模拟增加潜在巢捕食风险对2种鸟类雏鸟生长发育产生不同影响

捕食风险是影响鸟类生活史对策的重要因素之一。为应对捕食风险,鸟类进化出多样的反捕食策略。为探究北热带石灰岩森林地区鸟类生长发育对高巢捕食风险的适应机制,本研究通过利用蛇类模型模拟巢捕食者,增加潜在巢捕食风险,设置实验组,将未放置蛇类模型的实验设置为对照组。测量育雏期内黄腹山鹪莺（Prinia flaviventris）雏鸟与红耳鹎（Pycnonotus jocosus）雏鸟每日的体重、跗跖长与翼长,分别比较分析黄腹山鹪莺雏鸟与红耳鹎雏鸟上述体型特征在实验组与对照组中的生长发育规律。对符合正态分布的各日龄雏鸟生长参数进行t检验,不符合正态分布的参数进行非参数检验（Wilcox秩和检验）并求均值,使用SPSS 26.0统计软件包对雏鸟各日龄参数均值进行Logistic曲线拟合,比较各雏鸟相同生长参数在实验组和对照组实验的“S”发育曲线。结果显示,黄腹山鹪莺雏鸟在实验组的体重、跗跖长与翼长的渐近线分别占成鸟量度的71.57%、94.10%与55.29%,对照组分别占成鸟量度的78.05%、97.49%与55.67%,在实验组和对照组育雏期分别为11.1 d与10.6 d。实验组和对照组黄腹山...     

纯色鹪莺繁殖行为观察

纯色鹪莺(Prinia inornata)具有繁殖期身体量度小于非繁殖期的特性,开展相关研究对了解鸟类新的生存和繁殖策略有重要意义。从2005年12月至2006年6月在广东省肇庆地区对纯色鹪莺的繁殖行为开展了研究。结果如下:①繁殖期体长和尾羽长均极显著(P<0·01)小于非繁殖期,翼长显著(P<0·05)小于非繁殖期;②每年3月下旬开始出现筑巢现象,雌雄共同筑巢,筑巢时间4～5d,巢多建在禾本科植物上,巢址选择与水源关系密切,主成分分析显示,距水边距离占主导地位(34·88%),之后依次是距地面高度(18·55%)、距最近水面高度(12·95%)、巢口方向(9·08%)和巢区植被盖度(8·51%);③筑巢后1～2d产卵,窝卵数(4·7±1·6),孵卵期持续8～10d,亲鸟轮流坐巢,亲鸟会根据窝卵数的不同调整坐巢次数和坐巢时间。窝卵数越多,亲鸟坐巢时间越长;④纯色鹪莺的巢成功率为43·75%,繁殖失败的主要原因是外界干扰。据此,我们认为纯色鹪莺在繁殖期的高投资和繁殖行为,可能是研究该物种生存对策的重要线索。     

黄腹山鹪莺成鸟的秋季换羽

黄腹山鹪莺(Prinia flaviventris)具有冬羽尾羽长于繁殖羽尾羽的特点,可能意味着一种新的生存和繁殖策略。为此,从2006年9月～2007年2月,在广东省肇庆市江溪村对黄腹山鹪莺的秋季换羽进行研究。结果显示:(1)黄腹山鹪莺成鸟繁殖羽体长和尾羽长皆极显著短于冬羽(P<0.01),繁殖羽翼长显著短于冬羽(P<0.05),其余身体量度的差异均不显著(P>0.05)。(2)9月17日获得第一个黄腹山鹪莺换羽个体,初级飞羽已更换到P5,次级飞羽已更换到S6,11月20日后所获样本均已完成羽毛的更换。(3)初级飞羽的换羽模式为递降换羽,次级飞羽为递升换羽,尾羽为离心型换羽。(4)换羽期间,10月的个体平均体重最大,显著(P<0.01)重于11月的体重,其他各月无显著性差异(P>0.05)。据此,推测黄腹山鹪莺秋季种群换羽的持续时间约100d;相对其他羽毛而言,尾羽更换对黄腹山鹪莺生长发育的影响更为明显。     

