两种雀形目鸟类的窝雏数处理实验:检验Lack假说

于1997—1999年在位于青海省北部的中国科学院海北高寒草甸生态系统定位站进行。选择地面筑巢的小云雀(Alauda gulgula)和灌丛筑巢的黄嘴朱顶雀(Acanthis flavirostris)为代表进行窝雏数处理实验。根据Lack假说的预报检验(1)常见窝卵数是否是最大生产力窝卵数;(2)窝雏数处理对雏鸟质量和亲鸟投入是否产生影响;(3)两种鸟的响应方式是否相同。其结果如下：①小云雀和黄嘴朱顶雀的常见窝卵数分别是3和5枚。年间变化不明显,用幼鸟出飞率作为生产力,两种鸟的扩增窝幼鸟出飞率下降,常见窝卵数(分布频率最高)等同于最大生产力窝卵数;②小云雀的幼鸟的生长参数不随窝雏数的改变而变化．而黄嘴朱顶雀有明显变化．说明窝雏数处理对后者幼鸟质量有明显影响。③用递食率作为亲鸟投资指标,小云雀亲鸟的递食率随窝雏数的增加而增加,但雏期不变;而黄嘴朱顶雀递食率不变,但雏期延长。④扩增窝雏数后,两种亲鸟表现出不同的响应方式,小云雀表现为提高单位时间递食次数,而黄嘴朱顶雀延长育幼时间。这两种方式不是通过影响雏鸟质量就是通过影响亲鸟存活率来降低子代和亲代的适合度。结果支持了自然选择将窝卵数调节到亲鸟能喂活最大数量子代的限度。即常见窝卵数就是最大生产力窝卵数的Lack假说。     

陕西红碱淖遗鸥育雏行为和雏鸟生长

2012年5～7月,应用e-Science信息技术和标记法,对陕西神木县红碱淖(N 38°13′～ 39°27′,E 109°42′～110°54′)遗鸥(Larus relictus)的育雏行为和雏鸟生长发育进行了研究.结果表明,雏鸟由双亲共同承担喂食.育雏前期,亲鸟采取直接喂食、食物呕吐于巢边和在巢中间断性喂食这3种喂食模式;亲鸟昼间平均喂食(0.706±0.036)次/h,夜间平均喂食(0.469±0.024)次/h,双亲在喂食频次上无显著差异(F=32.54,P＞0.05).育雏后期,主要采取双亲直接喂食和亲鸟把食物呕吐于地面上,由雏鸟自己取食的喂食模式;亲鸟昼间平均喂食(0.416±0.021)次/h,夜间平均喂食(0.331±0.018)次/h,亲鸟喂食次数与雏鸟的日龄存在相关性(r =0.074,P＜0.05).随着雏鸟日龄的增长,暖雏次数趋于减少,而在炎热晴天、降雨和大风等天气状况下,暖雏时间和护雏行为都增强.雏鸟20日龄后未再观察到暖雏行为.雏鸟体长及外部器官的形态学参数适合用Gompertz曲线方程拟合.同时,与其近缘种黑嘴鸥(L.saundersi)的育雏行为和雏鸟生长进行了比较.     

杂色山雀亲代喂食与子代乞食间的行为

在育雏期,晚成鸟的子代一般都是由双亲共同来抚育,子代为了更好地存活,会用自己的方式竞争获得更多的食物和更好的生存空间,同时亲代也会根据子代的乞食信号来分配食物。2011年3～7月采用针孔摄像技术录制了杂色山雀(Parus varius)育雏期巢内亲代与子代间的行为,统计了亲鸟站位、雏鸟站位、雏鸟乞食强度及亲鸟的喂食情况等数据。分析结果表明:(1)雌雄亲鸟在巢中的站位各有特点,雄鸟在整个育雏期都喜欢站在距离巢口较近的位置;雌鸟站位不太固定,前期离巢口相对较远,中期和后期离巢口相对较近;(2)雏鸟离亲鸟越近,乞食强度越大,获得食物的机会就越多;离亲鸟越远的雏鸟越不爱乞食,所以站位对雏鸟的食物获得影响最大;(3)雌鸟承担主要的育雏任务,喂食频率远大于雄鸟;(4)育雏期的不同阶段雏鸟乞食强度、亲鸟喂食频率变化很大:中期雏鸟乞食强度最大,亲鸟喂食频率最高,后期雏鸟乞食强度最弱;(5)整个育雏期雌性亲本没有表现出明显的偏爱行为,但雄性亲本在中、后期更偏爱体型大的雏鸟。可见杂色山雀子代的行为和体型大小影响着亲代的食物分配,亲代也会根据雏鸟日龄调整站位和喂食行为。     

