pattern at parasitized and unparasitized nests, indicating that the latter may act as a potential reserve for egg-laying.     

Cuckoo females preferentially use specific habitats when searching for host nests

Egg mimicry is an important adaptation of common cuckoos, Cuculus canorus, against rejection of eggs by their respective hosts. A precondition for the maintenance of egg mimicry is that female cuckoos find hosts with a matching egg type. Experimental evidence indicated that habitat imprinting may be important for host selection. We tested whether the spacing and laying patterns of female cuckoos in the field are compatible with the supposed habitat-imprinting mechanism. We observed 16 females, with the help of radiotelemetry; of seven females, we observed directly 26 egg layings and 27 nest visits without laying. As expected if females were imprinted on different vegetation types, (1) the distribution of vegetation types differed between female home ranges, (2) female habitat use differed from average habitat availability within the egg-laying area (habitat preference), (3) females visited nests and deposited their eggs in the habitat they preferred, and (4) females laid their eggs consistently in a particular habitat type, irrespective of the host species. These results indicate that cuckoo females show habitat preference when searching for suitable host nests. Hence our data are compatible with the habitat-imprinting hypothesis, but owing to the habitat specificity of hosts the data cannot disprove a potential role of host specificity in cuckoo females.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Common Cuckoos Cuculus canorus change their nest‐searching strategy according to the number of available host nests

In recent decades, numerous studies have examined factors affecting risk of host nest parasitism in well‐known avian host–parasite systems; however, little attention has been paid to the role of host nest availability. In accordance with other studies, we found that nest visibility, reed density and timing of breeding predicted brood parasitism of Great Reed Warblers Acrocephalus arundinaceus by the Common Cuckoo Cuculus canorus. More interestingly, hosts had a greater chance of escaping brood parasitism if nesting was synchronized. Cuckoo nest searching was governed primarily by nest visibility at high host‐nest density. However, even well‐concealed nests were likely to be parasitized during periods when just a few hosts were laying eggs, suggesting that Cuckoos adjust their nest‐searching strategy in relation to the availability of host nests. Our results demonstrate that host vulnerability to brood parasitism varies temporally and that Cuckoo females are able to optimize their nest‐searching strategy. Moreover, our study indicated that Cuckoos always manage to find at least some nests to parasitize. Thus, in this case, the co‐evolutionary arms race should take place mainly in the form of parasitic egg rejection rather than via frontline pre‐parasitism defence.     

Nest sharing under semi-natural conditions in laying hens

Under natural conditions, the feral hen (Gallus gallus domesticus) will choose a nest location away from the flock, whereas under commercial conditions, the domestic hen will often choose the same nest as other hens have used or are still using. Simultaneous nest sharing causes several welfare problems to laying hens, and egg production may also be negatively affected. Understanding what causes this difference in nest location selection may provide solutions to the problems associated with simultaneous nest sharing. The aims were to investigate whether a commercial strain of laying hens normally housed in intensive production systems share nests under semi-natural conditions and to describe the behaviour if this behaviour occurred. Twenty 15 weeks old hens were released into an 840 m² enclosure with multiple options for natural and semi-natural nest sites. Over a 63-day period records were made daily of each nest with regard to number of eggs, position, and materials used. On five mornings nesting behaviour was observed. Nest sharing occurred on all but the first 5 days of egg-laying. The majority of hens (n = 14) chose to visit an occupied nest at least once, but no hens exclusively used occupied nests. Visits in shared nests lasted longer than visits in undisturbed nests (13 min 50 s (±4 min and 57 s) vs 30 min 44 s (±4 min and 55 s); P < 0.001). Fifteen nests were used. All shared nests (n = 5) were placed up against the borders, whereas the majority of non-shared nests (n = 7 out of 10) were placed more than 1 m away from the borders (P = 0.002). Some results indicate that nest sharing was caused by environmental restrictions.     

Social behaviour ofRopalidia fasciata (Hymenoptera: Vespidae) females on satellite nests and on a nest with multiple combs

The social behaviour ofRopalidia fasciata females on 3 satellite nests and on a nest with multiple, independent combs was observed. Many females often visited satellite nests, and even dominant foundresses, who usually spent more of their time on the nest, became active when satellite nests were being constructed. Three foundresses laid eggs on a satellite nest and eggs laid by 2 of these (who performed foraging) survived. On another satellite nest, only the dominant foundress laid an egg which survived. On a nest consisting of 2 independent combs, females frequently shifted between the 2 combs even after the emergence of many adults. Constructions of satellite nests and of multiple independent combs are considered to be adaptive for this species living in Okinawa, where most of nests are destroyed by frequent typhoons or abandoned after predation by ants.     