两种山鹪莺的春季换羽及尾羽变化

换羽是鸟类为保证持续生存的重要过程。换羽策略与鸟类进化及对环境的适应紧密相关,研究鸟类换羽特征,对于了解鸟类的分类、系统发育、进化历史及其对环境的适应性等方面都有重要意义。2007年3月至9月,在广东肇庆市江溪村对黄腹山鹪莺(Prinia flaviventris)和纯色山鹪莺(P.inornata)的春季换羽进行了研究。通过设置雾网捕捉2种山鹪莺,对捕捉到的成体进行体重及身体量度的测量;对飞羽及尾羽进行标记:初级飞羽以翅尖的第一枚羽毛标记为"P1",次级飞羽以翅中部最外一枚标记为"S1",向内依次递增标记;尾羽以中央两根最长尾羽为"T1",分别向两侧递增标记为"T2~T5"。采用单因素方差分析(One way ANOVA)对不同月份山鹪莺的体重值进行差异性检验,对体重与月份进行Pearson相关分析,对尾羽的长度和宽度进行Pearson偏相关分析(控制变量:体长)。研究结果表明:1)两种山鹪莺换羽期为3至5月,持续时间约为60 d;2)两种山鹪莺春季换羽仅更换尾羽,换羽模式均为离心型,即中央一对尾羽最先开始替换,然后向两侧由内到外逐次更替;3)两种山鹪莺的尾羽长度和宽度同步变化,但绝大部分山鹪莺非繁殖期尾羽长度与繁殖期尾羽长度之比大于非繁殖期尾羽宽度与繁殖期尾羽宽度之比,即繁殖期尾羽相对较宽;4)两种山鹪莺换羽期间体重大致呈现下降趋势,但变化不显著(P0.05)。推测两种山鹪莺通过增加食物的摄入来抵抗换羽期和繁殖期重叠而导致的能量消耗,这可能与该地区丰富的食物资源有关,并在一定程度上体现了两种山鹪莺换羽策略对环境的适应性。     

黄腹山鹪莺的营巢特征

营巢对鸟类的生长繁殖有着重要影响。为此,从2007年3-9月在广东省肇庆江溪村对研究地中的黄腹山鹪莺(Prinia flaviventris)的巢进行标记和测量,并以巢址为中心做5m×5m样方调查,通过主成分分析研究其巢址选择,结果表明:1)黄腹山鹪莺营巢时间始于3月中旬,4月达到高峰,至7月底结束;2)在13种植物上发现44个巢,其中在象草(Pennisetum purpureum)上最多,有27个,其次为加拿大飞蓬(Erigeron canadensis),4个,其他植物皆为1—2个;3)巢皆为不规则的球状巢,巢材除动植物性材料外,均有人工制品;巢内空间与巢整体大小较为一致;4)影响巢址选择的主要因素4种,依次为:距最近水源距离(29.89%)、距最近道路距离(16.45%)、距最近灌木距离(12.92%)、距水面高度(11.69%)。据此认为黄腹山鹪莺的营巢是对草本植物环境的适应,而其尾羽的逆向变化对营巢是有利的,起到增加飞行灵活性和减少筑巢投资的效果。     

高寒草甸几种雀形目鸟类的卵反射率分析

于2010~2013年的繁殖季节,在中国科学院海北高寒草甸生态系统定位站收集了9种青藏高原常见雀形目鸟类的卵,分别进行了反射率测量。测量光谱范围为可见光(400~750 nm)和短波近红外(750~1050 nm)。卵在两个波段光谱的反射率分别是0.066和0.123,二者差异极显著。在可见光范围内,除赭红尾鸲Phoenicurus ochruros的波峰在绿色光范围内,其他物种都在黄色光范围内;在短波近红外范围内,反射率明显分为3个组,以角百灵和小云雀的地面开放巢较高,黄嘴朱顶雀和黄腹柳莺的灌丛巢较低,其他种居中;开放巢及封闭巢的卵反射率在可见光范围无显著差异,但在短波近红外范围有显著差异。我们认为,在青藏高原高寒草甸生态系统中,植被郁闭度低,地面营巢环境不利于巢与卵的遮阳,造成营开放度高的地面巢的鸟卵近红外反射率相对较高,而灌丛巢和封闭巢的雀形目鸟类鸟卵相对较低,但对可见光谱的卵反射率各物种差异不显著。     

