Intraspecific brood parasitism: a strategy for floating females in the European starling

In many bird species, there is a floating population of females that are excluded from breeding because of competition for limited breeding resources. Female floaters may enhance their reproductive success by engaging in intraspecific brood parasitism. We studied female floaters in a population of European starlings, Sturnus vulgaris, in order to determine their identity and potential parasitic behaviour. Females were caught after being attracted to nestboxes with artificial nests during 1993-1995. None of the females was known to have a nest of her own at capture but 47% of the females either laid an egg in the nest or carried a fully developed egg within the reproductive tract, indicating that they were intraspecific brood parasites. The floating females were significantly younger and smaller than breeding females. Of 13 females equipped with radiotransmitters and followed daily, all but one started a breeding attempt of their own after 3-8 days and the majority settled as secondary females or mated with males where the original female had disappeared. This suggests that females that are unable to compete successfully for nest sites or males early in the breeding season may use intraspecific brood parasitism to enhance reproductive success during the period that they are constrained from breeding. The importance of settling rapidly because of a seasonal decline in reproductive success may also promote the evolution of intraspecific brood parasitism in the starling. The relative reproductive success of combining egg dumping with breeding compared with traditional breeding will depend on the costs of delaying breeding as well as the probability of finding a mate later in the breeding season. Copyright 1999 The Association for the Study of Animal Behaviour.     

Reduced reproductive performance associated with warmer ambient temperatures during incubation in a winter‐breeding,food‐storing passerine

Timing of reproduction can influence individual fitness whereby early breeders tend to have higher reproductive success than late breeders. However, the fitness consequences of timing of breeding may also be influenced by environmental conditions after the commencement of breeding. We tested whether ambient temperatures during the incubation and early nestling periods modulated the effect of laying date on brood size and dominant juvenile survival in gray jays (Perisoreus canadensis), a sedentary boreal species whose late winter nesting depends, in part, on caches of perishable food. Previous evidence has suggested that warmer temperatures degrade the quality of these food hoards, and we asked whether warmer ambient temperatures during the incubation and early nestling periods would be associated with smaller brood sizes and lower summer survival of dominant juveniles. We used 38 years of data from a range‐edge population of gray jays in Algonquin Provincial Park, Ontario, where the population has declined over 50% since the study began. Consistent with the “hoard‐rot” hypothesis, we found that cold temperatures during incubation were associated with larger brood sizes in later breeding attempts, but temperatures had little effect on brood size for females breeding early in the season. This is the first evidence that laying date and temperature during incubation interactively influence brood size in any bird species. We did not find evidence that ambient temperatures during the incubation period or early part of the nestling period influenced summer survival of dominant juveniles. Our findings provide evidence that warming temperatures are associated with some aspects of reduced reproductive performance in a species that is reliant on cold temperatures to store perishable food caches, some of which are later consumed during the reproductive period.     

Early springs and breeding performance in two sympatric duck species with different migration strategies

The capacity of migratory species to adapt to climate change may depend on their migratory and reproductive strategies. For example, reproductive output is likely to be influenced by how well migration and nesting are timed to temporal patterns of food abundance, or by temperature variations during the brood rearing phase. Based on two decades (1988–2009) of waterfowl counts from a boreal catchment in southern Finland we assessed how variation in ice break‐up date affected nesting phenology and breeding success in two sympatric duck species, Mallard Anas platyrhynchos and Eurasian Teal Anas crecca. In Fennoscandia these species have similar breeding habitat requirements but differ in migration distance; Teal migrate roughly seven times as far as do Mallard. Annual ice break‐up date was used as a proxy of spring ‘earliness’ to test the potential effect of climate change on hatching timing and breeding performance. Both species were capable of adapting their nesting phenology, and bred earlier in years when spring was early. However, the interval from ice break‐up to hatching tended to be longer in early springs in both species, so that broods hatched relatively later than in late springs. Ice break‐up date did not appear to influence annual number of broods per pair or annual mean brood size in either species. Our study therefore does not suggest that breeding performance in Teal and Mallard is negatively affected by advancement of ice break‐up at the population level. However, both species showed a within‐season decline in brood size with increasing interval between ice break‐up and hatching. Our study therefore highlights a disparity between individuals in their capacity to adjust to ice break‐up date, late breeders having a lower breeding success than early breeders. We speculate that breeding success of both species may therefore decline should a consistent trend towards earlier springs occur.     