Egg Coloration and Recognition of Conspecific Brood Parasitism in Eastern Bluebirds

Individual eastern bluebird (Sialia sialis) females produce clutches of eggs with unique coloration and older females and females in better body condition lay more pigmented blue‐green eggs. Conspecific brood parasitism in this species is not uncommon and bluebirds occasionally reject what appear to be normal eggs by moving them to the periphery of the nest. I used UV‐visual reflectance spectrometry to objectively measure coloration of eggs and nest material. To estimate the conspicuousness of the trait, I calculated the contrast between eggs and background nest material. I found high achromatic and chromatic contrast between the coloration of eggs and of the nests, suggesting that bluebird eggs are highly conspicuous. To test the hypothesis that expression of blue‐green coloration eggs facilitates recognition of eggs laid by conspecific brood parasites, I cross‐fostered individual eggs into host nests during egg laying and monitored the fate of those eggs. I found no support, however, for the hypothesis that egg coloration facilitates discrimination of parasitic eggs from host eggs.     

Multiple effects of brood parasitism reduce the reproductive success of prothonotary warblers, Protonotaria citrea

Avian brood parasitism often has multiple negative effects on the reproductive success of the host. Most studies have focused on one or two of these effects, but rarely have they all been studied simultaneously for one species. I studied prothonotary warblers to quantify the effects of different intensities of (i.e. multiple) brood parasitism by brown-headed cowbirds, Molothrus ater, on the production of host and cowbird young and on the between-year returns of adult warblers. Host clutch size decreased with an increase in the number of cowbird eggs laid in nests. The hatching success of warbler and cowbird eggs decreased with increased cowbird eggs in nests, but was always higher for cowbird eggs than warbler eggs. The survival of warbler nestlings, but not cowbird nestlings, decreased with increased cowbird nestlings in the brood. An increase in the number of cowbird nestlings in broods resulted in a reduction in the average mass of warbler nestlings but not cowbird nestlings. The number of cowbird eggs or nestlings present did not affect nest predation, and the fledging of cowbirds did not influence the renesting interval of female warblers. In addition, the between-year returns of adult warblers were not negatively affected by brood parasitism. Decreased hatching success and nestling survival reduced the reproductive output of the warblers the most. These effects were substantial and appear to favour the evolution of behavioural responses that reduce the effects of brood parasitism on prothonotary warblers. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour      

Intraspecific nest parasitism in the grey starling (Sturnus cineraceus)

We studied intraspecific nest parasitism in the grey starling (Sturnus cineraceus) in 1992 and 1993. We used three criteria to detect nest parasitism: (i) the appearance of more than one egg per day while the host was laying; (ii) the appearance of extra eggs after the host completed its clutch; and (iii) the appearance of eggs which were of a different shape, size and color to other eggs in the clutch. There were 290 nests (157 nests in 1992; 133 nests in 1993) in which the clutch was completed early (clutches initiated before May 10). Twenty-nine (1992) and 32 (1993) nests contained at least one parasitic egg. Parasitic eggs hatched if they were laid during the laying period and early in the incubation period of their host, and a few of them fledged. Fledging success of parasitic eggs was not different from that of eggs in non-parasitized nests if parasitic eggs were laid during the host's laying period. However, fledging success of all parasitic eggs was fewer than that of eggs in non-parasitized nests. By comparison, fledging success of parasitized nests was not a great as that of non-parasitized nests.     

Predation risk affects trade-off between nest guarding and foraging in Seychelles warblers

The fitness costs of egg loss for Seychelles warblers (Acrocephalus sechellensis)on Cousin Island are considerable because warblers have a single-eggclutch and no time to lay a successful replacement clutch. Onthe islands of Cousin and Cousine, with equal densities of Seychellesfodies (Foudia sechellarum), nearly 75% of artificial eggs placedin artificial nests were predated by fodies after 3 days. OnAride Island with no fodies present, loss of artificial eggswas not observed. Female warblers incubate the clutch, and malewarblers guard the clutch when females are absent. Deterrenceof fodies by male warblers is efficient: loss rate of eggs fromunattended warbler nests was seven times as high as from attendednests, and the more nest guarding, the lower the egg loss andthe higher the hatching success. Egg loss is independent ofthe amount of incubation by females. There is no trade-off betweenincubating and foraging by females. Nest guarding competes withforaging by males, and this trade-off has a more pronounced effecton egg loss when food availability is low. The transfer of breeding pairsfrom Cousin to either Cousine with egg-predating fodies or toAride without fodies allowed us to experimentally investigatethe presumed trade-off between nest guarding and foraging. OnCousine, individual males spent the same amount of time nestguarding and foraging as on Cousin, and egg loss was similarand inversely related to time spent nest guarding as on Cousin.Males that guarded their clutch on Cousin did not guard theclutch on Aride but allocated significantly more time to foragingand gained better body condition. Loss of warbler eggs on Aridewas not observed. Time allocation to incubating and foragingby individual females before and after both translocations remainedthe same.     

