How can I stop nesting birds

Brood loss in blue tits

When all the young birds die in a nest, the death of one of the parent animals is almost always the cause

In blue tits, the death of all chicks in a nest is almost always related to the sudden and irrevocable disappearance of a parent. Peter Santema and Bart Kempenaers from the Max Planck Institute for Ornithology in Seewiesen show that the remaining parent then tries harder to get at least some of the chicks through, which is also possible with two thirds of the nests. Single males are generally less successful, which could be due to the fact that they cannot breed and therefore cannot keep the chicks warm.

Blue tits typically lay 8-15 eggs in a nest, from which the young fly ideally 21 days after hatching. In some nests, however, all the young die before they are old enough to leave the nest. The reasons for such a complete loss of brood have so far been unclear. For example, one parent could have left the rearing of the young entirely to their partner, or both parents could have jointly decided to leave the brood. In their study, Peter Santema and Bart Kempenaers from the Max Planck Institute for Ornithology in Seewiesen developed a method with which they could find out exactly when the parents stop bringing food to the nest and when the offspring is lost.

To do this, the two scientists have equipped all the adult blue tits in their study area with a tiny microchip transponder. An automatic monitoring system was installed in all nest boxes in the area, which recorded every visit by a bird with a transponder throughout the year. This enabled them to analyze every visit to the 277 nest boxes and determine when a parent was last at the nest. In the case of a parent suddenly disappearing, they looked at the number of times the remaining parent visited the nest before and after their partner's disappearance.

Loss of a parent

Of the 684 nests that they were able to analyze over seven years, thirteen percent have completely lost the brood. In almost all of these nests, one of the parents disappeared while the young were still alive. “With one exception, none of the missing birds reappeared in the study area,” says Bart Kempenaers, who led the study. It is therefore unlikely that one parent has left the brood and left the rearing to the partner. The visit rates of the parents in the nest were also normal up to the time of the disappearance. This indicates otherwise healthy individuals who did not have to give up their brood exhausted. "With all of this evidence, it is clear that a parent's sudden absence was caused by their death," says Peter Santema, lead author of the study. The uninterrupted approach to and from the nest makes the parents susceptible to aerial predators, especially the sparrowhawk.

The absence of a parent does not necessarily mean the end of the whole brood. In more than two thirds of the cases in which one parent had previously disappeared, at least some of the nestlings reached the age of egress. The probability of a successful brood was higher if the male had disappeared. “The nestlings cannot keep the temperature in their bodies constant as long as they have no feathers. Only females have a brood spot and thus the opportunity to keep them warm, ”says Peter Santema. That could be one reason single males have generally been less successful than females.

One parent tries alone

In the other cases, the retarded parents appear to have been unable to bring the brood through on their own. After the partner's disappearance, the number of visits to nests increased considerably in both males and females, but then steadily decreased again in the case of ultimately unsuccessful broods. It seems that the abandoned parents tried at least on their own, but ultimately couldn't keep up with the high demand. It is very unlikely that the parents will give up their nestlings voluntarily. Since blue tits only have offspring once a year and are short-lived, they have little chance of reproducing again in the future.

In contrast to nests that failed completely, nests with only a partial loss of brood were generally cared for by both parents. “Our results show that there are other reasons for the death of individual nestlings than for the loss of the entire brood, such as food shortages or illness,” summarizes Bart Kempenaers.