Nest Defense Research Articles

Honey robbing causes coordinated changes in foraging and nest defence in the honey bee, Apis mellifera

Individuals and social groups balance foraging activities with other energetically demanding behaviours. In honey bee colonies, foraging and nest defence behaviours are performed by similar-aged bees, and so hives must adjust their workforce investment to fulfil both tasks. We investigate this balance in the context of honey robbing, a tactic in which foragers invade a victim hive, kill worker bees and steal honey stores. Robbing is highly beneficial because stored honey is a plentiful, concentrated food resource. However, robbing requires a large workforce to overwhelm the defences of the victim hive; it is unknown how robbing hives adjust other behaviours to accommodate this demand. We developed a method to provoke a hive to engage in simulated robbing and measured rapid changes in foraging activity and nest defence. Surprisingly, robbing hives increased both behaviours. Guards, the individuals responsible for nest defence, specifically increased defensiveness towards their own nestmates as they returned from a robbing trip. We found that increased foraging activity and changes in forager odour profiles from prolonged exposure to victim hive honeycomb were insufficient to explain robbing-induced changes in guard defensiveness. However, brain gene expression profiles of robbing foragers suggest these bees are unusually aggressive, and thus more likely to provoke aggression from nestmate guards. Increased forager aggression occurred even in the absence of direct competition with victim bees. Thus, although increased guard defensiveness may be costly in terms of increased nestmate mortality, because the ecological conditions that promote robbing simultaneously increase the likelihood a hive will become a robbing target, guards may use cues from returning nestmates to determine invasion risk and adjust their defensiveness accordingly. These results suggest that colonies use social information to dynamically optimize both foraging and defensiveness in order to maximize the benefits and minimize the costs of this high-risk tactic.

State dependence explains individual variation in nest defence behaviour in a long-lived bird.

Parental care, such as nest or offspring defence, is crucial for offspring survival in many species. Yet, despite its obvious fitness benefits, the level of defence can consistently vary between individuals of the same species. One prominent adaptive explanation for consistent individual differences in behaviours involves state dependency: relatively stable differences in individual state should lead to the emergence of repeatable behavioural variation whereas changes in state should lead to a readjustment of behaviour. Therefore, empirical testing of adaptive state dependence requires longitudinal data where behaviour and state of individuals of the same population are repeatedly measured.Here, we test if variation in states predicts nest defence behaviour (a ‘risky’ behaviour) in a long‐lived species, the barnacle goose Branta leucopsis. Adaptive models have predicted that an individual's residual reproductive value or ‘asset’ is an important state variable underlying variation in risk‐taking behaviour. Hence, we investigate how nest defence varies as a function of time of the season and individual age, two state variables that can vary between and within individuals and determine asset.Repeated measures of nest defence towards a human intruder (flight initiation distance or FID) of females of known age were collected during 15 breeding seasons. Increasing values of FID represent increasing shyness.We found that females strongly and consistently differed in FID within‐ and between‐years. As predicted by theory, females adjusted their behaviour to state by decreasing their FID with season and age. Decomposing these population patterns into within‐ and between‐individual effects showed that the state‐dependent change in FID was driven by individual plasticity in FID and that bolder females were more plastic than shyer females.This study shows that nest defence behaviour differs consistently among individuals and is adjusted to individual state in a direction predicted by adaptive personality theory.

Variation in alarm calls during different breeding stages of the common kestrel (Falco tinnunculus).

ABSTRACTAcoustic signals play a key role in animal communication. Animals usually use alarm signals to warn mates or offspring of the presence of threats or to intimidate or distract predators. Birds commonly use acoustic signals as a means of communication. Alarm calls in passerines at different breeding stages can reflect their nest defense intensity. However, little is known about the characteristics, plasticity, and impact factors of alarm calls during the reproductive period in raptors. Here, from March to July in 2019, the alarm calls of eight pairs of common kestrels (Falco tinnunculus) during the breeding period were recorded using a portable recorder with a strongly directed microphone in the Zuojia Nature Reserve, Jilin province, China. The differences in acoustic parameters of parental alarm calls in different breeding stages were analyzed. The results showed that the alarm calls of common kestrels were composed of multi-harmonic arched frequency modulation with the maximum energy distribution in the second harmonic. The duration and rate of the alarm calls increased significantly as the breeding season progressed, showing that parents spent increasing amounts of time on nest defense. Additionally, the acoustic parameters of alarm calls in common kestrels were significantly different depending on offspring numbers, suggesting that offspring numbers influenced parental nest defense. These results showed that differences in alarm calls during different breeding stages may reflect a trade-off between defense costs and reproductive benefits.