杂色山雀(Parus varius)亲代喂食与子代乞食间的行为交流

在育雏期,晚成鸟的子代一般都是由双亲共同来抚育,子代为了更好地存活,会用自己的方式竞争获得更多的食物和更好的生存空间,同时亲代也会通过子代的乞食信号来分配食物。2011年3月-7月采用针孔摄像技术录制了杂色山雀(Parus varius)育雏期巢内亲代与子代间的行为交流,统计了亲鸟站位、雏鸟站位、雏鸟乞食强度及亲鸟的喂食情况等数据。分析结果表明：(1)雌雄亲鸟在巢中的站位各有特点：雄鸟在整个育雏期都喜欢站在距离巢口较近的位置;雌鸟站位不太固定,前期离巢口相对较远,中期和后期离巢口相对较近;(2)雏鸟离亲鸟越近,乞食强度越大,获得食物的机会就越多;(3)杂色山雀主要是雌鸟担任育雏任务,喂食频率远大于雄鸟;(4)育雏期的不同阶段雏鸟乞食强度、亲鸟喂食频率变化很大：中期雏鸟乞食强度最大,亲鸟喂食频率最高,后期雏鸟乞食强度最弱;(5)整个育雏期雌性亲本没表现出明显的偏爱行为,但雄性亲本在中期表现出了偏爱大的雏鸟。     

荒漠伯劳的递食位置与同胞竞争

亲鸟递食位置可以调控雏鸟之间的竞争,而异步孵化会使不同孵出顺序雏鸟的体况产生等级差异。雏鸟可以通过竞争巢中有利位置来获得更多的食物资源,从而提高自身生长发育速度和存活率。本研究于2013和2014年两年的4至8月对甘肃安西荒漠保护区桥子片区22巢荒漠伯劳（Lanius isabellinus）的递食行为进行研究。结果发现,荒漠伯劳亲鸟递食的位置主要集中在1个（n = 13,比例超过50%）或2个（n = 9,合并比例超过60%）位置上,具有一定的可预测性;处于亲鸟递食位置直线方向上的雏鸟获食次数显著高于两侧位置（n = 22,P < 0.001）。但异步孵化雏鸟的孵出顺序与其在巢内占据直线（df = 4,F = 0.211,P > 0.05）和两侧位置（df = 4,F = 0.068,P > 0.05）的次数都不具相关性;并且不同孵出顺序雏鸟间平均获食次数无明显差异（df = 4,F = 0.090,P > 0.05）;拟合不同孵出顺序的雏鸟生长曲线发现,不同孵出顺序雏鸟的体重生长率也没有表现出明显差异（df = 4,F = 0.637,P > 0.05）。研究结果表明,荒漠伯劳亲鸟趋于固定递食位置并向直线方向递食;但雏鸟间具有较小的同胞竞争,出飞等级差异不显著,这可能与繁殖地短暂的食物丰富度和雏鸟本身对于乞食与获食之间的利弊权衡有关。     

大鵟繁殖期的行为及时间分配

2015年4～9月,采用焦点动物取样法,通过人工观察及监控设备记录,在青海省祁连县研究了2窝在人工巢中繁殖的大(鵟)(Buteo hemilasius)行为.构建了大(鵟)亲鸟及雏鸟在繁殖期的行为谱,将亲鸟繁殖期内的行为划分为12项30种,将雏鸟的行为划分为9项25种.研究发现,大(鵟)繁殖期开始于4月中下旬,持续至8月中旬结束,平均(112.0±2.0)d(n=2);将繁殖期划分为孵卵前期、孵卵期、育雏期及雏鸟成熟期.利用单因素方差分析(One-way ANOVA)比对了雌雄亲鸟之间,以及不同时期间亲鸟、雏鸟的行为时间分配.结果显示,(1)雌雄大(鵟)之间的行为时间分配在孵卵前期及孵卵期差异不显著(P＞0.05),在育雏期及雏鸟成熟期差异显著(P＜0.05).在这两个时期,雌性栖停行为所占比例显著高于雄性(P＜0.01),而捕食行为占比显著低于雄性(P＜ 0.01).(2)雌性大(鵟)行为时间分配在不同时期均变化显著(P＜ 0.05),雄性大(鵟)行为时间分配在育雏期与雏鸟成熟期间差异不显著,其余各个时期间差异显著(P＜ 0.05).(3)大(鵟)雏鸟行为时间分配在育雏期与成熟期之间差异显著(P＜0.05).     

