Limitations of methods to test density-dependent fecundity hypothesis

1. Two main hypotheses are usually invoked to explain density dependence in fecundity: the habitat heterogeneity hypothesis (HHH) and the individual adjustment hypothesis (IAH). Although simple methods have been proposed to discriminate between the two hypotheses, their adequacy was tested for only a limited set of real and model populations. 2. In a computer simulation study based on a stochastic territory-based approach, Ferrer, Newton & Casado (2006, Journal of Animal Ecology, 75, 111-117) argued that a strong negative relationship between mean fecundity and its skewness in stable or increasing populations provides critical support for HHH, as this relationship should be lacking under IAH. A negative relationship between mean fecundity and its coefficient of variation (CV) was predicted under both hypotheses, although with a lower slope under IAH. 3. We used a comparable simulation approach, with model populations parameterized from an increasing Bonelli's eagle Hieraaetus fasciatus population (1992-2006), to show that both HHH and IAH can produce indistinguishable relationships between mean fecundity and both its CV and its skewness. 4. Strong negative correlations between the mean and both its CV and its skewness can emerge as statistical artifacts under biologically plausible assumptions, and so they may be largely inadequate to infer mechanisms underlying density dependence in demographic parameters.     

The regulation of brood reduction in Booted Eagles Hieraaetus pennatus through habitat heterogeneity

Brood reduction, the death of one or more chicks through siblicide or starvation, can occur through density‐dependence in fecundity. Brood reduction may arise in territorial breeding systems either as a response to a high level of territorial interference in a situation of high density or as a result of habitat heterogeneity. To test the predictions of the two main hypotheses that attempt to explain how density‐dependent fecundity is generated, the Habitat Heterogeneity Hypothesis (HHH) and the Individual Adjustment Hypothesis (IAH), we analysed the relationship between density and fecundity in an expanding population of Booted Eagles in Doñana National Park, Spain, using an 18‐year data series. We also studied the occurrence and frequency of brood reduction in the same Booted Eagle population to appreciate further its effects and the factors that influence its occurrence and frequency. Our results support the HHH in the present situation of high density, as fecundity in the better territories (older and more frequently occupied) was higher than in low quality territories and was not affected by population density in high density periods. Nevertheless, the fecundity of high quality territories was affected (although not significantly) by population density in periods of low density, suggesting that the IAH was supported when only high quality territories were occupied. Older territories were used more frequently and chicks in these areas hatched earlier and suffered lower mortality than in new territories. We found a significant negative relationship between mean fecundity and its skewness, a finding that also supports HHH. During years of food shortage, less frequently occupied territories suffered higher rates of brood reduction. Brood reduction in this Booted Eagle population was a consequence of the heterogeneous structure of the habitat, with some territories having a higher probability of brood reduction than others. Parental nutritional condition did not affect brood reduction. The effect of brood reduction on nestling quality and population dynamics is also discussed.     

How to test different density-dependent fecundity hypotheses in an increasing or stable population

1. We report on a simulation study of increasing and stable populations working under two different hypotheses of density dependence of fecundity: the habitat heterogeneity hypothesis (HHH) and the individual adjustment hypothesis (IAH). Our aim is to find critical differences between the two regulatory hypotheses in natural populations. 2. Populations under HHH show a strong negative relationship between fecundity and the coefficient of variation of fecundity. We also found a strong negative relationship between fecundity and skewness, demonstrating that, as fecundity decreases, the form of the distribution of brood sizes changes, being more left-skewed due to more territories failing to produce any offspring. 3. This strong relationship was found only in the simulations of populations under HHH; whether increasing or stable, and under different ratios of good: poor territories and different population sizes. In contrast, no relationship between mean fecundity and skewness was found among simulations under IAH. 4. Populations under IAH also showed a significant relationship between mean fecundity and the coefficient of variation of fecundity, but with a lower slope than in populations under HHH. 5. In conclusion, skewness was found to be an adequate critical test that showed significant and strong relationships with mean fecundity only in populations under HHH, whether increasing or stable. This test is useful for species with a discrete distribution of offspring with a small number of integer categories, including most of the bird and mammal species.     