Do shiny cowbird females adjust egg pecking behavior according to the level of competition their chicks face in host nests?

Interspecific brood parasites, like the shiny cowbird (Molothrus bonariensis), lay eggs in nests of other species. Shiny cowbird females peck and puncture eggs when they parasitize host nests. This behavior increases the survival of cowbird chicks when they have to compete for food with larger nestmates. However, cowbird chicks may benefit from smaller nestmates as they increase food provisioning by parents and the cowbird chicks secure most extra provisioning. We investigated whether egg-pecking behavior by female shiny cowbirds might be adjusted to the competition that their chicks face in host nests. We found that more host eggs are destroyed per cowbird egg laid in a larger-bodied host (chalk-browed mockingbird, Mimus saturninus, 70-75 g) than a smaller-bodied host (house wrens, Troglodytes aedon, 12-13 g). We also tested egg-pecking preferences in choice experiments with female cowbirds in captivity and found cowbirds presented with eggs in artificial nests pecked first and more frequently, and punctured more frequently the larger egg when this was a host egg, but not when this was a cowbird egg. Our results are partially consistent with the hypothesis that shiny cowbird females adaptively adjust their egg pecking behavior according to the competition that their chicks face in host nests.     

Brood-parasitism by the Shining Cuckoo Chrysococcyx lucidus at Kaikoura, New Zealand

For three seasons starting in 1976 I studied the breeding of Shining Cuckoos Chrysococcyx lucidus in forest near Kaikoura, New Zealand. There is no evidence that the cuckoo parasitizes any host on mainland New Zealand other than the Grey Warbler Gerygone igata. A nestling cuckoo returned to within 1 km of its natal site in a subsequent breeding season, presumably after migrating beyond New Zealand. Empirical and theoretical estimates of the area occupied by Shining Cuckoos while breeding are given. Cuckoos near Kaikoura laid during ten weeks, the modal week of laying following seven weeks after the presumed peak of arrival of birds in New Zealand. First clutches of the host escaped parasitism because they were laid before most cuckoos arrived. Parasitized clutches received one cuckoo egg which replaced a host's egg. It was laid before, just after or long after the host began incubating, and mimicry was lacking. Cuckoo eggs, which were about 8% of the adult cuckoo's weight, hatched in 14–17 days. The frequency of parasitism near Kaikoura was 55% of late clutches (n = 40).
At 3–7 days old, nestling Shining Cuckoos evicted from the nest all other contents. The nestling period was at least 19 days. Growth in weight followed a logistic curve and the equation is given. Just over half the cuckoo eggs produced fledglings. The effect of brood-parasitism on the Grey Warbler's productivity was small. Only 17% of late warbler eggs, and late eggs only, were prevented by parasitism from yielding fledglings. Late laying by some Shining Cuckoos (relative to the host's incubational cycle), and late eviction, often led to brief inter-specific competition among nestlings for food. The brief coexistence of young Warblers and Cuckoos in the nest may explain the apparent mimicry by newly-hatched Shining Cuckoos of the host's young.     

Relative reproductive success of female moorhens using conditional strategies of brood parasitism and parental care

In a population of moorhens (Gallinula chloropus), at least27% of netting females laid one or more eggs in a neighbor'snest Females laid parasitically under three conditions: 56%of parasitic eggs were from nesting females that preceded layinga dutch in their own nest by a parasitic laying bout, 19% werefrom females whose nests were depredated before clutch completionand that laid the following egg parasiticaDy, and 25% were froma small number of females without territories, "non-nesting"parasites, that each laid a series of parasitic eggs. Clutchsizes varied greatly between females, but nesting females eachlaid a consistent clutch size both within and between seasonsfor a given mate and territory. Nesting females that employeda dual strategy of brood parasitism and parental care producedextra eggs that they laid in the nests of neighbors before layinga dutch in their own nests. Two out of ten females whose dutchesI experimentally removed during the laying period were successfullyinduced to lay their next egg in the nest of a neighbor. Nestingfemales that laid parasitically selected their hosts opportunisticallyfrom among the nests dosest to their territories. An experimentin which parasitic eggs were removed and hosts left to rearonly their own young showed that parasites did not choose hoststhat were better parents than pairs with contemporary neststhat were not parasitized. Females that only laid parasiticaDywithin a given season timed their parasitic laying bouts poorlyand achieved no reproductive success. Parasitic young rarelyfledged, and the mean seasonal reproductive success of nestingbrood parasites did not differ from that of nonparasitic females.However, the variance in reproductive success of nesting broodparasites was significantly higher than that of nonparasiticfemales.     