Heavy decline of the largest European Arctic Skua Stercorarius parasiticus colony: interacting effects of food shortage and predation

ABSTRACT Capsule The number of breeding pairs of Europe’s largest Arctic Skua Stercorarius parasiticus colony at Slettnes, Norway, showed a dramatic decline of at least 50% over two decades, with food shortage in four years and increasing predation by Red Fox Vulpes vulpes leading to total breeding failure in five out of six recent study years. Aims To document the decline of Europe’s largest Arctic Skua colony and quantify bottom-up and top-down effects on reproduction. Methods We compared nest counts between 1997–1998 and 2014–2019 and collected data on egg size, clutch size and nest success for all years, and adult body mass, nest attendance, at-sea activity, aggressive nest defence, Red Fox Vulpes vulpes encounters, daily nest survival and adult survival for 2014–2019. We deployed nest cameras to identify predators in 2018–2019. In addition, we developed a demographic model to estimate the fecundity required for a stable population. Results A higher proportion of time spent at sea, small eggs, low adult female body mass and indirect assessment of foraging fish availability suggested food shortages in four of six recent study years. At the same time, nest predation by Red Foxes, the likely predator involved, increased during the six-year study. The combined effects of food shortage and nest predation led to total breeding failures in 2017–2019. Conclusion We provide evidence of both bottom-up (food shortage) and top-down (predation) effects on reproductive investment and hatching success in this colony. The reproductive output in recent years is far too low to sustain a stable population. The severe decline of the Arctic Skua colony at Slettnes fits reported trends for this species across most of its European breeding range, as well as for its important host species, the Arctic Tern Sterna paradisaea and the Black-legged Kittiwake Rissa tridactyla.

Pathways linking female personality with reproductive success are trait- and year-specific

AbstractPersonality (i.e., among-individual variation in average behavior) often covaries with fitness, but how such personality–fitness relationships come about is poorly understood. Here, we explore potential mechanisms by which two female personality traits (female–female aggression and female nest defense as manifested by hissing behavior) were linked with annual reproductive success in a population of great tits (Parus major), a socially monogamous species with biparental care. We hypothesized that personality-related differences in reproductive success result from variation in reproductive decision (lay date, brood size) and/or parental provisioning rates. Relative support for these mechanisms was evaluated using path analysis on data collected in two successive years. We reveal that larger broods were provisioned at a higher rate by both parents and that female, but not male, provisioning rate was involved in the trade-off between offspring number (brood size) and fledgling mass. Among-individual variation in female aggression, via its association with female provisioning rate, was negatively linked to fledgling mass (i.e., indirect effect), yet only in one of the study years. Male provisioning rate did not influence these relationships. In contrast, among-individual variation in hissing behavior was directly and negatively linked with fledgling mass in both years, via an underlying mechanism that remains to be identified (i.e., direct effect). Together, our findings emphasize that personality–fitness relationships may come about via different mechanisms across personality traits and/or years, thereby illustrating additional complexity in how selection might act on and maintain among-individual variation in behavioral phenotypes in the wild.

Defensive biting by Tetragonisca angustula is dangerous but not suicidal

Suicidal defense is an important and effective attribute of some highly social animals. It has been suggested that the defensive behavior of biting onto raiders is suicidal in stingless bees; however, the mechanisms causing death are unclear. We conducted the first test of the effect of agonistic encounters on death and injury rates of guards of a stingless bee, Tetragonisca angustula, that bit both conspecific and heterospecific invaders within plastic tubes. Tetragonisca angustula guards bit and immobilized invading bees for long durations and had the ability to release their hold, but rarely did so unless the other bee died. In our assays, which lasted hours to up to 1 day, the probability of death during nest defense by T. angustula was 5–7%. Bitten invading bees fought back, often causing injury and sometimes death of the biting T. angustula. Tetragonisca angustula foragers can survive for 7 days without food and so death by starvation of guards during day-long encounters seems unlikely. Death rates of biting guards holding non-nestmates on the ground may be higher in the field due to susceptibility to predation. Defensive biting of stingless bees effectively protects the nest, and most guards likely are able to return to their tasks after a raid is thwarted, though studies in a more naturalistic setting are needed.