大■繁殖期的行为及时间分配

2015年4~9月,采用焦点动物取样法,通过人工观察及监控设备记录,在青海省祁连县研究了2窝在人工巢中繁殖的大■(Buteo hemilasius)行为。构建了大■亲鸟及雏鸟在繁殖期的行为谱,将亲鸟繁殖期内的行为划分为12项30种,将雏鸟的行为划分为9项25种。研究发现,大■繁殖期开始于4月中下旬,持续至8月中旬结束,平均(112.0±2.0)d(n=2);将繁殖期划分为孵卵前期、孵卵期、育雏期及雏鸟成熟期。利用单因素方差分析(One-way ANOVA)比对了雌雄亲鸟之间,以及不同时期间亲鸟、雏鸟的行为时间分配。结果显示,(1)雌雄大■之间的行为时间分配在孵卵前期及孵卵期差异不显著(P0.05),在育雏期及雏鸟成熟期差异显著(P0.05)。在这两个时期,雌性栖停行为所占比例显著高于雄性(P0.01),而捕食行为占比显著低于雄性(P0.01)。(2)雌性大■行为时间分配在不同时期均变化显著(P0.05),雄性大■行为时间分配在育雏期与雏鸟成熟期间差异不显著,其余各个时期间差异显著(P0.05)。(3)大■雏鸟行为时间分配在育雏期与成熟期之间差异显著(P0.05)。     

黄腹山鹪莺育雏行为和雏鸟生长

2007年3~9月,在广东省肇庆市江溪村对黄腹山鹪莺(Prinia flaviventris)的育雏行为和雏鸟生长进行了研究。通过取食行为观察、育雏食物分析和雏鸟身体量度的测量来研究黄腹山鹪莺亲鸟繁殖投资和雏鸟的生长规律。研究期间,利用隐蔽帐观察窗进行行为观察,观察距离在5 m以内;在雏鸟身体上用无味彩笔标号以区别雏鸟个体:10日龄前,标记于雏鸟背部,10~12日龄,标记在雏鸟跗跖处;对部分数据进行双变量相关分析,利用Logistic曲线拟合雏鸟形态增长,并比较每个回归方程斜率间的差异。结果显示:1)黄腹山鹪莺育雏由雌雄共同承担,育雏期(11.9±0.4)d(n=7巢)。幼雏出壳后亲鸟早晚暖雏,第7天起亲鸟白天不再暖雏;2)随雏鸟的生长,喂食次数和食物种类逐渐增加,雏鸟日龄与喂食次数极显著相关(r=0.995,P0.01);3)育雏期雏鸟食物皆为动物性食物,以蜘蛛目物种所占比例最大(40.95%),其他包括幼虫及直翅目、鳞翅目、鞘翅目、蜻蜓目等节肢动物;4)Logistic曲线方程中,体重的生长率常数k值最大,与其他k值之间存在显著性差异(P0.05);5)黄腹山鹪莺体重、体长、尾长、翼长、嘴峰、嘴裂、第三根初级飞羽(简称为P3)、跗跖及爪各参数间的相关系数均为0.9以上(P0.01),参数之间在一定程度上可相互代替;6)黄腹山鹪莺雏鸟的发育遵循最重要的功能优先发育的原则,符合能量分配假说。黄腹山鹪莺喂食次数、雏鸟生长速率(k值)相对较高,可能与当地丰富的食物资源有关,也可能是对巢址环境多变的适应。     