The comparative breeding behaviour of two sympatric cuckoos, Horsfield's Bronze-Cuckoo Chrysococcyx basalts and the Shining Bronze-Cuckoo C. lucidus, in Western Australia: a new model for the evolution of egg morphology and host specificity in avian brood parasites

The breeding behaviour of two similarly sized sympatric cuckoos, Horsfield's Bronze-Cuckoo Chrysococcyx basalts and the Shining Bronze-Cuckoo C. lucidus, was studied over four breeding seasons at Gooseberry Hill, Western Australia. Both cuckoos usually began laying in late August; Shining Bronze-Cuckoos laid for up to 13 weeks and Horsfield's Bronze-Cuckoos for up to 15 weeks. Four host species were parasitized and major hosts were parasitized throughout most of their laying periods. The frequency of parasitism varied between hosts and between years, but Splendid Fairy-wrens Malurus splendens and Yellow-rumped Thornbills Acanthiza chrysorrhoa (major hosts) were always parasitized more heavily than Western Thornbills A. inornata and Scarlet Robins Petroica multicolor. Western Thornbills were parasitized by both cuckoos. Horsfield's and Shining Bronze-Cuckoos laid monomorphic eggs; those of Horsfield's Bronze-Cuckoos were highly mimetic whereas those of Shining Bronze-Cuckoos were non-mimetic and dark in colour. Both cuckoos laid one egg per host nest, deposited eggs directly into the nest, laid very quickly in the early morning, removed at least one host egg at laying, laid eggs small for the size of the birds, hatched after 12 days and evicted nest companions shortly after hatching. Laying was well synchronized with the start of incubation by hosts. Field observations and experiments with egg models indicated that neither of the major hosts, nor the secondary host in common, discriminate against foreign eggs. The nestling period for Horsfield's Bronze-Cuckoo was 17 days, and for the Shining Bronze-Cuckoo 20 days. There was a corresponding difference in nestling growth rate between the cuckoo species. About 50% of cuckoo eggs produced fledglings. Reproductive success for both cuckoos was highest in nests of the secondary host in common, the Western Thornbill. Young cuckoos reached independence 5–6 weeks after hatching. The adaptive significance of competition between cuckoos as a selective agent for cuckoo egg morphology and host specificity is discussed.     

Infanticide in great reed warblers: secondary females destroy eggs of primary females

In 1994–1995 artificial nests with attached model eggs were put into territories that were known to have been occupied by male great reed warblers,Acrocephalus arundinaceusin previous years. Because the eggs were made of soft plasticine, predators left peckmarks in them and this enabled us to identify predators by comparing peckmarks with reference marks made by various species. Previous field data had suggested that infanticidal behaviour existed in our study population, as nests of primary females suffered a three times higher rate of nest loss during the egg-laying period than nests of secondary and monogamous females. The presence of infanticide was supported by the experiment. Small peckmarks resembling those of a great reed warbler occurred almost exclusively in territories occupied by great reed warblers, in particular when a new female settled in the territory. The newly settled females built nests closer to depredated than non-depredated nests. That small peckmarks occurred when new females settled strongly suggests that it is secondary female great reed warblers that commit infanticide on eggs of primary females. Females of low harem rank are expected to gain from infanticidal behaviour because a low ranked female gets a higher proportion of male parental investment when the nest of the primary female fails.     

Intraspecific nest parasitism in the European starling Sturnus vulgaris

Biochemical genetic markers were used along with conventional methods (abnormal laying sequence/clutch size, unusual egg shape/pigmentation) to identify intraspecific nest parasitism at two British nestbox colonies of the European starling. Between 11 and 37% of first clutches were parasitized during 1977–1979. Parasitic females probably comprised all of the following categories: (1) paired females contesting a nestbox occupied by another pair; (2) previously paired females who had laid a clutch but had been unsuccessful; (3) unpaired females who had copulated with males that already had a mate and nest site; and (4) ‘professional’ nest parasites who distributed at lest some of their eggs in one or more nests other than their own. Although parasitized nests had higher clutch sizes, parasitism led to fewer host young fledging per egg laid, mainly through the eviction of eggs and subsequent nest desertion. Number of parasitic young fledged per egg laid was highest when eggs were laid synchronously with the host, when host clutches were larger, or a smaller number of parasite eggs were added to a nest, thus favouring parasites that distribute their eggs amongst a number of nests. A greater pressure on nest sites may have accounted for the higher levels of parasitism at the Aberdeen colony and for the greater number of parasite eggs laid in a nest. Although most parasitic female starlings appeared to be much less successful than non-parasitic ones, nest parasitism in the starling might evolve directly when one or more of the following advantages are present. (1) There are no constraints on the number of eggs a female may lay but there are constraints on the number of young she may feed adequately. (2) Female survival is increased by having fewer or no eggs/young to care for. (3) Current feeding conditions favour the survival of more young than would be produced by the most common clutch size. Intraspecific nest parasitism is considered to be a first stage in the evolution of interspecific nest parasitism.