FREQUENCY-DEPENDENT FITNESS CONSEQUENCES OF INTRASPECIFIC NEST PARASITISM IN SNOW GEESE

The reproductive efficiency, defined as the number of breeding recruits produced per egg laid; of intraspecific nest parasites; of hosts in parasitized nests; and of unparasitized nesting females, was measured for 14 years for lesser snow geese Anser caerulescens caerulescens nesting near Churchill, Manitoba, Canada. Relative efficiencies were 0.71–0.88, 0.91, and 1.0 for eggs of parasites, hosts, and unparasitized birds, respectively. Differences in the hatching probabilities of the three classes of eggs produced the efficiency differences. Parasitic success was limited by the parasites' failure to place more eggs than expected by chance into nests at the appropriate time relative to host incubation. Host nesting success was lower when more than one parasitic egg was added to the clutch. No differences in gosling survival and breeding recruitment probabilities were detected among any categories of goslings. Thus, hatching parasitic young are at no disadvantage relative to parental young, and there is no support for the hypothesis that increased success of host young at later stages of reproduction might offset negative effects at the egg stage. The hatching efficiency of parasitic eggs declined more rapidly than that of parental eggs as the parasitism rate increased. Efficiencies were similar when 3–4% of the eggs laid per year were parasitic, but relative parasitic efficiency was significantly lower when the parasitism rate was 8–9% or more. Using ancillary information and assumptions about the fecundity, viability, and behavioral flexibility of parasitic and parental females, we conclude that intraspecific nest parasitism could compete with nesting as a reproductive strategy in this population. The conditional use of parasitism by a large component of the population in certain years, however, combined with negative-frequency dependent success, limits the potential spread of a purely parasitic strategy in this population.     

Consequences of a changing environment on the breeding phenology and reproductive success components in a long-distance migratory bird

Migratory birds have a narrow time window to breed, especially in the Arctic, where early nesting typically yields the highest reproductive success. We assessed temporal changes (1991–2015) in reproductive success components in relation to timing of breeding in greater snow geese (Chen caerulescens atlantica). This species breeds in the Canadian Arctic, a region that has experienced a strong warming trend. We tested the effect of laying or hatching date, year and their interaction on six reproductive components: Total clutch laid, nesting success, egg survival, hatching success, prefledging, and postfledging survival. Over 25 years, mean laying date changed little, even though it advanced 1.8 days in early breeders and was delayed 3 days in late breeders. Likewise, the number of eggs in nests initiated early in the season decreased by 0.6 egg, whereas in late nests it increased by 0.3 egg. Success of nests initiated early and late in the season was lower than nests initiated near the population mean, and consistently increased over time. The proportion of eggs surviving to partial predation and postfledging survival decreased with laying date but the pattern did not change over time. In contrast, prefledging survival was not affected by laying date initially but declined in nests initiated late in the season toward the end of the study. Overall, nests initiated close to the population mean showed little temporal change for most components of reproductive success and seem to be less affected by environmental change than nests initiated early and late in the season.     

Perceived risk of ectoparasitism reduces primary reproductive investment in tree swallows Tachycineta bicolor

Many birds have been shown to reduce their reproductive investment in response to infestation of nest sites by ectoparasites. Nest-dwelling parasite populations increase throughout the breeding season, and can reduce the condition and future survival of both breeding adults and their offspring. Thus, avian hosts should be capable of assessing early cues that predict future ectoparasitism risk, and should be able to facultatively adjust their primary reproductive investment in response to anticipated future costs of parasites. We tested this hypothesis in the tree swallow Tachycineta bicolor , a cavity nesting passerine, by presenting a visual cue of avian fleas on the outer surface of nest boxes. This treatment manipulated perceived ectoparasitism risk without exposing birds to parasites, thereby allowing us to examine facultative responses in the absence of early physiological effects of parasites on female reproductive investment. During one of the study years, birds preferentially occupied control boxes, however, across both years, birds nesting in treatment boxes produced significantly smaller clutches, resulting in smaller broods at hatching, relative to those in control boxes. This difference in clutch size could not be explained by differences in phenotypic quality of breeding birds, indicating that for cavity nesting birds such as tree swallows, the perception of future ectoparasitism risk may be sufficient to induce a facultative reduction in reproductive investment early in the breeding season, before nest-dwelling parasite populations have grown very large.     