Comparative analysis of breeding rates of rural and urban grey starling colonies in Tokyo area: The second report (Part 2)

1. The rates of clutch size, brood size and brood-size fledged recorded during the period 1956≈63 (2 years interrupted) in the rural and the urban colony of the grey starling were analysed comparatively.
2. The clutch size was significantly larger in the urban than in the rural one and the brood size was also larger significantly in the urban population. But, the difference in brood-size fledged was insignificant.
3. This reflected that the hatching rate was similar in both colonies (the rates in total differed but possible unusual rate was included in the clutch size of 5 eggs) and this may be determined physiologically but not depending upon food supply since smaller clutches showed higher hatching rate.
4. However, the fledging rate was higher in the rural and lower and more variable in the urban colony. This is apparently due to food which was nutritive animal food in the rural but largely mixed with fruits in the urban.
5. The fledging rate was rather irregularly variable with brood size suggesting that this is dependent upon parents' adaptability in feeding. However, the rate of 100% fledging becomes higher from brood sizes of 6 and less chicks.
6. Thus, in general larger clutch and brood sizes produced larger absolute numbers of chicks and chicks fledged respectively. But, 5 was the most frequent size in the clutch and brood sizes and 4 in the brood-size fledged.
7. From the above, the ecological evolution of urban population was discussed.

     

Brood size, sibling competition, and the cost of begging in great tits (Parus major)

Evolutionary theory of parent-offspring conflict explains beggingdisplays of nestling birds as selfish attempts to influenceparental food allocation. Models predict that this conflictmay be resolved by honest signaling of offspring need to parents,or by competition among nestmates, leading to escalated beggingscrambles. Although the former type of models has been qualitativelysupported by experimental studies, the potential for a beggingcomponent driven by scramble competition cannot be excludedby the evidence. In a brood-size manipulation experiment withgreat tits, Parus major, we explored the scramble componentin the begging activity of great tit nestlings by investigatingthe mechanisms of sibling competition in relation to brood size.While under full parental compensation, the feeding rate pernestling will remain constant over all brood sizes for bothtypes of models; the scramble begging models alone predict anincrease in begging intensity with brood size, if begging costsdo not arise exclusively through predation. Great tit parentsadjusted feeding rates to brood size and fed nestlings at similarrates and with similar prey sizes in all three brood-size categories.Despite full parental compensation, the begging and food solicitationactivities increased with experimental brood size, whereas nestlingbody condition deteriorated. These findings support a scramblecomponent in begging and suggest that the competition-inducedcosts of food solicitation behavior play an important role inthe evolution of parent-offspring communication.     

Is the shape of the density-growth relationship for stream salmonids evidence for exploitative rather than interference competition?

1. Empirical studies show that average growth of stream-dwelling salmon and trout often declines with increasing density in a characteristic concave relationship. However, the mechanisms that generate negative density-growth relationships in populations in natural streams are not certain. 2. In a recent study, Imre, Grant & Cunjak (2005; Journal of Animal Ecology, 74, 508-516) argue that density-dependent growth due to exploitative competition for prey causes the negative density-growth relationships for stream salmonids. They argue that the concave shape of empirical density-growth relationships is consistent with a simple model of exploitative competition and not consistent with interference competition for space. 3. We use a simple model to show that competition for space can yield concave density-growth relationships consistent with the empirical pattern when individuals compete for foraging sites that vary spatially in quality and lower-quality sites predominate. Thus, the predictions of the exploitative competition and spatial competition models overlap. 4. The shape of the density-growth relationship does not differentiate between candidate mechanisms underlying density-dependent growth for stream salmonids. Our results highlight the general problem with determining the mechanism driving an ecological process from patterns in observational data within the context of linking population demographics to habitat structure and animal behaviour.     

The Divergence of Echolocation Frequency in Horseshoe Bats: Moth Hearing,Body Size or Habitat?

