“Strange behaviours” of parents at the nest in an avian prey species as a potential source of bias when studying nest predation and signs left by different predators

In most passerines, parent birds clean their nests. Egg shells, fecal sacs, and dead nestlings, as well as foreign objects, for example, leaf and twig debris, as they appear, are usually removed from nests in one or another way. If, for one reason or another (nest predation, inclement weather, starvation etc.), all offspring die, parents abandon the nest with egg or/and nestling remains. Finding one or another nest empty and intact before the earliest possible fledging date, observers who monitor nests usually attribute the failure to predation. Automated cameras placed at 148 Fieldfare (Turdus pilaris) nests in 2016–2020 to study nest predation in Moscow City, Russia documented two cases of sanitation (by females) of entire and almost entire (all but one dead nestling) broods of dead nestlings that had died due to inclement weather. This information is one of the very few, probably the second, and for Eurasian species probably the first published evidence of such a behaviour in passerines. In addition, eight cases of removal by parents of the traces of predation were recorded immediately or soon after (within 0.1– 4.2 hours) complete depredation of nest contents: eating at the nest or the removal by parents of egg shells, remains of egg contents, as well as feathers lost by parents as a result of predator attacks, an active or passive elimination of disturbances in the lining of the nest cup. Complete or partial “concealment of evidence” by parents occurred in about every four depredated nests, including those in which predators did not leave any “evidence”. The removal by parents of all evidence of clutch/brood failure that had happened for any reason, not only due to predation, was recorded in approximately every third nest from which all offspring disappeared without a trace before the earliest possible fledging date. These behaviours of parents could possibly be considered as bringing them to the point of absurdity, “by inertia”, a sequence of stereotypic actions to maintain cleanliness, as well as the lining of the nest. An observation was also made of a female adding fresh lining to the nest cup on top of a dead, ca. 7-day old nestling from the failed previous brood and then laying a replacement clutch. All these cases are interesting not only because they provide new information on parental behaviour in the Fieldfare. These “strange behaviours” are also a potential source of bias when studying nest predation and signs left by different predatory species with traditional methods for monitoring the nests, with neither video monitoring nor automatic photography. Furthermore, it must not be excluded that, under some circumstances, even estimates of the relative frequency of different causes of nest failure can be biased due to these behavioural curiosities. That is why it is important to know how many and how frequently do bird species show similar behaviours.