小杜鹃对强脚树莺的巢寄生及其卯色模拟

于1999-2009年的鸟类繁殖季(4-8月)对贵州宽阔水自然保护区内的强脚树莺(Cettia fortipes)的繁殖进行监测,并采用光纤光谱仪,通过_芏成分分析、反射光谱、罗宾逊投射等方法对强脚树莺的卵色与小杜鹃(Cuculuspoliocephalus)对其寄生的卵色模拟程度进行分析.研究结果表明,强脚树莺的繁殖成效在不同年份间均尤显著差异,但其巢被捕食率和巢被寄生率都较高,分别为49.26%和9.18%.通过反射光谱分析,表明小杜鹃卵在色调和色度上高度模拟强脚树莺的卵,但其亮度高于宿主卵,而且在人眼无法探测的紫外光部分存在差异.     

小杜鹃对强脚树莺的巢寄生及其卵色模拟

于1999-2009年的鸟类繁殖季(4-8月)对贵州宽阔水自然保护区内的强脚树莺(Cettia fortipes)的繁殖进行监测,并采用光纤光谱仪,通过主成分分析、反射光谱、罗宾逊投射等方法对强脚树莺的卵色与小杜鹃(Cuculus poliocephalus)对其寄生的卵色模拟程度进行分析。研究结果表明,强脚树莺的繁殖成效在不同年份间均无显著差异,但其巢被捕食率和巢被寄生率都较高,分别为49.26%和9.18%。通过反射光谱分析,表明小杜鹃卵在色调和色度上高度模拟强脚树莺的卵,但其亮度高于宿主卵,而且在人眼无法探测的紫外光部分存在差异。     

纯色山鹪莺的领域鸣叫

鸟类鸣叫是鸟类行为研究的一项重要内容,为探究纯色山鹪莺(Prinia inornata)的鸣叫模式与其尾羽逆向变化的关系,于2007年8-10月在广东省肇庆江溪村对繁殖期纯色山鹪莺的领域鸣声等行为进行研究。运用焦点动物观察法,通过Olympus DS-20数码录音笔(100—17100Hz)和直径50cm声音收集器采集声音。行为统计以直接观察和SonyDCR-VX2000E数码摄像机录像相结合。结果如下:⑴共采集到620个鸣句,分属6种鸣句类型,其中4中为常见类型,1种为过渡类型,1种少见类型。(2)纯色山鹪莺的鸣声结构简单,变化较多,能够根据环境改变鸣声,具有识别危险程度和危险对象种类的能力。(3)鸣声均和一定的行为具有联系,4种主要鸣声类型均伴有抖尾行为出现。据此认为,纯色山鹪莺鸣唱结构简单但变化较多,而尾羽在其领域鸣叫行为中发挥了一定作用。     

蓝尾石龙子杭州和宁德种群繁殖生活史特征的差异

测定处于不同纬度的浙江杭州和福建宁德的蓝尾石龙子(Eumeceselegans)种群的个体大小和繁殖特征。宁德种群的产卵时间为5月27日—6月22日,早于高纬度杭州种群(6月4日—7月12日)。宁德种群最小繁殖雌体及性成熟个体大小均显著小于杭州种群。宁德和杭州两种群的相对窝卵重无显著差异;当统计去除母体体长的影响之后,两地种群的窝卵数和窝卵重也无显著差异,但杭州种群的卵重量显著大于宁德种群。蓝尾石龙子窝卵数和卵重量呈负相关,窝卵数和卵大小的权衡存在种群间差异。特定窝卵数条件下,杭州种群的卵重量显著大于宁德种群。由此可见,蓝尾石龙子种群间的繁殖生活史特征存在显著差异,而且与母体大小的差异密切相关。推测不同纬度地区的蓝尾石龙子种群的繁殖策略存在差异。     

Maturation increases early reproductive investment in Adélie Penguins Pygoscelis adeliae

Correlations between female investment in egg production and age, breeding experience and laying date have been reported in several seabird species. In general, clutch and egg sizes increase with female age and breeding experience but decrease with laying date. Positive correlations of clutch and egg size with age and breeding experience can be caused by an increase in reproductive investment with maturation or they may be an artefact of lower survival rates for individuals with poor-quality phenotypes. Negative correlations of clutch and egg size with laying date might signal an adaptive reduction in egg production or be due in part to variation among individuals. We examined the interactions of female age, breeding experience, laying date and clutch and egg size in Adélie Penguins Pygoscelis adeliae . Breeding experience strongly affected clutch size with 87.3% of all one-egg clutches laid by first-time breeders. In addition, increasing age had a positive influence on egg size and was associated with earlier laying dates. However, there was little evidence to suggest that either clutch or egg sizes are influenced by laying date. Laying dates and clutch and egg sizes did not affect a female's probability of returning to breed in the following year, indicating that increased investment is a product of maturation and not of the loss of poor-quality breeders from the population. Our results suggest that as female Adélie Penguins gain foraging and breeding experience they are able to initiate breeding earlier, to lay complete clutches of two eggs and to lay larger eggs.     