Some effects of inclement weather conditions on the survival and condition of bull-headed shrike nestlings

Inclement weather struck Japan in 1993, permitting a natural experiment for examining the effect of weather conditions on nesting success and nestling growth of the bull-headed shrike, Lanius bucephalus. Aspects of the breeding biology of bull-headed shrikes in relation to weather conditions and timing of breeding in 1992 and 1993 were examined. The two breeding seasons were divided into two periods, early and late, in each year. While the probability of nest survival in nestling stages during both periods was almost equal, the probability of nest survival in the egg stage during the early period was significantly lower than that during the late period. In 1993, the probability of nestling survival during the late period was significantly lower than during the early period; the late period had larger fluctuations of precipitation and was colder than the same period in 1992. The number of disappeared nestlings positively correlated with mean precipitation per day. The greater part of the disappeared nestlings was the lightest in each brood. Late breeders fledged lighter young than the early breeders. Although shrikes adopted hatching asynchrony, the late breeders could not surmount the unpredictable inclement weather in 1993.     

Errors in egg-laying by female Common Cuckoo Cuculus canorus in nests of its common host

Dozens of studies have documented that brood parasites are well adapted to a brood parasitic lifestyle but not all parasitism events are successful. Co-evolution between brood parasites and their hosts is a dynamic process so it is reasonable to expect that a female brood parasite may commit errors during egg deposition by laying her eggs outside the laying period of the host, with consequent impacts on her fitness. Using an extensive dataset from a long-term study, we evaluated egg-laying patterns and errors related to the timing of egg-laying in the Common Cuckoo Cuculus canorus (hereafter ‘Cuckoo’). Specifically, we tested whether the Cuckoo avoids laying before or on the day of host clutch initiation to reduce the risk of rejection of parasitic eggs, whether laying errors will be more frequent in periods with a lack of active host nests, and whether the laying errors will be more frequent in periods with intense Cuckoo parasitism and a consequent lack of suitable host nests. We found that about one-third of Cuckoo eggs were laid on the host clutch initiation day or 1 day before, and the percentage of Cuckoo eggs laid decreased thereafter. Surprisingly, the probability of Cuckoo egg acceptance by the hosts was not affected by the egg-laying stage of the host clutch. Errors in the timing of egg-laying with fatal consequences (i.e. those precluding Cuckoo hatching because of laying in incubated or deserted clutches) were recorded in about 5% of cases. Only laying date of a Cuckoo egg had a significant effect on the probability of errors, which increased during the breeding season. This may be related to the higher number of deserted and incubated host nests at the site at the end of the breeding season. Errors in egg-laying may be attributed to young and inexperienced females but also impaired body condition or intraspecific competition may cause this behaviour. Future studies, which will test these possible explanations, will help to understand better the mechanism of co-evolutionary arms races and differences between host specialist and generalist brood parasites in various host–parasite systems.     

Hormone profiles of obligate avian brood parasites during the breeding season

The ultimate explanations for avian brood parasitism have been studied intensively as a model system for coevolution, but little is known about the proximate mechanisms, for example hormonal regulation, underlying brood parasitic behaviour. In this study, we explored seasonal hormone profiles in two brood parasitic Cuculus species breeding in the Republic of Korea. As brood parasites have relatively simple breeding stages without incubation and provisioning, we predicted that during the breeding season individuals would exhibit similar levels of testosterone (T) and stress‐induced corticosterone (CORT), hormones that are known to be closely related to the transition of breeding stages. We also assessed how these hormone profiles were associated with traits such as body size and sex. Overall, male cuckoos showed similarly high T levels throughout the breeding season, as predicted, but individual variation became greater as the season progressed. Individual CORT levels tended to decrease as the season progressed, although the decrease was significant only in Common Cuckoos Cuculus canorus. We also found that male Lesser Cuckoos Cuculus poliocephalus showed a much higher level of T than females, as expected, but this sexual difference was not observed in Common Cuckoos. Our results suggest that the seasonal hormone profiles of avian brood parasites are likely to be similar to typical hormone profiles expected for non‐brood parasites during the breeding season. This may suggest that not only the breeding cycle but also other factors such as social interaction may be affected by hormonal changes. Further studies are needed to fully understand the proximate mechanism of avian brood parasitism.     