A phylogenetic approach was used to test three hypotheses regarding the evolution of diversity in the echolocation frequencies used by horseshoe bats (family Rhinolophidae, genus Rhinolophus): 1) Allotonic Frequency Hypothesis (high frequency echolocation in the Rhinolophidae resulted from coevolution with moth hearing); 2) Allometry Hypothesis (echolocation frequency is negatively scaled with body size and evolutionary changes in echolocation frequencies are correlated with changes in body size in the Rhinolophidae); and 3) Foraging Habitat Hypothesis (evolution of echolocation frequency is associated with changes in habitat type). Both discrete and continuous character sets were used for ancestral state reconstructions and for investigating patterns of evolution between frequency and body size, and frequency and habitat type. Contrary to the prediction of the Allotonic Frequency Hypothesis, echolocation frequency in the Rhinolophidae did not increase over time, which would be expected if moth hearing and bat echolocation frequency coevolved. The number of extant species that exhibit calls within moth hearing ranges was not significantly different from the number of species that echolocate outside of moth hearing range. There was also no correlation between changes in frequency and changes in habitat type as predicted by the Foraging Habitat Hypothesis. Instead, the evolution of echolocation frequency within the Rhinolophidae was correlated with changes in body size as predicted by the Allometry Hypothesis.     

Rearing environment effects on immune defence in blue tit Cyanistes caeruleus nestlings

Rearing conditions may influence ontogeny and functioning of the immune system. Activation of different mechanisms involved in host disease resistance and their internal regulation can be affected by intrinsic and extrinsic factors influencing development. I investigated how rearing environment can influence associations between humoral and cellular constituents of immune defence in nestling blue tits (Cyanistes caeruleus). The ability to mount a cell-mediated immune response was estimated as a hypersensitivity reaction to phytohaemagglutinin, and the ontogeny of humoral immunity was determined by assessing circulating levels of total IgG in 15-day-old nestlings. Heterogeneity in rearing conditions was evoked by placing nest-boxes in areas differing in habitat structural characteristics, and examining natural variation in nest ectoparasite infestations, hatching date, brood size and brood sex-ratio. Habitat characteristics, parasitism and hatching date may shape associations between different components of the immune system in developing birds. I discuss the effects of rearing conditions on the interaction between different arms of the immune system and the implications for understanding negative correlations within the immune system at the individual and brood level.     

Maternal care in the rockpool amphipod Apherusa jurinei: developmental and environmental cues

There is evidence that active, pre-emergence maternal brood care in amphipod crustaceans may be associated with ‘harsh’ environmental conditions. We examined, in the rockpool amphipod Apherusa jurinei, behavioural activities that may function as a form of active brood care. Only ovigerous females showed ‘curl’ and ‘stretch’ activities, with consequent flushing of the brood pouch and cycling of the eggs therein. There was a significant decline in these activities as embryonic development advanced and brood care almost ceased when well-developed embryos showed a heart pulse and self-ventilation. We propose that this pattern of brood care reflects changes in the physiological requirements of embryos as they develop within the egg membrane. In addition, ovigerous females showed significantly higher levels of brood care under lowered oxygen conditions. They achieved this by increasing the average duration of the ‘stretch’ component, with other brood care components remaining constant. Thus, developmental and environmental cues alter the components of active brood care in distinct ways. Experimental removal showed that the physical presence of eggs in the brood pouch is important in controlling the expression of brood care activities. However, females with all of their eggs removed continued to brood at low levels, suggesting that a maternal state also controls brood care. The sophisticated expression of active maternal brood care in amphipods under ‘harsh’ environmental conditions such as rockpools has implications both for individual reproductive success and the distribution and abundance of brooding versus nonbrooding species. Copyright 2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour     

The application of habitat templets and traits to hyphomycete fungi in a mid-European river system

1. The general knowledge about hyphomycetes is summarized and traits of some of these organisms in the Rhône floodplain are described to assess compliance with the river habitat templet (RHT) and patch dynamics concept (PDC) hypotheses. This investigation corresponds with those for other taxonomic groups reported in a recent Freshwater Biology Special Issue on the Ecology of the Upper Rhône River (Statzner, Resh & Dolédec, 1994). 2. Three groups of fungi were considered: aquatic, terrestrial, and aero-aquatic hyphomycetes. 3. The main factors controlling distribution of these micro-organisms are food (mostly organic matter), dissolved oxygen and biotic interactions. 4. With contrasting strategies, hyphomycetes exploit both fallen leaves, a discrete ephemeral resource, and wood fragments, which are more durable. 5. Fungal propagules (spores, hyphae) are distributed in the water, and even the air, largely throughout the year. This explains the resilience of these organisms to disturbance. 6. A major problem is the spatial (boundary layer, dead zones) and temporal (leaf input) scale at which these micro-organisms live and their interactions at the scale adopted for the other organisms referred to in the Special Issue on the Ecology of the Upper Rhône. 7. Our scant knowledge of the microbial ecology in the Rhône floodplain, together with the scale problem and high microbial resilience to environmental stress, render the RHT and PDC hypotheses difficult to test. Only three traits could be tested on hyphomycetes, but all three confirmed the RHT hypotheses. 8. Hyphomycete communities appear to be biologically controlled in more favourable and temporally stable environments, and physico-chemically controlled in harsher and more perturbed environments.     

