Nest Protection Hypothesis Research Articles

Is there a role for aromatic plants in blue tit (Cyanistes caeruleus) nests? Results from a correlational and an experimental study

The utility of fresh green material in avian nests is still not fully understood. Potential explanations include the effects of plants’ volatile compounds on parasite reduction (nest protection hypothesis) or direct beneficial effects on nestling condition (drug hypothesis). We used correlative data collected during 2020 and 2021 in a Mediterranean population of blue tits (Cyanistes caeruleus) as well as experimental data (aromatic nest content manipulation) to assess the effects of aromatic plant use on nestling physiological condition and survival, nest-dwelling ectoparasitic pressure and its relationship with breeding parameters. We found that aromatic plants were disproportionally used in relation to their abundance in the environment and that their use was positively related to egg mass (but only in 2020). Blowflies and facultative parasitic mites were more frequent in nests with aromatics compared to nests without aromatics, but obligatory parasitic mites were less abundant in nests with aromatics. However, no effects of aromatic plants were observed on nestling haemoglobin levels nor erythrocyte sedimentation rate or other physiological health metrics, but the heterophil to lymphocyte ratio was higher in nests with the highest quantity of aromatics. The artificial addition of mint reduced the flea abundance, but 7-day old nestlings showed significantly lower colour saturation and brightness in the mouth flange. Nestling survival to fledging was not related to aromatic plant use. Therefore, our results partially support a beneficial effect of aromatic plants in blue tit nests because some ectoparasite groups were reduced. Immediate effects on nestling physiology or survival could not be established.Significance statementSome avian species place fresh aromatic plant material in their nests, and several non-mutually exclusive hypotheses have been proposed to explain its potential functions. In this study, we use both correlational and experimental data from a blue tit population to test two hypotheses, namely the ‘nest protection hypothesis’ and the ‘drug hypothesis’. The first one proposes that aromatic plants have direct repellent effects against ectoparasites, while the latter poses that these plant components benefit nestling condition through the stimulation of some components of the immune system. Our results suggest that some ectoparasites, such as fleas and obligatory parasitic mites, were less abundant in nests where aromatic plants were artificially or naturally added, respectively, but no relationships were detected with nestling physiology or survival, which could be partially explained by our small sample size. Our study partially supports a beneficial effect of aromatic plants in avian nests.

Effects of nest-box environment on fledgling success rate and pathogen load.

Nest boxes have been used for many decades as tools for conservation and to study avian population dynamics. Plastic is increasingly used as a material for nest boxes, but no studies have investigated effects of this different material. Two consecutive studies were conducted to investigate effects of nest-box environment on nidicolous parasites, bacteria and fungi, as well as nest success, in blue tits Cyanistes caeruleus and great tits Parus major. The first compared microclimate and parasite and pathogen load in plastic and wooden nest boxes. The second tested the nest protection hypothesis – that birds naturally incorporate aromatic herbs into nests to decrease nest parasites and pathogens – by comparing parasite and pathogen load in plastic nest boxes to which aromatic or non-aromatic plant material was added. No significant difference in nest-box temperature or relative humidity was found between plastic and wooden boxes. Wooden boxes, however, contained 30-fold higher numbers of fleas and a higher total bacterial load on chicks. Fledging success for blue tit broods was significantly higher in wooden boxes. Parasites and bacteria did not decrease by the inclusion of aromatic herbs. The results increase the evidence base for nest-box design in support of plastic, which can provide an appropriate alternative nest-box material to wood, with apparently no difference in microclimate and no increase in the load of measured parasites and pathogens.

Testing the Value of Prickly Pear Cactus as a Nest-Predator Deterrent for Northern Bobwhite

Recent research indicates that northern bobwhites (Colinus virginianus) in Texas commonly nest in prickly pear cactus (Opuntia sp.) instead of conventional bunchgrass habitat. We hypothesized that bobwhites nested in prickly pear because it served as a deterrent to nest predators thereby increasing probability of nest success (nest-protection hypothesis; Slater et al. 2001). We experimentally tested the nest-protection hypothesis by providing 50 wild-caught, captive raccoons (Procyon lotor) with combinations of simulated, bobwhite nests. Nest combinations included either 1 nest in bunchgrass (e.g., little bluestem [Schizachryium scoparium]) and 1 nest in prickly pear cactus (partial [75%] or full [100%] protection), or 2 separate nests in prickly pear (partial and full protection). Raccoons depredated 97%, 33%, and 14% of simulated nests constructed of bunchgrass (n = 35 nests), partial protection prickly pear (n = 30 nests), and full protection prickly pear (n = 35 nests), respectively. Prickly pear nests that provided full protection exhibited better survival against raccoon depredation than other nest types. Our study provides support for the nest-protection hypothesis regarding why northern bobwhite possibly nest in prickly pear cactus.