Habitat-specific clutch size and cost of incubation in eiders reconsidered

The energetic incubation constraint hypothesis (EICH) for clutch size states that birds breeding in poor habitat may free up resources for future reproduction by laying a smaller clutch. The eider (Somateria mollissima) is considered a candidate for supporting this hypothesis. Clutch size is smaller in exposed nests, presumably because of faster heat loss and higher incubation cost, and, hence, smaller optimal clutch size. However, an alternative explanation is partial predation: the first egg(s) are left unattended and vulnerable to predation, which may disproportionately affect exposed nests, so clutch size may be underestimated. We experimentally investigated whether predation on first-laid eggs in eiders depends on nest cover. We then re-evaluated how nesting habitat affects clutch size and incubation costs based on long-term data, accounting for confounding effects between habitat and individual quality. We also experimentally assessed adult survival costs of nesting in sheltered nests. The risk of egg predation in experimental nests decreased with cover. Confounding between individual and habitat quality is unlikely, as clutch size was also smaller in open nests within individuals, and early and late breeders had similar nest cover characteristics. A trade-off between clutch and female safety may explain nest cover variation, as the risk of female capture by us, mimicking predation on adults, increased with nest cover. Nest habitat had no effect on female hatching weight or weight loss, while lower temperature during incubation had an unanticipated positive relationship with hatching weight. There were no indications of elevated costs of incubating larger clutches, while clutch size and colony size were positively correlated, a pattern not predicted by the ‘energetic incubation constraint’ hypothesis. Differential partial clutch predation thus offers the more parsimonious explanation for clutch size variation among habitats in eiders, highlighting the need for caution when analysing fecundity and associated life-history parameters when habitat-specific rates of clutch predation occur.     

Life-history and ecological correlates of geographic variation in egg and clutch mass among passerine species

Broad geographic patterns in egg and clutch mass are poorly described, and potential causes of variation remain largely unexamined. We describe interspecific variation in avian egg and clutch mass within and among diverse geographic regions and explore hypotheses related to allometry, clutch size, nest predation, adult mortality, and parental care as correlates and possible explanations of variation. We studied 74 species of Passeriformes at four latitudes on three continents: the north temperate United States, tropical Venezuela, subtropical Argentina, and south temperate South Africa. Egg and clutch mass increased with adult body mass in all locations, but differed among locations for the same body mass, demonstrating that egg and clutch mass have evolved to some extent independent of body mass among regions. A major portion of egg mass variation was explained by an inverse relationship with clutch size within and among regions, as predicted by life-history theory. However, clutch size did not explain all geographic differences in egg mass; eggs were smallest in South Africa despite small clutch sizes. These small eggs might be explained by high nest predation rates in South Africa; life-history theory predicts reduced reproductive effort under high risk of offspring mortality. This prediction was supported for clutch mass, which was inversely related to nest predation but not for egg mass. Nevertheless, clutch mass variation was not fully explained by nest predation, possibly reflecting interacting effects of adult mortality. Tests of the possible effects of nest predation on egg mass were compromised by limited power and by counterposing direct and indirect effects. Finally, components of parental investment, defined as effort per offspring, might be expected to positively coevolve. Indeed, egg mass, but not clutch mass, was greater in species that shared incubation by males and females compared with species in which only females incubate eggs. However, egg and clutch mass were not related to effort of parental care as measured by incubation attentiveness. Ecological and life-history correlates of egg and clutch mass variation found here follow from theory, but possible evolutionary causes deserve further study.     