Maternal nutritional condition and genetic differentiation affect brood size and offspring body size in Nicrophorus

In most animal species, brood size and body size exhibit some variation within and between populations. This is also true for burying beetles (genus Nicrophorus), a group in which the body size of offspring depends critically on the number of offspring competing for food due to the discrete nature of resource used for larval nutrition (vertebrate carcasses). In one species, brood size and body size are correlated with population density, and appear to be phenotypically plastic. We investigated potential proximate causes of between-population variation in brood size and body size in two species, Nicrophorus vespilloides and Nicrophorus defodiens. Our first experiment supported the notion that brood size is phenotypically plastic, because it was affected by environmental variation in adult nutritional condition. We found that the pre-breeding nutritional status of female N. vespilloides affected the number of eggs they laid, the number of surviving larvae in their broods, and the body size of their offspring. We do not know whether this plasticity is adaptive because greater offspring body size confers an advantage in contests over breeding resources, or whether starved females are constrained to produce smaller clutches because they cannot fully compensate for their poor pre-breeding nutritional status by feeding from the carcass. Our second experiment documents that brood size, specifically the infanticidal brood-size adjustment behavior, has undergone genetic differentiation between two populations of N. defodiens. Even under identical breeding conditions with identical numbers of first-instar larvae, females descended from the two populations produced broods of different size with corresponding differences in offspring body size.     

Maternal correlates of brood sex ratio variation in the lekking lance-tailed manakin Chiroxiphia lanceolata

Theory predicts that overall population sex ratios should be around parity. But when individual females can receive higher fitness from offspring of one sex, they may benefit by biasing their brood sex ratios accordingly. In lekking species, higher variance in male reproductive success relative to that of females predicts that male offspring gain disproportionately from favorable rearing conditions. Females should therefore produce male-biased broods when they are in a position to raise higher quality offspring: i.e., in better body condition or when they reproduce earlier in the breeding season. To investigate these hypotheses, we studied brood sex ratios of lance-tailed manakins Chiroxiphia lanceolata . We found that overall sex ratios and mean brood sex ratios were not different from random expectation. Brood sex ratios were not related to laying date or female body condition. However, we detected a quadratic relationship between brood sex ratios and maternal age: both young (1–2 years) and old (8+ years) females produced female-biased brood sex ratios. This relationship was most clear in a year also distinguished by early rainy and breeding seasons. We suggest that breeding inexperience in young females and senescence in older females is the most plausible explanation for these results, and that the relationship between female age and brood sex ratio is mediated by environmental conditions.     

Ectoparasite load is linked to ontogeny and cell-mediated immunity in an avian host system with pronounced hatching asynchrony

Several contrasting hypotheses have been proposed to account for host age-biased parasite distribution, with some of them suggesting a key role of ectoparasites in the evolution and maintenance of weight hierarchies within broods. We examined parasite distribution among individual hosts across the whole period of host exposure to the parasite in a host system that shows distinct within-brood differences in age and age-related mortality. By contrast to previous hypotheses, we found that the abundance of a haematophagous, mobile ectoparasite Carnus haemapterus on nestling European rollers ( Coracias garrulus ) was highest approximately during the mid-nestling stage of their host, coinciding with the inflection point of the host growth phase. Parasite load increased neither with absolute resource availability (i.e. body size), nor body condition index. By contrast to previous evidence, higher parasite load under natural conditions was associated with a stronger cell-mediated immune response. However, this association was moderated by low parasite densities, as well as a better brood body condition index. Overall, although we revealed remarkable host ontogenetic effects on parasite distribution, the present study suggests that a highly mobile ectoparasite generally prefers healthier hosts. We propose that, in host systems with a marked asynchrony of hatching and background mortality within the brood, parasites favour persistence rather than nutritional attractiveness of the host.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 463–473.     

