Droppings activate the immune system in nestlings

In the animal world, strict rules are followed to deal with sources of contamination and potential dangers such as predation. In the case of birds, parents remove their chicks' droppings from the nest on a daily basis to conserve hygiene. A new study carried out in Spain and published in the journal Frontiers in Zoology demonstrates that the presence of feces in nests attracts insects and provokes the activation of nestlings' immune systems. This provides important information on the reason for this sanitation behaviour.

Until now, the predominant hypothesis in ornithology (the branch of zoology dedicated to the study of birds) was that birds removed droppings from the nest in order to avoid attracting parasite species to the nest. However, the new research refutes this. "Our study demonstrates that parasites being attracted by feces does not appear to be the reason for which evolution has favoured this behaviour, despite this having been traditionally assumed to be the case," Juan Diego Ibáñez-Álamo said, the paper's main author and an investigator at the Spanish National Scientific Research Council's (CSIC) Doñana Biological Station and at the University of Groningen (Netherlands).

Immune system response

The scientists ran three different experiments using insect traps, artificial nests and real blackbird nests in order to observe the attractant effect of droppings on parasites.

Although the experimental predictions were for a higher quantity of parasites when feces were present, "the fecal sacs did not attract a higher number of parasites," says the researcher.

The chicks' immune systems were affected by the presence of feces; specifically, there was a change in the ratio of heterophils to lymphocytes (blood cells that fight against pathogens such as parasites), a physiological indicator of birds' response to stress. "This ratio was significantly higher in nestlings that lived near the fecal sacs than in those which did not have feces near them," state the authors. The scientists also observed that the chicks' droppings caused an increase in the appearance of flies and a reduction in the number of acarids. The authors indicate the capacity of flies to act as vectors for the transmission of damaging microorganisms as a cause of immune system activation.

Fecal sacs, isolation against bacteria

Nestlings produce feces enclosed in a mucous covering. This unique structure, shown in a previous study to function as isolation against bacteria, may also be responsible for preventing parasites being attracted to the nest. "It is possible that the mucus acts as a barrier, blocking the spread of chemical signals that parasites may use to locate the chicks," says Ibáñez-Álamo. "The findings of our study appear to indicate that microorganisms may play a very significant role in relation to the cleanliness of birds' nests," he adds.

Even so, the authors consider this habit of parent birds to be the result of several factors: "We cannot rule out that parental behaviour may be altered by the presence of feces near the nest," they state. Even predation could be another determinant cause for this behaviour.