Interactive effects of time and vegetation on reproduction of redshanks (Tringa totanus) breeding in Wadden Sea salt marshes

As shown for various species, nesting waders are non-randomly distributed on wetlands and preferentially select riparian nest-sites adjacent to limnic or marine waterbodies. Studying the redshank Tringa totanus, we tested the hypotheses that, in a coastal wader species which conceals its clutch in vegetation, predation and hatching success are affected by vegetation zonation, and that breeding in lower salt marsh areas has negative consequences for reproduction. We further predicted effects of timing of breeding and breeding experience/age of adults potentially reflected by egg biometrics both on nest-site selection and reproduction. Effects of vegetation, space, time and individual quality on hatching success of redshanks were studied in the German part of the Wadden Sea. Dominant plant species, vertical vegetation structure and nest concealment varied significantly between nests. Variation in nest concealment was relatively low: about 90% of clutches were classified as being well concealed. This variation was explainable by vegetation structure but not by vegetation composition at the nest-site, distance to shoreline, and time of clutch initiation. Vertical vegetation structure varied by dominant plant species but not by distance to shoreline and time of clutch initiation. Hatching success of clutches was low (10.6%) due to high predation (daily predation rate: 7.4%). Hatching success and duration of clutch survival were negatively and predation positively related to the date of clutch initiation. Furthermore, negative relationships were found between egg size and predation and duration of survival, respectively. We assume that concealed nests, early breeding and breeding experience diminish predation in salt marsh breeding redshanks. Thus, redshank reproduction appears to be affected by interactive effects of timing of breeding and vegetation facilitating early breeding. In contrast to open-nesting species, breeding in riparian habitats next to waterbodies may be disadvantageous for species breeding concealed in vegetation if these are covered by less structured vegetation.     

Reproductive tactics under severe egg predation: an eider’s dilemma

Parental defence against predators may increase offspring survival but entail other costs. Egg predation is frequent early in the laying sequence of the common eider, which differs in this and in several other ways from most other waterfowl. We test the hypothesis that permanent presence at the nest from the second or third egg is an adaptation for reducing egg predation in eiders. Two other alternative hypotheses for lower predation at later nest stages are early predation loss of the most vulnerable nests and seasonal decrease in predation risk. Analyses of predation rates at the one-egg and later stages refute these two alternatives. Early nest attendance by eider females is estimated to increase clutch survival by about 20% in four-egg and 35% in five-egg clutches, albeit probably at a cost of smaller clutch size.     

Clutch size variation in passerine birds: The nest predation hypothesis

Summary The hypothesis that a negative relationship exists between clutch size and the probability that the nest will be robbed is tested, using data for passerine birds given in the literature. The data for four separate groups of species, viz. hole-nesters, semi hole-nesters and open-nesters nesting above and on the ground, respectively, were examined in relation to geographical gradients and seasonal and annual variation. In general, the data analysis results support the hypothesis, but cannot yet be considered as proven. More data on the riks of nest predation are needed. The most serious discrepancy is that for the Fieldfare, the protection from predation provided by nesting in colonies does not seem to be accompanied by a corresponding increase in clutch size. However, the clutch size of the Brambling, a species with seeks out such colonies for its breeding sites, does tend to increase in these hatitats.The pattern of clutch size variation was similar for the two groups of hole-nesting species, but differed significantly from that found for the two groups of open-nesters. The difference in the clutch size variation of the two groups of open-nesting species predicted from the hypothesis, viz. that, in northern regions, both latitudinal and altitudinal increases in clutch size should more commonly be found for those species which nest on the ground, compared to those nesting above ground level, was confirmed.A seasonal decrease in clutch size, in temperate regions, was found to be typical for species whose nests are subject to relatively little predation, particularly for those species which have a short breeding season. Advantages of laying small clutches, resulting in fewer nestlings than the number which would be possible for the parents to rear successfully, are discussed, and a simple model is presented which does not assume that nest predation is dependent on clutch size.     

Evolution of avian clutch size along latitudinal gradients: do seasonality,nest predation or breeding season length matter?

Birds display a latitudinal gradient in clutch size with smaller clutches in the tropics and larger in the temperate region. Three factors have been proposed to affect this pattern: seasonality of resources (SR), nest predation and length of the breeding season (LBS). Here, we test the importance of these factors by modelling clutch size evolution within bird populations under different environmental settings. We use an individual‐based ecogenetic simulation model that combines principles from population ecology and life history theory. Results suggest that increasing SR from the tropics to the poles by itself or in combination with a decreasing predation rate and LBS can generate the latitudinal gradient in clutch size. Annual fecundity increases and annual adult survival rate decreases from the tropics to the poles. We further show that the annual number of breeding attempts that (together with clutch size) determines total annual egg production is an important trait to understand latitudinal patterns in these life history characteristics. Field experiments that manipulate environmental factors have to record effects not only on clutch size, but also on annual number of breeding attempts. We use our model to predict the outcome of such experiments under different environmental settings.     