A review of the nest protection hypothesis: does inclusion of fresh green plant material in birds' nests reduce parasite infestation?

The use of aromatic plants and their essential oils for ectoparasite treatment is a field of growing interest. Several species of birds regularly introduce aromatic herbs into their nests putatively to reduce parasites. The behaviour is most often seen in cavity nesting birds and after nest building has finished. The plants are included in a non-structural manner and are often strongly aromatic. Various different hypotheses have been proposed regarding the function of this behaviour; from the plants altering some non-living factor in the nest (crypsis, water loss and insulation hypotheses) to them being involved in mate selection (mate hypothesis) or even having a beneficial effect, direct or indirect, on chicks (drug or nest protection hypothesis, NPH). Many studies have been carried out over the years observing and experimentally testing these hypotheses. This review focuses on studies involving the most popular of these hypotheses, the NPH: that plants decrease nest parasites or pathogens, thereby conveying positive effects to the chicks, allowing the behaviour to evolve. Studies providing observational evidence towards this hypothesis and those experimentally testing it are discussed.

Green Plant Material in Avian Nests

Some bird species place in their nests fresh green plant materials, e.g. leaves, sprigs or branches of herbs, shrubs and trees, which are not a part of the basic nest structure. These additional materials are often characterised by a high content of volatile secondary metabolites and constitute a non-random, small fraction of plants available in the habitat. Several non-mutually exclusive hypotheses have been proposed to explain the function of green material in avian nests with three of them attracting the most attention. The courtship hypothesis proposes that green material is used by males in order to attract females to nesting sites. The nest protection hypothesis posits that birds place in the nest green material rich in volatile compounds to reduce the abundance of parasites and pathogens which, in turn, should mitigate their negative impact on the host. According to the drug hypothesis green material directly positively affects the health and development of nestlings, e.g. through stimulation of some components of the nestlings’ immune system. Here, we present an overview of hypotheses explaining the phenomenon of green material in avian nests with a thorough description of three most frequently tested hypotheses and suggest the directions for future studies.

Aromatic Plants in Eurasian Blue Tit Nests: The ‘Nest Protection Hypothesis’ Revisited

The ‘Nest Protection Hypothesis’ suggests that some birds add aromatic plants to their nests to repel or kill ectoparasites. This behavior has been described for several species, including the Eurasian Blue Tit (Cyanistes caeruleus). We studied the reproductive performance, based on 26 nests (in nest boxes), of this species in mixed forested areas of Quercus spp. and Pinus pinea in the ‘Parque Florestal de Monsanto’, the largest park of Lisbon, Portugal. The frequency of aromatic plants in nests was compared with frequency of these plants in the study area. The three most frequent aromatic plants (Dittrichia viscosa, Lavandula dentata, Calamintha baetica) in nests were used more than expected from their availability in the study area. We could not reject the null hypothesis that nest survival rate is independent of the presence of aromatic plants in the nest.

Interacting effects of aromatic plants and female age on nest-dwelling ectoparasites and blood-sucking flies in avian nests

