To attain my goals, I have followed the fate of 225 breeding events of starlings nesting in nest boxes. They produced a total of 1112 eggs, from which 925 nestlings hatched.
In March, before starlings start laying eggs, we trapped the adults in their nest boxes. Trapping has several purposes. On the one hand, it serves to get important information about the condition of the animals. We weigh them, we measure them, and we individually mark them with leg rings. Moreover, we insert a small transponder, which will be very useful to track the individual throughout its life. In addition, trapping provides information about potential social pairs. Stable pairs sleep together in the nest box before reproduction begins, and thus, those animals trapped together in the nest box, have spent the night together, and are good candidates to breed together. An additional source of information that trapping provides is that we can assess the personality of the birds. Animals differ in the way they react to being handled; some scream, some struggle, and some are aggressive towards the observer, whereas others stay apparently calm. Those reactions are repeatable, which means that birds that attack in one trapping event, do attack too when trapped again. Those behaviours that are repeatable across time and contexts conform what we call the personality of the individual.
In April starlings synchronously start laying eggs. By daily checking the nest boxes we can assess the onset of egg laying, the number of eggs each pair lays, and the number of nestlings that hatch from those eggs. Nestlings are individually marked, so we can follow the fate of each individual. By doing so, we can know who are the nestlings that survive and the body condition they attain before fledging. From each nestling, we take a blood sample, which is later analysed in the genetic laboratory. From all those blood samples we can know the genetic father and mother, which sometimes differ from the mother and father that they had at the nest. This is very laborious, but the results are sometimes very surprising.
We found that female personality affected reproductive success. More explorative females were less successful, they raised less nestlings and less fledglings than less explorative ones. In addition, neophobic individuals, those that react strongly to novelty, started reproducing later in the season, with the concomitant risk of having less food available to feed the young. We could show that parents had a strong impact on the development of their young. Young development was affected by the time of the year they were born (starlings lay two broods in a year, one in April and a second one in May-June), the body condition of the mother, the number of siblings, and the promiscuity of the mother. We found that males attended the nest less than females, and that females paired with a male that attended less than average, compensated by increasing their own share. However, the compensation was partial, and low paternal care was translated into higher chick mortality. Compensation, in any case, is costly, and it can compromise the future reproduction of the female. We explored the costs of reproduction for females, and we found that high-quality females could heavily invest in one brood at apparently no cost. However, when extra-costs were added, in terms of enlarged broods (some females do not defend a nest and lay their eggs in others nests, what is called brood parasitism), they paid the cost later in life, by reducing the investment in future reproduction.