Columbian sharp-tailed grouse female and nest survival in northwestern Colorado

Wildlife management and conservation can be challenging when the demography of a focal species is unknown or limited given that fecundity and adult survival influence population growth. The Columbian sharp-tailed grouse (Tympanuchus phasianellus columbianus) have been reduced to ≤10% of their former range since the early 1900s. We conducted a 3-year study (2015–2017) across 4 study sites in northwestern Colorado, USA, to evaluate female hazard and nest survival. We trapped and marked 270 female sharp-tailed grouse and identified 275 nests for our hazard and survival analyses. Females during the breeding stage of the reproductive season experienced more hazard compared to the nesting and the early and late post-nesting stages for females without broods. Females experienced a higher degree of hazard during the breeding stage and mortality risk was >3 times higher than the nesting stage, >7 times higher than early post-nesting (EPN)-no brood stage, and >5 times higher than the late post-nesting (LPN)-unsuccessful stage. Two reproductive season stages (LPN-successful and EPN-brood) provided marginal inference. Nest incubation initiation date and nest age best described nest daily survival. Females that initiated incubation of initial nests earlier in the season experienced lower nest daily survival than later in the incubation season. Because female Columbian sharp-tailed grouse hazard varied among different reproductive season stages, we recommend that wildlife managers develop management actions that reduce hazard during the specific reproductive season stages (i.e., the breeding season). For Columbian sharp-tailed grouse in Colorado, we recommend that Colorado Parks and Wildlife collaborate with federal farm program agencies to implement a no-tillage restriction from 1 May through 30 June for active agricultural fields within 2 km of active Columbian sharp-tailed grouse leks to enhance nest survival.     

Sexual aggression by intruders in hooded crow <Emphasis Type="Italic">Corvus cornix</Emphasis>

The hooded crow Corvus cornix is a west Palaearctic, solitary nesting, monogamous corvid. In the breeding season, populations are characterized by a social organization wherein breeding pairs are territorial and non-breeding individuals, called floaters, live in flocks. During a study of the breeding ecology of the hooded crow, conducted in a protected flooded area, we monitored nests with video cameras. We recorded two separate incidents when intruders attacked a female at the nest. We believe that she remained in the nest in order to prevent the strangers cannibalizing the nestlings by mantling over the brood. The spatio-temporal occurrence of these attacks suggests that the observed behaviour is intraspecific sexual aggression wherein non-breeding males mounted an immobilized female.     

BREEDING BIOLOGY OF HOUSE MARTINS DELICHON URBICA IN RELATION TO AERIAL INSECT ABUNDANCE

The breeding biology of House Martins nesting in artificial boxes was examined in relation to aerial insect abundance and weather. The insect food supply was continuously monitored with a suction-trap at 40 ft above ground level. Aerial insect abundance started to rise from the winter level in April and reached a high stable state in June. Insects remained abundant until September; the subsequent decline continued at least until the end of October. The size of first clutches (mean size 3.87) was correlated with the abundance of aphids in spring, though not with temperature. It is suggested that this was an adaptive response because high food levels in May were associated with high levels in the main nesting period. Clutch-size declined through the season but was not matched by changes in food abundance. Second clutches (mean size 2–95) were most frequent following a high food level in June. No correlation was found between egg-weight and preceding food conditions, nor with season, clutch-size or order of laying within clutches. If food was scarce on the day of laying of the first egg, these clutches suffered a suspension of laying of subsequent eggs. Some of the young in these nests showed a reduced growth rate. The duration of incubation (mean 14.6 ± 1.1 days) was not apparently correlated with food abundance, however it must have been potentially extended by extreme shortages because incubation duties were neglected at such times. Infertility was probably the main cause of hatching failure (14.2%). A model was used to predict the date of onset of laying. It was based on the assumption that nestling tissue was produced with the same energetic efficiency as eggs. The model indicated that House Martins could collect enough food to lay eggs earlier than the observed date in most years. The high probability of food shortage before the laying period, however, appeared to discourage laying at this time. The observed onset of laying coincided with the appearance of aphids in the air, probably because they comprised an abundant and stable food source for egg formation. Breeding generally occurred in the period of highest food abundance. Pairs rearing only a single brood each year did so in July when food levels were highest. The growth of first brood nestlings was more closely associated with food levels than any other factor investigated, while in second broods, rainfall was found to have the greatest influence. The total variance in growth explained by environmental factors in the first broods (41%) was greater than in second broods (22%). This relative independence of second broods from adverse environments probably arose from more abundant food and the feeding of some of these broods by the young of first broods. An exceptionally late brood showed an extended nestling period indicating deteriorating conditions later in the breeding season. It is proposed that the spread of laying may reflect differences in the feeding efficiency of nesting House Martins, because the growth of two early first broods was more rapid at given food levels than two late broods. Laying patterns conformed to a distribution based on a progressive threshold mechanism. The more efficient pairs may be able to attain breeding condition and lay at low levels of food abundance, and hence breed earlier in the season. Subsequent layings are facilitated by the rise in food levels in early summer. Mortality within the nest was low (5.8%) and associated with food shortage. Nestling periods (mean 30.6 ± 2.3 days) were not shown to be correlated with food abundance; they were more dependent on brood-size. Recruitment into House Martin populations is thus widely influenced by food supply, particularly through an influence on clutch-size, the occurrence of second clutches and nestling mortality.     