Some avian species incorporate aromatic plants to their nests. The "nest protection hypothesis", which posits that volatile secondary compounds contained in these plants may have antiparasite properties, has not received full support. All previous tests of this hypothesis have only considered effects on nest-dwelling ectoparasites, but not on blood-sucking flies. The "drug hypothesis" posits that aromatic plants may stimulate nestling immune system, development, or condition. We tested these hypotheses experimentally in wild blue tits, Cyanistes caeruleus, a species that adds aromatic plants to their nests. We supplemented aromatic plants to half of a sample of nests, while adding grass to the other half of nests. We quantified abundance of two groups of blood-sucking flies (blackflies and biting midges) at two different stages of the reproductive period, and abundance of three nest-dwelling ectoparasites (fleas, mites, and blowflies). Experimental supplementation of aromatic plants reduced abundance of fleas only in nests of yearling females and not in nests of older females. Blackflies and biting midges were both more numerous in nests of yearling females than in nests of older females. Mass of aromatic plants added by females was negatively related with abundance of fleas in control nests but not in experimental nests supplied with aromatic plants. Mass of plants added by females was also positively related with abundance of blackflies during the nestling stage. Finally, aromatic plants did not affect nestling growth or immune responses. We conclude that several factors such as female experience and their ability to add plants to the nest interact to explain effects of aromatic plants on different parasites.

Experimental addition of greenery reduces flea loads in nests of a non‐greenery using species, the tree swallowTachycineta bicolor

Several bird species, including cavity‐nesters such as European starlingsSturnus vulgaris, add to their nests green sprigs of plants such as yarrowAchillea millefoliumthat are rich in volatile compounds. In this field study on another cavity‐nester, tree swallowsTachycineta bicolor, we tested whether yarrow reduced ectoparasite loads (the nest protection hypothesis), stimulated nestling immune systems (the drug hypothesis), or had other consequences for nestling growth or parental reproductive success (predicted by both preceding hypotheses). Tree swallows do not naturally add greenery to their nests, and thus offer several advantages in testing for effects of greenery independent of other potentially confounding explanations for the behaviour. We placed fresh yarrow in 23 swallow nests on the day the first egg was laid, replenishing every two days until clutch completion (=three times), and at 44 control nests, nesting material was simply touched. At 12 days of age, we measured nestling body size and mass, and took blood smears to do differential white blood cell counts. We subsequently determined the number and proportion of young fledging from nests and the number of fleas remaining after fledging. Higher humidity was associated with higher flea numbers whereas number of feathers in the nest was not. Our most significant finding was that an average of 773 fleasCeratophyllus idiuswas found in control nests, versus 419 in yarrow nests. Possibly, parents compensate for blood that nestlings lose to ectoparasites by increasing food delivery, because we detected no differences between treatments in nestling mass, nestling leukocyte profiles, or proportion of young fledging, or relative to flea numbers. Our results provide no support for the drug hypothesis and strong support for the nest protection hypothesis.

Nest Green Plants as a Male Status Signal and Courtship Display in the Spotless Starling

AbstractThe use of green nest plants by birds has received much attention in recent years. Although many hypotheses have been put forward to explain the functional significance of this trait, three of them,the courtship hypothesis,the nest protection hypothesisandthe drug hypothesis, have particularly captured the interest of researchers. In this paper we demonstrate that in the spotless starling,Sturnus unicolor(1) most of the green plants were carried to nests during the 10 d that preceded the start of egg laying, (2) males that controlled a greater number of nest boxes, so that they mated with more females, also carried a larger amount of green material to each of their nests, (3) males carried larger amounts of green plants in second than in first clutches, and (4) monogamous males did not exhibit this last tendency when they mated with inexperienced females. Thus, plants seem to exert a function during the female fertile period but not during incubation or nestling stages. The amount of green plants added by a male may be seen as a honest signal of attractiveness and status. We suggest that males uses green plants to stimulate females to breed in it so that they carry more plants in second clutches, when females might be more reluctant to breed. This strategy was apparently constrained in monogamous males mated with inexperienced females probably because parental duties, rarely recorded in polygynous males, limits the time available to carry plants to nests. We also discuss to what extent other hypotheses may further explain the evolution of green plants carrying in starlings.