一窝三趾啄木鸟雏鸟的死亡事件与双亲育幼的关系

三趾啄木鸟的夜间孵卵和抱暖通常由雄性亲鸟承担。我们使用两台数码相机同时对1个三趾啄木鸟巢口和巢室进行了监视,记录了该窝雏鸟的死亡过程。该窝1只雏鸟在孵出的第1天死亡,并在第2天被雌性亲鸟移出巢外。其余两只雏鸟连续两个晚上没有亲鸟的照料,于第9天死亡。在这个巢内,由于雄鸟未归,使得雌鸟不得不连续两个晚上照顾雏鸟;而突发的暴风雨可能是导致雄鸟回巢失败的原因[动物学报52 (2) : 410 -414 , 2006]。     

食物、捕食和种间竞争对东方田鼠种群动态的作用

采用2×2×2析因实验设计,在野外围栏条件下,测定食物、捕食和竞争物种黑线姬鼠(Apodemus agrarius)对东方田鼠(Microtus fortis)种群动态作用的格局.食物可利用性、捕食及种间竞争的独立作用对种群最小存活数均具有极显著的效应,除捕食与种间竞争的交互作用接近显著水平外,食物与种间竞争、食物与捕食者以及三者间的交互作用均不显著;三类外部因子对种群补充量的独立作用效应均达到极显著水平,且对种群补充量的作用具有累加效应;食物可利用性、捕食及种间竞争对种群繁殖成体的比例具有极显著的作用;三类外部因子对种群幼体与成体的比例具有极显著的作用.对种群年龄结构而言,与捕食者及种间竞争比较,食物可利用性是相对较弱的影响因子,在任何捕食与种间竞争交互作用条件下,食物的作用均不显著;三类外部因子均能显著地影响东方田鼠的体重增长率,但三者的交互作用对其影响不显著;MANOVA结果表明,捕食对成体存活率的作用最强烈,其次,为食物可利用性,种间竞争的作用最弱,但三者的交互作用效应不显著.对幼体的存活时间,除捕食的作用接近显著水平外,食物可利用性及种间竞争的作用均不显著.结果提供了食物可利用性、捕食和种间竞争对东方田鼠种群动态作用的充分证据,验证了食物、捕食和种间竞争对田鼠类种群动态具有独立或累加效应的总假设.     

Sibling competition in a brood-tending leech

Conflict among siblings over parental investment, particularly over parental feeding, is a feature of family life in many kinds of animals. In some bird species, the size of prey items provided to juveniles has been implicated as a cause of aggressive competition among sibling chicks, because prey size determines whether dominance allows monopolization of parental offerings. Our experiment was meant to test the generality of this factor in creating intrafamilial conflict. We investigated sibling competition in relation to prey size using the carnivorous, brood-tending leech Helobdella papillornata. We equalized the total amount of food available to H. papillornata broods, but varied the size of individual prey items. Competition, measured by disparity in body size at independence, was more intense in broods provisioned with small items than in broods receiving large items, but similar between broods receiving large items and broods fed ad libitum. These patterns suggest that the intensity of conflict did not depend only on the total food amount, but was enhanced by small prey size. Our results indicate that conflict over the provision of parental resources to offspring can have a similar basis across very dissimilar organisms.     

Parental versus offspring control on food division within the brood: the role of hatching asynchrony

Using an individual-based simulation model we study how different mechanisms of food division among multiple offspring influence nestling number and quality, as well as parental effort. We consider the combination of different scenarios of food availability (feeding conditions), hatching asynchrony and food division. If parents have full control on how to divide food among offspring, asynchronous broods have higher breeding performance than synchronous ones in a wide range of feeding conditions, giving theoretical support to empirically proved benefits of hatching asynchrony. If parents accept the outcome of sibling competition there is a threshold in feeding conditions below which asynchronous broods produced more fledglings and the reverse was true above the threshold. Interestingly, parents relying on the outcome of nestling competition do not necessarily differ in breeding performance from those which have full control over food allocation. Our study combines hatching asynchrony, provisioning behaviour of parents, jostling behaviour of nestlings and feeding conditions as a network of interacting processes of enormous interest to fully understand the parent–offspring conflict.     