Congeneric phylogeography: hypothesizing species limits and evolutionary processes in Patagonian lizards of the Liolaemus boulengeri group (Squamata: Liolaemini)

In poorly known groups for which data are insufficient to develop biologically plausible model‐based approaches to phylogeographical analyses, a ‘first hypotheses’ protocol is suggested as offering the best way to generate hypotheses for subsequent model‐based tests. Preliminary hypotheses are formulated about species boundaries and population processes in three species complexes of the Liolaemus boulengeri group in the context of mtDNA ‘congeneric phylogeography’. The temperate South American Liolaemus provides a model with ancient and recent allopatric divergence across ecologically and geologically complex landscapes, incipient speciation, secondary contact, and discordance between molecular and morphological patterns of variation. Moderately dense sampling of widely distributed ‘inertial’ species in the Patagonian Steppe has revealed hidden genetic and probably species diversity, and also hinted at demographic and historical processes that may have shaped the histories of these taxa. Five of the seven focal species of the present study were paraphyletic for mtDNA genealogies, suggesting that they represent complexes of species, and nested clade phylogeographical analysis (NCPA) analyses suggest that different historical and demographic processes have shaped the observed patterns. Introgression and incomplete lineage sorting are hypothesized as being the cause of some of the observed paraphyly. Provisional delimitations of species are proposed and NCPA is used to generate hypotheses of population history, all of which are subject to further testing. Multi‐faceted studies, involving phylogenetic assessments of independent molecular markers and morphological variation across codistributed taxa with estimates of niche breadths in a landscape context, will likely yield the most promising returns for cross‐validation of hypotheses of population and speciation histories. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89 , 241–275.     

The diversity and evolution of recruitment behaviour in ants,with a discussion of the usefulness of parsimony criteria in the reconstruction of evolutionary histories

Summary Three alternative hypotheses about the evolution of recruitment behaviour in ants, based on accounts in the literature, are compared by means of a cladistic analysis. The three hypotheses are the following:Hypothesis 1. Increasingly efficient recruitment behaviours exhibited by different ant species have been shaped by or are correlated with ant phylogeny.Hypothesis 2. Increasingly efficient recruitment behaviours represent necessary evolutionary steps independently followed during the evolution of different ant clades.Hypothesis 3. Differently efficient recruitment behaviours have been selected in a convergent way among different species by similar population/environmental constraints.In a first stage of the analysis, these hypotheses have been compared in terms of parsimony (i.e. in terms of tree length = TL) of alternative cladograms based on recruitment behaviour only. The analysis gave the following results: Hypothesis 1, TL = 4; Hypothesis 2, TL = 18; Hypothesis 3, TL = 11. At least in terms of parsimony, hence, Hypothesis 1 appears to be the best. This hypothesis, however, cannot be retained for its total lack of congruence with current views on ant phylogeny. Among the remaining two hypotheses, Hypothesis 3 is again much (ca. 40%) more parsimonious than Hypothesis 2, but the retention index for recruitment behaviour on the relative cladogram is 0.2 as compared with 0.7 for Hypothesis 2. Practically, this implies biologically very implausible behavioural evolution indicated by very improbable ancestors for the species included in the analysis. In the case of recruitment evolution the biological credibility of each hypothesis is inversely proportional to its parsimony.The three hypotheses on the evolution of recruitment behaviour are compared again taking into account the morphological and behavioural correlates of recruitment. The results confirm those obtained by simple cladistic analysis of behaviour alone, namely that an obligatory (i. e. neither reversible nor random) increase in recruitment efficiency has been repeatedly selected within different ant clades. Inclusion of the recruitment correlates allows, in addition, a more precise formulation of the implications of each hypothesis and a tentative test of two other alternatives deduced from the literature. Most papers dealing with recruitment assume this behaviour to be controlled by a single gland, while at least two experimental analyses show that more than one gland is likely to be involved as behavioural releaser. A cladistic approach allowed testing of the following two adaptational hypotheses: A) Synergic behavioural control by several glands, allowing shift of the dominant role from one gland to another. B) Single gland control, making improbable the replacement of one gland by another that performs the same function. The results of the analysis appear to favour alternative A slightly, though neither alternative results in implausible evolutionary paths.It is stressed that parsimony remains the sole decisional criterion when no other criteria are available but it can by no way be preferred to the slightest trace of biological common sense.     