北热带石灰岩地区红耳鹎的繁殖生态和巢址选择

热带鸟类的生活史进化策略与温带鸟类的不同,而迄今国内对热带鸟类的研究却非常缺乏,红耳鹎(Pycnonotus jocosus)在我国北热带地区分布广泛,是较为理想的研究对象。2010年至2014年春夏季,对北热带石灰岩地区红耳鹎的繁殖生态和巢址选择进行了研究。采用系统搜寻法并根据亲鸟行为寻巢,应用方差分析和主成分分析对相关数据进行处理。结果显示,红耳鹎的产卵期集中在4月中旬至5月下旬,喜筑巢于灌木和人工种植的苹婆(Sterculia nobilis)树。平均窝卵数为(3.4±0.5)枚(3~4枚),卵重(2.59±0.29)g,卵大小(21.10±1.73)mm×(15.35±1.50)mm(n=31)。总的繁殖成功率为36.16%,繁殖失败的主要原因是天敌捕食、弃巢和人为干扰。一年繁殖一次和较低的繁殖成功率是研究地的红耳鹎有较大窝卵数的主要原因。红耳鹎在巢址选择时主要考虑避雨因子、避敌因子和灌木因子。     

Ecological predictors of reduced avian reproductive investment in the southern hemisphere

Despite intensive research, the factors driving spatial patterns in life‐history traits remain poorly understood. One of the most frequently documented, and paradoxically, least understood patterns, is the latitudinal gradient of increasing avian clutch size at higher latitudes. These gradients are less marked in the southern hemisphere, thus clutch sizes tend to be smaller at southern latitudes than at equivalent northern ones. We exploited a natural experiment provided by the introduction of European passerines to New Zealand (NZ) to test three widely proposed ecological drivers of this pattern, i.e. the nest predation, Ashmole’s seasonality, and the breeding density hypotheses. We focus on the blackbird Turdus merula and the song thrush T. philomelos as founder effects do not have a major influence on the reproductive traits of their introduced populations. Both species laid smaller clutches in NZ than in Europe. These reductions had stabilised within one hundred years and were not associated with a compensatory increase in investment in individual offspring by laying larger eggs. In contrast to the nest predation hypothesis, daily nest predation rates were lower in NZ than in Europe. Smaller southern hemisphere clutches were associated with higher conspecific population densities and a relaxation of seasonal clutch size trends. These findings thus provide some support for both Ashmole’s seasonality and the breeding density hypotheses. Analyses across 11 European passerines introduced to NZ suggest, however, that neither of these hypotheses provide general explanations of smaller clutches in the southern hemisphere. We suggest that reduced seasonality and lower nest predation promote increased breeding densities and adult survival in the southern hemisphere. The later may drive smaller southern clutch sizes by generating spatial variation in the outcome of the trade‐off between reproductive investment and longevity.     

Costs of large communal clutches for male and female Greater Rheas Rhea americana

The breeding system of the Greater Rhea Rhea americana is almost unique among birds as it combines harem polygyny and sequential polyandry, with communal egg-laying and uniparental male care. In this species, large communal clutches (more than 30 eggs) are rare and have a lower hatching success than smaller clutches. Here we analyse the proximate causes of hatching failures and the costs of large communal clutches (and therefore the costs of extensive polygyny) for males and females. We evaluated if length of the nesting period, egg viability, egg losses during incubation and male parental activity at the nest were affected by clutch size. We also evaluated if chicks hatched from large clutches have a lower survival during the first 2 months after hatching. Large clutches had longer nesting period and lower hatching success, mainly as a result of bacterial contamination of the eggs and increased hatching asynchrony. In addition, large clutches tended to lose more eggs as a result of accidental breakage or predation. Male activity at the nest and chick survival were not related to clutch size. Low hatching success, nest predation risk and energetic costs associated with large clutches penalize females that join large harems and males that accept additional eggs into the nest.