Repetitive confrontations between Australian Ravens (Corvus coronoides) and lone Australasian Darters (Anhinga novaehollandiae)

Aggressive confrontations between different species of birds are commonly reported in scientific literature. In particular, the Australian Raven (Corvus coronoides) engages in confrontations with other birds, including competitions for food resources, kleptoparasitism, nest defense, and retaliation against mobbing birds. Herein, I report repeated observations of multiple ravens staging confrontations against lone Australasian Darters (Anhinga novaehollandiae) in an urban park in Sydney, Australia. During these confrontations, ravens enclosed around the darters and both species exhibited bill gaping and snap-threats. In some confrontations, ravens divebombed the darters from behind. These behaviors are found in scientific literature for each species, respectively; however, the motivation behind these confrontations was not clear.

Assessing and mitigating impacts of motorboat noise on nesting damselfish

Motorboats are a pervasive, growing source of anthropogenic noise in marine environments, with known impacts on fish physiology and behaviour. However, empirical evidence for the disruption of parental care remains scarce and stems predominantly from playback studies. Additionally, there is a paucity of experimental studies examining noise-mitigation strategies. We conducted two field experiments to investigate the effects of noise from real motorboats on the parental-care behaviours of a common coral-reef fish, the Ambon damselfish Pomacentrus amboinensis, which exhibits male-only egg care. When exposed to motorboat noise, we found that males exhibited vigilance behaviour 34% more often and spent 17% more time remaining vigilant, compared to an ambient-sound control. We then investigated nest defence in the presence of an introduced conspecific male intruder, incorporating a third noise treatment of altered motorboat-driving practice that was designed to mitigate noise exposure via speed and distance limitations. The males spent 22% less time interacting with the intruder and 154% more time sheltering during normal motorboat exposure compared to the ambient-sound control, with nest-defence levels in the mitigation treatment equivalent to those in ambient conditions. Our results reveal detrimental impacts of real motorboat noise on some aspects of parental care in fish, and successfully demonstrate the positive effects of an affordable, easily implemented mitigation strategy. We strongly advocate the integration of mitigation strategies into future experiments in this field, and the application of evidence-based policy in our increasingly noisy world.

Fight or flight: Geographic variation in antipredator defenses by cinereous tits

In the face of death from predation, parent birds have to choose between prioritizing current versus future reproduction. Previous studies have shown that when exposed to predation risk, birds breeding in the tropics prioritize their own safety, whereas birds breeding at high latitudes prioritize the safety of future generations. However, this phenomenon has rarely been investigated among populations within a single bird species. Here, we studied the anti-predation behavior of female birds in response to a human observer of four cinereous tit (Parus cinereus) populations during their incubation period across different latitudes in China. When confronted with nest predation risk induced by the presence of a human, the probability of escape by cinereous tits decreased with increasing latitude, that is, the proportion of female birds that escaped from the nest was significantly larger in southern than in northern populations. For female birds that did not leave the nest, the proportion of individuals who produced a hissing call for nest defense was significantly lower in the southern than in the northern populations. In addition, nest defense behaviours were unrelated to nest predation rates, and clutch size in the southern cinereous tit populations was significantly smaller than that in the northern population, it was, therefore, likely that nest defense behaviours were related to clutch size. The present study confirms within a single species that under predation risk, female birds in the tropics prioritize future over current reproduction, while the birds breeding at high latitudes do the opposite.

Warblers perform less nest defense behavior and alarm calls to human intruders: A result of habituation

Human disturbance is caused by the anthropogenic activities in the environment. While human threat to wildlife is considered to be complex (nonlethal recreations and lethal hunting activities). To date, there has been few studies that focus on nest defense behavior of animals to human disturbance. In this study, we tested whether the behavioral response of Oriental reed warblers Acrocephalus orientalis against a human intruder are similar to other nest intruders. We used dummies of common cuckoo Cuculus canorus (the parasite), Eurasian sparrowhawk Accipiter nisus (the predator), Oriental turtle dove Streptopelia orientalis (the neutral control), also a human intruder to induce the nest defense behaviors and alarm calls of Oriental reed warblers. Our results showed that Oriental reed warblers performed less intense nest defense behavior and uttered the least number of calls and shortest duration of calls to a human intruder than those to other nest intruders, suggesting that human pose a low threat level to warblers. Warblers produced different number and duration of alarm calls against the cuckoos, sparrowhawks and doves, even though they performed similar behavioral responses to the three types of avian intruder, indicating that they could distinguish the three types of avian intruders. The present study provided experimental evidence for the different behavioral responses between human disturbance and other types of intruders and suggested that the less intense behavioral response to human disturbance may be the result of habituation.