Flexibility in the Foraging Behavior of Blue Tits in Response to Short‐Term Manipulations of Brood Size

Previous work suggests that short‐term changes in feeding rate are usually produced by the parent‐offspring interaction. However, few studies have properly tested this assumption. In this study, we attempt to explore the short‐term consequences of daily (within‐pair) brood size manipulations (reduced, original, and enlarged) on feeding behavior (provisioning rates, prey size, and prey type) of Mediterranean blue tits Cyanistes caeruleus. Total provisioning rates were lowest when broods were reduced in size and greatest when broods were enlarged. Mean prey size was also affected by the brood size changes: parents tended to bring larger prey when confronted with low brood demand reinforcing the view that a trade‐off exists between minimizing foraging time and maximizing food quantity. Such differences in feeding frequencies and the load sizes delivered may be explained by changes in the parents’ foraging tactic. Increase of brood size compelled parents to work harder and be less selective in prey choice; we found that stressed birds with a high level of feeding responsibility (hungry nestlings) opted to concentrate on more readily available food items (Tortricids). On the other hand, their immediate reaction when faced with a low level of feeding responsibility was to decrease this prey type in the diet, so that the percentage of other preys (Noctuids) in the diet increased. There was no intersexual difference in the way in which parents responded to the manipulation. In sum, our results revealed a flexibility in foraging strategies of blue tits to cope with changing scenarios, which supports the idea that provisioning behavior is largely governed by nestling demand.     

Effects of short-term hunger and competitive asymmetry on facultative aggression in nestling black guillemots Cepphus grylle

Siblings in a diversity of species are facultatively aggressive,yet the proximate control of the aggressive response and theecological conditions selecting for such systems are poorlyunderstood. In this study, we investigated the effects of foodamount (food amount hypothesis) and competitive asymmetry onsibling aggression in black guillemot broods. Parental provisioningrates were experimentally manipulated in broods comprisinga range of hatching intervals over a 12-h period. Aggressionbecame evident only after parental provisioning rates wereexperimentally reduced. When parental provisioning resumed,adults did not increase their feeding rate to compensate forthe induced food deficit, and the result of sibling rivalry was a change in the allocation of parental deliveries from oneof equality to one in favor of the dominant chick. Food-deprivedchicks from synchronous broods were more aggressive than thosefrom asynchronous broods, suggesting that one benefit of hatchingasynchrony in the black guillemot is to establish an efficientcompetitive hierarchy among siblings which minimizes the needfor costly aggressive interactions. On the following day, siblingaggression ceased, and chicks regained an equal share of parentalfeeds. Our results provide the first evidence that short-termfood shortage per se acts as an initial trigger for aggressionand also show that the aggressive response is complicated byfactors associated with hatching and laying order.     

Costs of growing up as a subordinate sibling are passed to the next generation in blue‐footed boobies

As stresses in early development may generate costs in adult life, sibling competition and conflict in infancy are expected to diminish the reproductive value of surviving low‐status members of broods and litters. We analysed delayed costs to blue‐footed booby fledglings, Sula nebouxii, of junior status in the brood, which involves aggressive subordination, food deprivation and elevated corticosterone, but little or no deficit in size at fledging. In ten cohorts observed for up to 16 years, juniors showed no deficit in breeding success at any age, independent of lifespan, including in a sample of sibling pairs. Among females, juniors actually outreproduced seniors across the 16‐year span. However, offspring produced by juniors in the first 3 years of life were less likely to recruit into the breeding population than offspring of seniors. Since junior fledglings survive, recruit and compete as well as seniors (shown earlier), and breed as successfully as seniors across the lifespan, it appears the delayed cost of subordination is passed to offspring, and only to those few offspring produced in the first 3 years of life. These correlational results indicate that systematic competition‐related differences in developmental conditions of infant siblings can alter their reproductive value by affecting the viability of their eventual offspring.     

Interaction between parental care and sibling competition: parents enhance offspring growth and exacerbate sibling competition

Species with elaborate parental care often also show intense sibling competition over resources provided by parents, suggesting joint evolution of these two traits. Despite this, the evolution of elaborate parental care and the evolution of intense sibling competition are often studied separately. Here, we examine the interaction between parental food provisioning and sibling competition for resources through the joint manipulation of the presence or absence of parents and brood size in a species with facultative parental care: the burying beetle Nicrophorus vespilloides. The effect of the interaction between the presence or absence of parents and brood size was strong; brood size had a strong effect on growth when parents provided care, but no effect when parents were absent. As in previous studies, offspring grew faster when parents were present than when parents were absent, and offspring grew faster in smaller broods than in larger broods. Our behavioral observations showed that brood size had a negative effect on both the amount of time parents spent providing resources to individual offspring and the offspring's effectiveness of begging, confirming that the level of sibling competition increased with brood size. Furthermore, offspring in larger broods shifted more from begging toward self-feeding as they grew older compared to offspring in small broods. Our study provides novel insights into the joint evolution of parental care and sibling competition, and the evolution of offspring begging signals. We discuss the implications of our results in light of recent theoretical work on the evolution of parental care, sibling competition, and offspring begging signals.     