Does fresh vegetation protect avian nests from ectoparasites? An experiment with tree swallows

Some species of birds incorporate fresh green vegetation into their nests. The adaptive significance of this behaviour, however, is unclear. The "nest protection hypothesis" suggests that volatile chemicals contained in green plants reduce populations of nest-dwelling ectoparasites. I tested this hypothesis with tree swallows, Tachycineta bicolor (Vieillot, 1808), a species that has large infestations of ectoparasites in its nests. I experimentally added green leaves of yarrow (Achillea millefolium L.) to nests and compared these nests to control nests. My results showed that nests with yarrow had significantly higher levels of infestation by fleas, and yarrow had no effect on the number of blow flies detected. There was no effect of yarrow on condition of parents, size of offspring, or the probability of nests fledging at least one young. I did, however, find that second-year (SY) females with yarrow added to their nests hatched significantly more eggs than control SY females. In addition, pairs with yarrow added to their nest were significantly less likely to suffer hatching failure and (or) brood reduction. These beneficial effects of yarrow cannot be explained by reductions in numbers of fleas or blow flies, and I suggest that these results may be due to reductions in numbers of other types of parasites not quantified in my study, such as flying insects or the diseases that they may vector. I discuss my findings for swallows in the context of results from previous studies of other species that habitually use green plant material such as yarrow in their nests.

Green nesting material has a function in mate attraction in the European starling

The function of fresh green nest material has long been debated. It has been suggested that it reduces the number of ectoparasites in nests and on nestlings (nest protection hypothesis), or is used by males to signal condition and paternal quality (male quality hypothesis) or is used as a sexually selected ornament to attract females (courtship hypothesis). We simultaneously tested these three hypotheses in the European starling, Sturnus vulgaris, in the field. Green material was carried by male starlings only, and mainly during nest building. It was not used to reduce ectoparasites. Males nesting in nestboxes that were experimentally contaminated with ectoparasites did not carry more green nest material than males nesting in control boxes, and experimental removal and addition of green material had no effect on the number of ectoparasites on the nestlings or on their body mass. Furthermore, the amount of green material carried into a nestbox was not associated with male body mass, paternal incubation attendance or nestling food provisioning. There is two-fold experimental evidence that males use green plant material to attract mates. First, removal of greenery resulted in a significantly lower percentage of nestboxes containing a clutch than the control or addition treatment. Second, unpaired male starlings sang more and carried more greenery into a nestbox when a caged female was positioned adjacent to the nestbox than when a caged male or an empty cage was present. Paired males, when subjected to the same experimental design, did not respond.

The Use of Prickly Pear Cactus as Nesting Cover by Northern Bobwhites

Years of research on the nesting ecology of the northern bobwhile (Colinus virginianus) have documented the use of bunchgrasses as common nesting cover. However, in the Rolling Plains of Texas, recent studies indicate a relatively high use of prickly pear cactus (Opuntia spp.) as suitable nesting cover. To explain this previously unreported use of prickly pear, we evaluated 2 hypotheses: (1) the limited-bunchgrass, and (2) nest-protection hypotheses. We speculated that bobwhites nested in prickly pear cactus because of low availability of bunchgrasses and/or structural protection against predators. We radioinarked 218 bobwhites during spring and summer of 1997 and 1998 on 4 sites in Shackelford County, Texas, USA. Bobwhites nested in prickly pear despite adequate bunchgrass cover (>600 potential nesting sites/ha). Of 81 nests located, 12% were in shrubs (e.g., catclaw [Acacia greggii]), 30% were in prickly pear, and 58% were in bunchgrass. Nest success did not differ among nesting substrates (X 2 = 2.1, df= 2, P= 0.35) but was dependent on the degree of protection provided by the nesting substrate (X 2 = 16, df= 2, P= 0.001). However, successful nests also had a higher density of bunchgrasses in the surrounding arca than unsuccessful nests (P = 0.001). Our data partially supported the nest-protection hypothesis and did not support the limited-bunchgrass hypothesis.

Blue tits use selected plants and olfaction to maintain an aromatic environment for nestlings

Animals use the chemical compounds of plants as a defence mechanism against enemies, and sometimes use olfaction to discriminate and select the chemical plant substances. Some birds bring to the nest plant material that has volatile compounds that protect the host and their offspring against parasitic organisms. Here we show that blue tits on the island of Corsica (Parus caeruleus ogliastrae) adorn their nests with fragments of aromatic plants. These plants have chemical compounds that are used by humans to make aromatic house cleaners and herbal medicines. We also show that individual blue tits maintain an aromatic nest environment when offspring are raised, using odour cues to determine the frequency with which they replenish the nest with fresh plant material. We provide an exceptional example of the ecologically relevant use of olfaction by birds under natural conditions. To our knowledge, we present the first experimental demonstration that a free‐ranging animal makes use of smell to maintain an aromatic environment for offspring with plants, supporting predictions of the nest protection hypothesis.