New finding of melanic three-spined sticklebacks Gasterosteus aculeatus in the Scottish Hebrides

Colour traits can be elaborated through sexual selection and have potential to drive reproductive isolation. Male three-spined sticklebacks (Gasterosteus aculeatus) express striking visual signals to attract choosy females during courtship, typically expressed as red carotenoid-based pigmentation on their throat and jaw during the breeding season, along with blue eyes and blue/green flanks. The extent and intensity of red colouration in males have been linked to fitness benefits to females, including body condition, parasite resistance, parental ability and nest defence. In some populations in the Pacific Northwest of North America, male three-spined sticklebacks express melanic nuptial colouration. In these populations, male possess black throats instead of red, and have dark or black bodies. Melanic males are associated with waterbodies that are red-shifted due to the presence of tannins, where the ambient light environment is dominated by long wavelengths. Here we report the first discovery outside North America of melanic populations of three-spined sticklebacks on the island of North Uist in the Scottish Hebrides, on the northwest Atlantic coast of Europe. These populations are associated with a hotspot of stickleback morphological diversity and occur in association with red-shifted waterbodies.

Nest defence and offspring provisioning in a cooperative bird: individual subordinates vary in total contribution, but no division of tasks among breeders and subordinates

Abstract: Research aimed at understanding the evolution of costly, seemingly altruistic helping behaviour in cooperative birds has strongly focused on a single aspect of helping—offspring provisioning. However, various other activities are also important for successful breeding, most notably nest defence, and emphasis on a single aspect of helping might hamper a full understanding of the evolutionary drivers of cooperation and seemingly altruistic helping behaviour. Nonetheless, the extent to which social classes (i.e. dominant breeders and subordinate helpers), as well as different individuals within social classes, divide tasks in cooperative groups remains largely unknown. Here we tested whether individual purple-crowned fairy-wrens, Malurus coronatus, show task division in the two forms of costly offspring care important for reproductive success: nestling provisioning and nest defence (to predator models presented at nests). Subordinates and breeders generally contributed to both tasks. Breeders defended marginally more than subordinates, and provisioned at a consistently high level, irrespective of their individual contribution to defence, whereas subordinates’ investment in both types of care was positively correlated. Thus, we found no evidence for division of tasks between or within social classes, highlighting it may be absent in facultatively cooperative birds even when activities are costly and important for reproduction. This suggests that in such species, cooperative improvement of overall investment may be more effective than enhanced individual efficiency at different tasks. Significance statement: In many social species, cooperation between individuals to achieve communal goals is important for successful reproduction. When multiple individuals are working together, division of tasks can occur, with tasks unevenly distributed among group members. Research on cooperative breeding, where subordinate helpers seemingly altruistically assist others in raising their young, has however mainly focused on offspring provisioning, while subordinates can often contribute to several breeding tasks. Thus, if task divisioning occurs, this would call into question conclusions about adaptive benefits of helping based on provisioning effort alone. Here we use nestling feeding watches and predator model presentations at nests to quantify nestling provisioning and nest defence effort by individuals in a classical cooperative breeder, the purple-crowned fairy-wren. We show a lack of task divisioning by breeders and subordinates, which is reassuring for our current understanding of the evolution of cooperative breeding.

Solitary breeding barn swallows pay a higher nest defense cost

Successful reproduction depends on the ability of parents to defend from nest predators. Breeding birds often put their life at risk to defend their offspring from predators. Communal nest defense has been proposed as an individual benefit of group living. We tested this by experimentally exposing a short-lived avian species to a potential risk of predation. We presented decoys of both a diurnal and a nocturnal predator to barn swallows Hirundo rustica breeding solitarily and in large colonies in South-western Spain. We found clear benefits of group living compared to solitary pairs in relation to investment in individual nest defense and the capacity to deter a challenging predator. Although we did not find differences in the time needed to detect the predator, we found that the number of individuals recruited to participate in nest defense was greater for colonial breeding pairs, leading to fewer attacks per individual in comparison to solitary breeding pairs. We also found that barn swallows defended their nests more vigorously against a nocturnal predator than against a diurnal predator. Therefore, barn swallows living in groups obtained clear benefits in communal nest defense by reducing the risk, energy and time dedicated to nest defense. Colonial breeding is thus a more effective strategy for decreasing nest predation rates.