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.     

Great tits, Parus major, lay too many eggs: experimental evidence in mid-boreal habitats

This study shows that great tits lay too large clutches in mid‐boreal habitats. First, breeding success, measured with number of fledglings or proportion of eggs that produce fledglings, in northern Finland (65°N) is much poorer than in central and western Europe. Second, brood size manipulations (ca ±30% of the natural mean) revealed that reduced broods produced equal numbers of and larger‐sized fledglings than control and enlarged broods, giving thus the best fitness value for reduced broods. Third, parents of enlarged broods could not adjust (i.e. increase) their feeding effort to the greater number of nestlings. Fourth, extra feeding (about 1/3 of the theoretical maximal needs of the nestlings) during the nestling period resulted in more numerous and larger‐sized fledglings in comparison to control broods. We suggest that the ultimate explanation for the too large clutches is gene flow from the southern population, which prevents local adaptations in the north. Consequently, the main reason for food limitation during the nestling period is that northern great tits apply “southern” decision rules for timing of breeding, clutch size and foraging behaviour. Thus, they tend to breed too early in comparison to the food abundance peak, lay too large clutches in comparison to the level of resources and, perhaps, forage on a too narrow diet (75% caterpillars). Since the late broods that matched the local food abundance peak did not succeed better than the mismatched earlier ones, the most crucial fault of northern great tits seems to be that they overestimate food abundance during peak demands and lay too large clutches. Another explanation for this could be that northern great tits have adopted a brood reduction strategy. However, the long‐term data reveal that years of high breeding success, which would maintain large clutches in the population, are very rare in the north. Therefore, it is unlikely that a brood reduction strategy per se could explain the phenomenon. Instead, it could work together with the gene flow against local adaptation for clutch adjustment.     

Hatching asynchrony in the house wren, Troglodytes aedon: a test of the brood-reduction hypothesis

We tested the brood-reduction hypothesis by adding three nestlingsto naturally occurring synchronous and asynchronous broods ofthe house wren (Troglodytes aedon) in order to mimic food shortagesfor the broods. Two types of controls were established in whichbrood size remained unchanged: those in which nestlings wereexchanged among broods and those in which no nestlings wereexchanged. The critical test of the hypothesis was in 1988 whenthere was a food shortage for enlarged broods. Although broodreduction occurred, enlarged synchronous broods produced asmany fledglings as did enlarged asynchronous broods, and fledgingmass was similar. Juvenile recapture 2-8 weeks after fledgingand offspring recruitment to subsequent breeding populationswere not related to treatment. The results are not consistentwith the brood-reduction hypothesis as an explanation for theoccurrence of hatching asynchrony in the house wren.     

Feeding decisions of eastern bluebirds are situationally influenced by fledgling plumage color

The relative amount of resources that avian parents provide to individual offspring within a brood represents a strategy that can have large effects on reproductive success. We tested whether parental feeding decisions of eastern bluebirds Sialia sialis are influenced by offspring plumage color by presenting pairs of differently colored fledglings side by side and observing how they were provisioned by parents. After a control period, we manipulated blue plumage color so that one sibling in each trial became relatively dark and one became relatively bright. During neither the control nor the experimental periods did either parent consistently feed naturally brighter or experimentally brightened sons more than drab sons. Under specific circumstances, however, both parents directed a higher proportion of their feeding attempts to more brightly colored sons. Paternal feeding attempts to brighter offspring during both the control and experimental periods increased in relation to the brightness of these fledglings relative to their brothers. Maternal feeding decision, on the other hand, were influenced by numerous variables during control and experimental periods including the date of the trial, the difference in mass between fledglings, the feeding behavior of fathers during the trial, the relative investment by fathers during the nestling stage, and the amount of UV chroma in fledgling plumage. Taken together, these results suggest that equal provisioning of offspring is the strategy most commonly adopted by eastern bluebirds but more brightly colored offspring will be fed preferentially when resources for offspring are limited.