Aromatic plants in bird nests as a protection against blood-sucking flying insects?

The nest protection hypothesis suggests that some of the fresh plant material, brought by some bird species to their nests, contain secondary compounds that repel parasites or mask the chemical cues that parasites use to find the host. Corsican blue tits (Paruscaeruleus) bring pieces of several species of aromatic plants to the nest at dusk, suggesting a protection against nocturnal agents such as blood-sucking flying insects. The potential repellent or masking effect of these aromatic plant species, alone or in mixture, was experimentally investigated in the mosquito Culexpipiens, using domestic chicks as bird hosts. Aromatic plants had a repellent effect against mosquitoes, although a masking effect could not be ruled out. The plant mixture was the most efficient, although some aromatic plants had a significant effect when used individually. These results are discussed in the context of the nest-protection hypothesis and of the impact of flying blood-sucking insects on bird behaviour and life history traits.

Green plants in starling nests: effects on nestlings

European starlings, Sturnus vulgaris, intermingle fresh herbs, especially species rich in volatile compounds, with their otherwise dry nest material. In this field study we investigated whether these herbs reduce ectoparasites and thereby protect nestlings (the nest protection hypothesis). We also considered whether volatile compounds in herbs improve the condition of nestlings (the drug hypothesis). As measures of condition we used body mass, haematocrit levels and immunological parameters. We replaced 148 natural starling nests with artificial ones: half contained herbs and half (controls) contained grass. The ectoparasite loads (mites, lice, fleas) in herb and control nests were indistinguishable. However, nestlings in herb nests weighed more and had higher haematocrit levels at fledging than nestlings in control nests. Fledging success was similar in herb and control nests, but more yearlings from herb nests were identified in the colony the year after hatching. The response of the immune system when challenged with phytohaemagglutinin did not differ in nestlings from herb and control nests. Nestlings from herb nests had more basophils and fewer lymphocytes in their blood than those from control nests, while the eosinophil and heterophil counts did not differ. We conclude that herbs do not reduce the number of ectoparasites, but they improve the condition of nestlings, perhaps by stimulating elements of the immune system that help them to cope better with the harmful activities of ectoparasites.

The Function of Green Plants in Nests of European Starlings (Sturnus Vulgaris)

AbstractEuropean starlings add fresh green plants to their dry nest material. Male starlings of our 60-nest-box colony carried 68 different plant species into their nests. Some males were polygynous and had 3-6 clutches, others were monogynous and had 1-2 clutches per reproductive season. The 'nest protection hypothesis' proposes that insecticidal compounds in green plants reduce the parasite load of the nests. The 'courtship hypothesis' predicts that carrying nest greenery is a courtship activity to attract females. The aim of this study was to collect field data suitable for distinguishing these two hypotheses. 1. Some plant species occurred more often in the nest-boxes than expected from their frequency in the nest-box environment. A significant number of these preferred plants were rich in volatiles, some of which are said to be insecticidal. But volatiles could also attract females and/or influence their breeding activity and the chicks' development directly. 2. The males carried greenery into their nest boxes maximal around 5 days before the onset of laying, when pair formation took place, and ceased this behaviour with egg deposition. The total amount of greenery deposited in a nest-box was a function of the number of days of courtship a male needed to attract a female. 3. Polygynous males deposited the same amount of greenery in their first nest as monogynous males. In additional nests polygynous males deposited more greenery. However, this was due to the fact that these additional nests were advertised for a longer time.

The Survival-Reproduction Selection Equilibrium and Reversed Size Dimorphism in Raptors