Morphological caste differences in primitively eusocial insects: the Van der Vecht organ ofMischocyttaruspaper wasps

AbstractNests of independent-founding primitively eusocial paper wasps are founded by one or a few females. Before worker emergence, foundresses must forage, sometimes leaving the nest unattended. Chemical defence - the application of ant repellent secretions of the Van der Vecht organ (VdVO) on the nest surface - allows brood protection when active defence is weak or not possible. After worker emergence, it is more likely that some wasps stay in the nest, so active nest defence may be more common. Given this constraint on the nest defence, selection for increasing the cuticular secretory area of the VdVO, to improve chemical defence, may be stronger in foundresses (queens) than in workers - morphological castes are hypothesized to evolve. We investigated the occurrence of morphological castes in the Neotropical paper wasp Mischocyttarus. Confirming our prediction, we found that in some (but not all) species, foundresses have a distinct increase in the secretory area of the VdVO in relation to workers; even though there is a strong overlap between caste phenotypes (incipient morphological castes). Implications of these findings for the evolution of morphological castes in primitively eusocial wasps are discussed.

Effect of Foraging and Nest Defense Tradeoffs on the Reproductive Success of Wood Storks (Mycteria americana)

In many species of birds, parental care is provided by both parents to maximize offspring survival, and there may be important trade-offs between maximizing food gathering and nest protection during the nesting period. The role of parental care in determining reproductive success was investigated in Wood Storks (Mycteria americana), and specifically how the trade-off between frequency and duration of foraging trips and nest protection contributed to the nesting outcome. Parental behavior of 85 pairs of Wood Storks was monitored throughout the nesting season in two breeding colonies in Palm Beach County, Florida, USA. Wood Storks gradually increased the frequency, but not the duration, of foraging trips as chicks developed. The ratio of hatchlings to fledglings was positively associated with the frequency of foraging trips during late chick development. Intra-specific aggressions resulting in nest takeovers affected 32% of the nests. Occurrence of nest takeovers were higher for later-breeding pairs, and happened primarily in the first few weeks of incubation, but was not affected by the degree of joint nest attendance of both parents. These results establish a functional link between parental effort and reproductive outcome in Wood Storks, and highlight the importance of frequent foraging trips, but not nest attendance, by parents.

Competition interactions among stingless bees (Apidae: Meliponini) for Croton yucatanensis Lundell resins

Stingless bees use extrafloral resources such as resins for nest construction, defense systems and chemical communication. We conducted the first study of the resin-collecting process from Croton yucatanensis Lundell shrubs by three stingless bee species in the tropical deciduous forest of the Guanacaste Conservation Area, Costa Rica. We performed systematic observations on 10 C. yucatanensis shrubs for 10-min periods between 8:20 a.m. and 3:00 p.m. for three days, and for 90 min we focused on behavior and interaction among bees on three shrubs. Trigona corvina Cockerell was the most frequent visitor. These bees made cuts on shrub stems and leaves to collect resin and transport it in their corbiculae. Others visitor species were Trigona fulviventris Guerin and Plebeia frontalis Friese, which tried to approach the resin remains left by T. corvina. While monopolizing the resource, T. corvina behaved aggressively if approached by T. fulviventris. Towards the last hour of sampling, T. fulviventris visits increased notably, which may indicate a decrease in the dominance or aggressiveness of T. corvina at that time. This tendency seems to be the beginning of temporal resource partitioning behavior commonly observed in bee communities. We provide novel observations on the ways in which stingless bees collect resin and the interactions that occur among them. We also propose and discuss working hypotheses to explain the fact that all the individuals observed almost exclusively visited a single C. yucatanensis specimen.

Caught on camera: circumstantial evidence for fatal mobbing of an avian brood parasite by a host

Hosts have evolved a multiplicity of defensive responses against avian brood parasites. One of them is mobbing behaviour which often includes direct contact attacks. These aggressive strikes may not only distract the parasites but may also be fatal to them, as documented by cases of dead brood parasite females found near host nests. Here, we present the first video-recording of a great reed warbler (Acrocephalus arundinaceus) host whose vigorous nest defence appears to directly lead to the death of a female common cuckoo (Cuculus canorus). We suggest that the chance of parasite death probably rises with the presence of unfavourable factors, such as water below the nest. Our observation supports previous suggestions that hosts may pose a lethal danger to their parasites.