Seasonal decline in provisioning effort and nestling mass of Blue Tits Cyanistes caeruleus: experimental support for the parent quality hypothesis

A common feature of many birds breeding in seasonal environments is that fitness‐related parameters such as nestling mass or survival decline as the breeding season progresses. Consequently, there is a tendency for early breeders to have better reproductive performance than individuals breeding later in the season. This variation could be caused by factors associated with the date of laying, such as changing environmental conditions (date hypothesis), or by differences in parental quality between early and late breeders (parent quality hypothesis). To evaluate the relative importance of both hypotheses, we manipulated hatch dates of Blue Tits Cyanistes caeruleus by exchanging clutches with different incubation stages and assessed the impact on nestling mass, nestling diet and provisioning rates. Mean nestling mass declined significantly over the season. This was the combined result of differences in parental quality, which dominated in the early part of the season, and the influence of hatching date (date effect per se), which prevailed later in the season. Nestling diet composition was apparently uninfluenced by the manipulation, suggesting that deteriorating food supplies are the primary reason for the seasonal variation in the nestling diet. Counter to the date hypothesis, delayed parents did not feed their young less than control pairs did, but in fact exhibited higher provisioning rates. Our results suggest that in this population, parental quality seems to constrain post‐hatching reproductive performance and such intrinsic limitations may help to explain why certain individuals breed later.     

Effect of intraspecific interactions on seasonal decline in productivity of Common Terns Sterna hirundo

Capsule Kleptoparasitic activities of older chicks from earlier nests did not contribute to late reproductive declines.

Aims To determine whether intraspecific interactions, such as kleptoparasitism and aggression, were experienced more frequently by birds breeding late in the season as a result of exposure to breeders at a more advanced stage. If so, to investigate whether this was the cause of the observed seasonal decline in reproductive parameters observed at Bird Island, where nesting density is high and interactions are more probable.

Methods Plots were fenced within the colony, exploiting natural variability in distribution of early and peak breeders to create two treatments: plots with only late-laying terns and those with a mixture of early-, peak- and late-layers. Hatching success, productivity and the growth and survival of chicks were measured for all late-laying pairs. Intraspecific interactions, adult attendance and provisioning of chicks were recorded during 9600 minutes of nest observations made within two periods: a few days after hatching and one week later.

Results The frequency of intraspecific interactions was maintained by the kleptoparasitic activities of older chicks within the mixed-laying-date treatment and was significantly lower in plots containing only late breeders with chicks of similar ages (mean 11.0 days). The overall rate was rarely greater than two interactions per nest per hour and there was no corresponding reduction in the growth or survival of chicks from late nests or any change in the provisioning activities of late-breeding adults.

Conclusion Increased frequency of intraspecific interactions experienced by late breeders in the presence of early-breeding conspecifics resulted from the kleptoparasitic activities of older chicks but was not sufficient to contribute to the observed seasonal reproductive decline at this dense breeding colony.     