A variety of avian taxa contain groups with 'reversed' (i.e. males smaller than females) sexual size dimorphism (Jehl and Murray 1986). Of all these taxa, its occurrence in the Falconiformes (hawks, eagles, falcons), the Strigiformes (owls) and the Stercorariidae (skuas) has received particular attention in the literature. These three independently evolved taxa have very different biologies but share predatory habits and, to a lesser degree, species in each show partial or complete sexual role specialization during breeding with males provisioning and females nest-tending. In other avian taxa reversed sexual dimorphism seems largely attributable to female-female competition for mates, but this explanation has seldom been applied to the hawks, owls and skuas (cf. Olsen and Olsen 1987). The diverse hypotheses that have been proposed (see reviews in Andersson and Norberg 1981, Mueller and Meyer 1985) are usually linked to the predatory nature of these birds. Many consider only a single selection pressure, operating only on one sex, without considering opposing selective forces. Arak (1988) has criticized this approach, pointing out that sexual dimorphism must arise from selection pressures for and against changes in body size acting differently on the two sexes (Ralls 1976, Price 1984). In this paper we use the framework for the evolution of sexual size dimorphism set out by Arak (1988) in an attempt to identify how different selection pressures acting on the two sexes could have resulted in the evolution of reversed sexual dimorphism in hawks, owls and skuas. We do not attempt a comprehensive review. Rather, we hope to convince the reader of the usefulness of Arak's approach: it easily accommodates the existing hypotheses, but helps make them more comprehensive, and it aids in identifying testable predictions not previously evident. We also modify and extend an existing hypothesis and show how, when used in the context laid out here, it provides explanations for broad patterns, as well as some of the exceptions, of sexual dimorphism in the groups considered. Hypothesis for reversed sexual dimorphism in predatory birds can be sorted into three categories, based on an evolutionary increase in female size, a decrease in male size, or a divergence in male and female size. The 'large female' hypotheses are (using the terminology in Andersson and Norberg's (1981) Table 1): the anticannibalism hypothesis, the male subordination hypothesis, the pair formation hypothesis, the egg size hypothesis, the incubation efficiency hypothesis, the nest protection hypothesis, the follicle protection hypothesis, and the starvation hypothesis. All these hypotheses assume that selection on breeding success favors large body size in females. In contrast, other hypotheses assume that the breeding success of pairs is enhanced when males are small. The 'small male', hypotheses are the energy savings hypothesis, the pyramid of prey hypothesis, the territorial defense hypothesis, and the courtship hypothesis. The 'dimorphic niche' hypotheses maintain that breeding pairs divergent in size are better able to exploit the prey base. They include the food competition and female supplementary feeding hypotheses. Descriptions of all these hypotheses, plus their sources, can be found in Andersson and Norberg (1981), with the exception of the starvation (see Lundberg 1986) and courtship (see Jehl and Murray 1986) hypotheses. Arak's (1988, see also Hedrick and Temeles 1989) framework for the evolution of sexual dimorphism considers natural selection acting on the survival and the reproductive consequences of body size. At the equilibrium body size in either sex, the fitness consequences of

Starlings as herbalists: countering parasites and pathogens.

The avian nest protection hypothesis explains the widespread behavior of incorporating fresh green plant material into an otherwise dry nest matrix as an evolutionary adaptation. Species that exhibit this behavior tend to breed in previously inhabited nest sites and experience high ectoparasite and pathogen loads. Metabolites in the green plants control parasite and pathogen populations and decrease the effects of these agents on nestlings. The choice of plants is not arbitrary. In the case of European starlings, a nonrandom subset of available vegetation is selected, and it is these plants that have the largest impact on ectoparasite and pathogen populations. Experimental and observational evidence suggests that starlings select plants on the basis of chemosensory cues.

Effect of biologically active plants used as netst material and the derived benefit to starling nestlings.

The European starling Sturnus vulgaris preferentially incorporates fresh sprigs of particular plant species for use as nesting material. Chemicals found in these plants may act to reduce pathogen and ectoparasite populations normally found in nest environments. The present experiments were performed to test this Nest Protection Hypothesis. In the fild, we experimentally determined that wild carrot Daucus carota, a plant species preferred as nest material, effectively reduced the number of hematophagous mites found within nests relative to control nests without green vegetation. Chicks from nests containing wild carrot had higher levels of blood hemoglobin than chicks from control nests. However, there were no differences in weight or feather development. In the laboratory, we found that wild carrot and fleabane, Erigeron philadelphicus, (also preferred by starlings as nest material) substantially reduced the emergence of feeding instars of mites, while garlic mustard, Alliaria officinalis, (commonly available but not preferred) had little effect on the emergence of mites. We infer that preferred plant material may act to inhibit feeding or otherwise delay reproduction of mites, thereby reducing risk of anemia to developing nestlings.