Predator defense is shaped by risk, brood value and social group benefits in a cooperative breeder

AbstractPredation is a major cause of mortality and nest failure in birds. Cooperative predator defense can enhance nest success and adult survival but, because it is inherently risky, dynamic risk assessment theory predicts that individuals modify defense behavior according to the risk posed by the predator. Parental investment theory, on the other hand, predicts that reproductive payoffs (brood value) determine investment in nest defense. We propose that, in cooperative breeders, fitness benefits deriving from the survival of other group members may additionally influence defense behavior (social group benefits theory). We tested predictions of these theories in the cooperatively breeding purple-crowned fairy-wren, Malurus coronatus, where brood value is higher for breeders, but social group benefits more important for helpers. We recorded experimentally induced individual defense behaviors in response to predator models presented near nests, representing differing levels of threat to nests and adults. As predicted, 1) individuals engaged in less risky defenses when encountering a more dangerous predator (dynamic risk assessment theory); 2) individuals defended older broods more often, and breeders defended more than helpers (parental investment theory); and 3) helpers were more likely to respond to a predator of adults (social group benefits theory). Our findings highlight that predator defense in cooperative breeders is complex, shaped by the combination of immediate risk and multiple benefits.

Conditional female strategies influence hatching success in a communally nesting iguana.

The decision of females to nest communally has important consequences for reproductive success. While often associated with reduced energetic expenditure, conspecific aggregations also expose females and offspring to conspecific aggression, exploitation, and infanticide. Intrasexual competition pressures are expected to favor the evolution of conditional strategies, which could be based on simple decision rules (i.e., availability of nesting sites and synchronicity with conspecifics) or on a focal individual's condition or status (i.e., body size). Oviparous reptiles that reproduce seasonally and provide limited to no postnatal care provide ideal systems for disentangling social factors that influence different female reproductive tactics from those present in offspring‐rearing environments. In this study, we investigated whether nesting strategies in a West Indian rock iguana, Cyclura nubila caymanensis, vary conditionally with reproductive timing or body size, and evaluated consequences for nesting success. Nesting surveys were conducted on Little Cayman, Cayman Islands, British West Indies for four consecutive years. Use of high‐density nesting sites was increasingly favored up to seasonal nesting activity peaks, after which nesting was generally restricted to low‐density nesting areas. Although larger females were not more likely than smaller females to nest in high‐density areas, larger females nested earlier and gained access to priority oviposition sites. Smaller females constructed nests later in the season, apparently foregoing investment in extended nest defense. Late‐season nests were also constructed at shallower depths and exhibited shorter incubation periods. While nest depth and incubation length had significant effects on reproductive outcomes, so did local nest densities. Higher densities were associated with significant declines in hatching success, with up to 20% of egg‐filled nests experiencing later intrusion by a conspecific. Despite these risks, nests in high‐density areas were significantly more successful than elsewhere due to the benefits of greater chamber depths and longer incubation times. These results imply that communal nest sites convey honest signals of habitat quality, but that gaining and defending priority oviposition sites requires competitive ability.

The effect of offspring value on variation of nest defense intensity in gray catbirds (Dumetella carolinensis)

Nest defense is an adaptive strategy used by adults to increase survival of independent young. However, the risk adults face by defending young from predators represents an important trade-off and parental investment theory predicts adults will modulate nest defense based on the value of offspring. We used a model of a black rat snake, a potential nest predator on our study site, to quantify nest defense in a population of gray catbirds in eastern Pennsylvania during the breeding seasons of 2017 and 2018. Specifically, we evaluated (1) the difference in nest defense between male and female parents, (2) the relationship between nest defense and nest success, and (3) how variation in nest defense was related to factors associated with offspring value. Although male catbirds tend to be larger than females, they did not defend their nests more aggressively than female catbirds. Response to the model predator we used was also not different between individuals whose nests were successful compared with individuals that failed. Although nests that contained more and older young were not defended more aggressively, our analyses did show that adult catbirds defended young that were laid earlier in the season more intensely than those laid later in the breeding season. In birds, earlier clutch initiation is associated with an increased chance of recruitment into the breeding population. Catbirds, therefore, follow predictions of parental investment theory because they defended offspring of higher value more aggressively.Significance statementDefense of young from predators is an important component of parental care. Nest defense represents a trade-off because although it should increase reproductive success by increasing offspring survival, it presents a risk to adults, thus lowering their survival. We studied variation in nest defense of gray catbirds in relation to offspring value. Although nest defense was not related to nest success, we did find that adult catbirds defended nests that were produced earlier in the year more aggressively than later nests. In birds, the timing of reproduction is linked to prospects of juvenile survival in such a way that offspring produced early in the breeding season are more likely to survive and recruit into the breeding population. As predicted by parental investment theory, catbirds defended young that represented a higher increase in fitness more aggressively.