Long-term brood division and exclusive parental care in a cooperatively breeding passerine

In some species of birds, individual adults feed different subsets of the brood after the young have left the nest. However, few studies have shown that such ‘brood division’ represents long-term separation into subfamilies, rather than short-term biases in care, and the function of brood division is unknown. Most functional hypotheses assume that brood division is an adult adaptation to increase reproductive success; others suggest that fledglings are responsible for brood division, or that brood division is the outcome of conflicts of interest within families. We tested for long-term brood division in the white-browed scrubwren, Sericornis frontalis, a cooperatively breeding passerine, and assessed functional hypotheses, using behavioural, morphological and molecular evidence. We found that 71% of scrubwren broods became divided in the second week after fledging, and remained divided for the following 4-5 weeks of care. Furthermore, most individual young were fed almost exclusively by a single adult, even in undivided broods and broods of one. Brood division did not arise through adults caring for young of a specific sex, nor did genetic paternity explain division. Tests of other hypotheses were indirect, relying for example on testing mechanisms rather than predictions. We rejected the hypotheses that brood division was primarily an adaptation to reduce the mean or variance in predation, or that it was likely to increase the rate at which young were provisioned. Fledglings probably had an active role in maintaining brood division, with fledglings that had been biggest in the nest tending to be fed by adults that had fed nestlings at the greatest rate. This suggests that the most dominant fledgling may sequester the care of the best feeder. We also found evidence that brood division might be an adult adaptation to reduce the effects of sibling competition, or that it might arise through conflicts of interest among adults. We further suggest that social specialization, resulting from learning about a particular individual, might bring additional advantages to long-term brood division. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Mitochondrial mismatch analysis is insensitive to the mutational process

Mismatch distributions are histograms showing the pattern of nucleotide (or restriction) site differences between pairs of individuals in a sample. They can be used to test hypotheses about the history of population size and subdivision (if selective neutrality is assumed) or about selection (if a constant population size is assumed). Previous work has assumed that mutations never strike the same site twice, an assumption that is called the model of infinite sites. Fortunately, the results are surprisingly robust even when this assumption is violated. We show here that (1) confidence regions inferred using the infinite- sites model differ little from those inferred using a model of finite sites with uniform site-specific mutation rates, and (2) even when site- specific mutation rates follow a gamma distribution, confidence regions are little changed until the gamma shape parameter falls well below its plausible range, to roughly 0.01. In addition, we evaluate and reject the proposition that mismatch waves are produced by pooling data from several subdivisions of a structured population.      

A goodness-of-fit test for capture-recapture model Mt under closure

A new, fully efficient goodness-of-fit test for the time-specific closed-population capture-recapture model Mt is presented. This test is based on the residual distribution of the capture history data given the maximum likelihood parameter estimates under model Mt, is partitioned into informative components, and is based on chi-square statistics. Comparison of this test with Leslie's test (Leslie, 1958, Journal of Animal Ecology 27, 84-86) for model Mt, using Monte Carlo simulations, shows the new test generally outperforms Leslie's test. The new test is frequently computable when Leslie's test is not, has Type I error rates that are closer to nominal error rates than Leslie's test, and is sensitive to behavioral variation and heterogeneity in capture probabilities. Leslie's test is not sensitive to behavioral variation in capture probabilities but, when computable, has greater power to detect heterogeneity than the new test.     

Trade-off between mating opportunities and parental care: brood desertion by female Kentish plovers.

Why do some parents care for their young whereas others divorce from their mate and abandon their offspring? This decision is governed by the trade-off between the value of the current breeding event and future breeding prospects. In the precocial Kentish plover Charadrius alexandrinus females frequently, but not always, abandon their broods to be cared for by their mate, and seek new breeding partners within the same season. We have shown previously that females'' remating opportunities decline with date in the season, so brood desertion should be particularly favourable for early breeding females. However, the benefits are tempered by the fact that single-parent families have lower survival expectancies than those where the female remains to help the male care for the young. We therefore tested the prediction that increasing the value of the current brood (by brood-size manipulation) should increase the duration of female care early in the season, but that in late breeders, with reduced remating opportunities, desertion and thus the duration of female care should be independent of current brood size. These predictions were fulfilled, indicating that seasonally modulated trade-offs between current brood value and remating opportunities can be important in the desertion decisions of species with flexible patterns of parental care.