Seasonal patterns in reproductive success of temperate‐breeding birds: Experimental tests of the date and quality hypotheses

For organisms in seasonal environments, individuals that breed earlier in the season regularly attain higher fitness than their late‐breeding counterparts. Two primary hypotheses have been proposed to explain these patterns: The quality hypothesis contends that early breeders are of better phenotypic quality or breed on higher quality territories, whereas the date hypothesis predicts that seasonally declining reproductive success is a response to a seasonal deterioration in environmental quality. In birds, food availability is thought to drive deteriorating environmental conditions, but few experimental studies have demonstrated its importance while also controlling for parental quality. We tested predictions of the date hypothesis in tree swallows (Tachycineta bicolor) over two breeding seasons and in two locations within their breeding range in Canada. Nests were paired by clutch initiation date to control for parental quality, and we delayed the hatching date of one nest within each pair. Subsequently, brood sizes were manipulated to mimic changes in per capita food abundance, and we examined the effects of manipulations, as well as indices of environmental and parental quality, on nestling quality, fledging success, and return rates. Reduced reproductive success of late‐breeding individuals was causally related to a seasonal decline in environmental quality. Declining insect biomass and enlarged brood sizes resulted in nestlings that were lighter, in poorer body condition, structurally smaller, had shorter and slower growing flight feathers and were less likely to survive to fledge. Our results provide evidence for the importance of food resources in mediating seasonal declines in offspring quality and survival.     

Speciation through temporal segregation of Madeiran storm petrel (Oceanodroma castro) populations in the Azores?

Madeiran storm petrels Oceanodroma castro breed on three small islets in the Azores: Vila, off Santa Maria, and Praia and Baixo, off Graciosa. Analysis of data on brood patch, incubation periods, chick body size and recaptures of adults provides evidence of the existence of two distinct populations (hot- and cool-season) breeding annually on Baixo and Praia, out of phase by four to five months and overlapping in colony attendance during August and early September; on Vila only the cool-season population is present. Analyses of adult morphology indicate highly significant phenotypic differentiation between the sympatric hot- and cool-season breeders, whereas an almost complete phenotypic uniformity characterizes allopatric breeders within the same season. The hot-season birds are 10% smaller in egg and body mass but have longer wings and tails than cool-season birds. The two groups were readily separated by discriminant analysis. The preference to breed in the cooler season is interpreted as a consequence of greater food availability in that period. Morphological differentiation between seasonal populations is interpreted as an adaptative response to different environmental conditions in the two seasons. The hypothesis is given that the hot-season population has evolved from the cool-season population owing to density-dependent constraints on crowded colonies, forcing birds to time-share nest sites. These populations may represent a case of sympatric speciation through temporal partitioning of reproduction and may be better treated as sibling species.     

Reduced immunocompetence and cost of reproduction in common eiders

Immunocompetence may be especially important in long-lived species where infectious organisms may have detrimental effects upon future reproductive value of hosts. The resource demand for immunocompetence may reduce resource availability for reproduction and a trade-off between these traits has therefore been proposed. Capital breeders, such as the common eider (Somateria mollissima), rely upon accumulated body reserves during reproduction. Eiders lose more than 40% of pre-breeding body mass during egglaying and incubation and many females abandon their ducklings to other females after hatching. Results from our observational study show that levels of leukocytes (i.e., lymphocytes, heterophils and heterophil/lymphocyte ratio) are not related to body mass early in the incubation period. However, eider females with low initial body mass showed signs of immunosuppression (i.e., decreased late levels of lymphocytes) and increased response towards stressors (i.e., increased heterophil/lymphocyte ratio) later in the incubation period. Moreover, females with low lymphocyte levels more frequently abandoned their brood, and females abandoning young had an increased return rate to the next breeding season. However, among brood abandoning females return rate was lower for the females with low lymphocyte levels. These results suggest that immunosuppression, as indicated by low lymphocyte levels, is a reproductive cost that may be partly compensated for by abandoning young.     

