Feather Nest Decoration Reduces Conspecific Brood Parasitism in Spotless Starlings
Feathers found in the nests of spotless starlings (Sturnus unicolor) may convey information that other members of the same species can interpret. A recent study found that nests containing feathers experienced lower rates of conspecific brood parasitism. Conspecific brood parasitism is a reproductive tactic in which some females lay eggs in the nests of other females of the same species, leaving the hosts to incubate the eggs and rear the resulting nestlings. Because spotless starlings are altricial birds whose chicks depend heavily on parental provisioning after hatching, receiving an additional parasitic egg can impose substantial costs on the host. More nestlings mean that parents must provide more food and care during the nestling period, increasing the burden of raising the brood.
Spotless starlings have consequently evolved defences against conspecific brood parasitism. They can sometimes recognize eggs laid by other females, and some hosts eject parasitic eggs when they differ in size from their own. Such egg rejection acts only after parasitism has already occurred. Preventing a parasitic female from approaching the nest in the first place could potentially reduce those costs even further.
Feathers may play a role in such defence. In this study, spotless starlings bred in artificial nestboxes, and the researchers observed that the birds actively carried feathers into their nests. Feathers were often placed outside the nest cup, between the nest and the wall of the nestbox, where they could easily be seen by birds inspecting the cavity from the entrance. Previous work on other bird species led to the proposal of the "fear of feathers hypothesis," according to which conspicuous feathers inside a cavity may be interpreted by visiting birds as evidence of a recent predation event. Because predators can leave feathers behind after killing and consuming birds, a cavity containing scattered feathers may be perceived as a risky place. Experiments with pied flycatchers (Ficedula hypoleuca), blue tits (Cyanistes caeruleus), and tree swallows (Tachycineta bicolor) have indeed shown that white feathers can make birds more hesitant to enter nestboxes.
But how many feathers do spotless starlings normally place in their nests? In 2023, the researchers photographed 242 nests in a long-term nestbox population in central Spain, obtaining a total of 499 photographs during the breeding period. Of the 242 nests, 235, or 97%, contained at least one feather. Pale feathers occurred in 71% of nests, dark feathers in 68%, and 48% contained both types. Feather numbers also increased as egg laying progressed. In a typical clutch of five eggs, nests contained an average of about six feathers by the sixth day after the first egg was laid, when laying had usually ended because females produce only one egg per day.
The researchers then used radiofrequency identification systems installed on the nestboxes to track visitors. Adult birds in the study population carried individually identifiable passive integrated transponder, or PIT, tags, allowing the system to record which bird perched at or entered a nestbox. The researchers analysed 69 nests and classified any spotless starling other than the breeding male or female as an intruder. The pattern was clear: the more feathers a nest contained, the fewer visits it received from intruders. This relationship occurred with both dark and pale feathers, suggesting that the effect was not restricted to the white feathers emphasized by the original fear of feathers hypothesis.
The next step was an experiment conducted when spotless starlings began laying their first clutches. Nests were assigned to three treatments. In one group, all feathers were removed daily. In another, the number of feathers was maintained at 10 by adding white feathers when necessary. A control group was checked at the same frequency, but its feather content was left unchanged. The experiment ultimately included 72 clutches, 23 of which were parasitized.
The number of white feathers did not have a statistically significant effect on the probability of parasitism. However, when the nests were reclassified simply according to whether they contained any feathers at all, nests with at least one feather were significantly less likely to be parasitized than nests containing none. Together with the observational data showing fewer intruder visits to feather-rich nests, this suggests that the presence of feathers is associated with a reduced likelihood of conspecific brood parasitism.
One possible explanation proposed by the researchers is that feathers provide information about the stage of the host's breeding attempt. Spotless starlings continue bringing feathers into their nests as egg laying progresses, so feather abundance could serve as a cue to how far laying has advanced. A parasitic female must lay during the host's laying sequence if her egg is to receive an appropriate period of incubation. A nest with few or no feathers may therefore indicate that laying has only recently begun, whereas a nest containing many feathers may indicate that the clutch is already close to completion. By selecting nests with few feathers, parasitic females may improve the timing of their own laying relative to that of the host.
Feathers may also provide information about the competitive ability of the nest owner. If females that are more successful at obtaining and carrying feathers are also more capable of defending their nests, avoiding feather-decorated nests could allow parasitic females to reduce the risk of encountering a particularly effective defender.
Feathers in a spotless starling nest may therefore form part of an information system between hosts and potential conspecific brood parasites. They could signal predation risk, the stage of the breeding attempt, the competitive ability of the nest owner, or some combination of several cues that are not yet fully understood. Potential brood parasites may use this information when deciding which nests to visit and where to lay their eggs.
Author: Shui-Ye You
Reference:
Liénard A et a. (2026). Feather nest decoration decreases conspecific brood parasitism rate in spotless starlings, Sturnus unicolor. Animal Behaviour.
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