ENVIRONMENTAL INFLUENCES ON GROWTH AND SURVIVAL OF NESTLING HOUSE MARTINS DELICHON URBICA

Growth of nestling House Martins was studied in relation to (a) conditions in the external environment and (b) aspects of their breeding biology. The dependence of growth performance on (1) hatchling weights, (2) relative difference in hatchling weights within broods, (3) brood size,(4) season, (5) earliness of breeding in relation to other pairs in the colony, (6) timing of breeding in relation to the median breeding week of the colony and (7) aerial food abundance, was investigated by step-down multiple regression analysis. Up to the stage of the peak brood weight, early laying, small brood sizes and high hatchling weights were associated with higher nestling growth rates. Large relative differences in hatchling weights however tended to depress mean brood weights and increase weight differences (= size hierarchies) within broods. These differences in hatchling weights were considered to contribute significantly to 23% of all nestling deaths, because small, late hatching nestlings suffered very high mortality even when food was abundant. The nestlings which died showed a progressive reduction on growth rates and all succumbed before the 11th nestling day. Because these differences in hatchling weights can be linked to the food supply during laying rather than immediately prior to their death, it is considered that the mortality of these nestlings can ultimately be attributed to the low quality of eggs from which they hatched. There was a tendency for pre-hatching factors to diminish in importance throughout growth, while post-hatching factors increased in importance and, with one exception, were responsible for explaining all the significant variance in the growth characteristics of fledglings. The exception was that differences in wing-lengths in broods could be linked with weight differences at hatching. Food shortages lowered average brood weights prior to fledging. Because pairs breeding during the median breeding week had lighter young, it was inferred that competition for food during this peak of breeding activity had the effect of lowering nestling growth performance, although the overall effect was considered to be small. Early breeding pairs tended to have larger broods, and these large broods showed a lowered growth performance. However, early breeding pairs had relatively smaller weight and wing-length differences, in broods of a given size, than occurred in broods of late breeders. It was therefore concluded that early breeding pairs had some attribute which tended to minimize certain disadvantages of large broods. This effect appeared to be linked to the pair, rather than to season or food supply.     

An experimental test on time constraint and sexual conflict over parental care

Because parental care is costly, a sexual conflict between parents over parental investment is expected to arise. Parental care behavior is an adaptive decision, involving trade‐offs between remating, and consequently desertion of the brood, and continuing parental effort. If the main advantage of desertion is remating, then this will be a time constraint, because the deserting individual will require a certain minimum period of time to breed again in the same breeding season. So, a short breeding season should force certain individuals to desert the first brood to have enough time to successfully complete their second breeding attempt. The rock sparrow, Petronia petronia, is an unusual species in which brood desertion can occur in both sexes and the breeding season is quite short so it is a good species to investigate the role of time constraint on brood desertion. For 3 years, I investigated the brood desertion modality of the rock sparrow. Then, for 2 years, I removed a group of experimental nest boxes during the autumn. Later, I re‐installed the experimental nest boxes after the start of the breeding season (2 weeks after the first egg was laid), mimicking a shortening of the breeding season for the (experimental) pairs that used experimental nest boxes. I found that in the experimental pairs, the percentage of deserting individuals was significantly higher than in the control groups, and the deserting individuals were older females. This experiment adds to our knowledge of timing of reproduction effects on individual decisions to desert by showing that a short and delayed breeding season may have different effects on males and females. To my knowledge, this is the first experimental study that demonstrates a direct link between time constraint and brood desertion.     

Contrasting breeding strategies of two sympatric orioles: first documentation of double brooding by Orchard Orioles

ABSTRACT.   Although many studies have revealed intraspecific variation in the number of broods raised per breeding season, less is known about the causes of variation between species. We monitored the breeding behavior of congeners traditionally thought to raise one brood per breeding season, Orchard Orioles ( Icterus spurius ) and Baltimore Orioles ( I. galbula ). Both species have similar habitat and nesting requirements. We captured and banded orioles at three study sites in Maryland in 2004 and 2005. No pairs of Baltimore Orioles ( N = 26) were double brooded, whereas 8 of 24 pairs (33%) of Orchard Orioles had a second brood and 7 of 16 pairs (44%) of Orchard Orioles that successfully raised a first brood also successfully raised a second brood during the same breeding season. Our findings provide the first documentation of double brooding by Orchard Orioles. The timing of molt and the amount of time and energy devoted to postfledgling parental care may contribute to this difference between species in terms of the number of broods per breeding season.