Blue-footed boobies (Sulla nebouxii) are relatively large and long-lived for a bird and it takes a lot of food for the adults to raise their chicks to independence. They nest in large colonies along the west coast of the Americas and feed on fish, caught in the tropical waters of the Pacific. Both parents care for the young and feed them for up to 6 months after hatching. As the chicks grow, so does the demand they place on their parents, who must make ever more frequent trips out to the ocean to find fish for the brood.
A new study published in Animal Behaviour investigated the physiological costs that the parents incur during this critical stage of their lives. Researchers monitored the behaviour of both male and female boobies in a large colony on
a Mexican island, where approximately 3,000 booby pairs nest at a density of 0.26 nests/m2. They also collected blood from the adults to measure their physiological condition and conducted an experiment to manipulate the need for provisioning. The experiment consisted of swapping chicks of different ages between nests. Chicks that were 1 week old were placed in the nests of parents who had 2-week old chicks and vice versa. The result of this manipulation was that parents who had younger chicks were suddenly faced with the significantly greater nutritional needs of the older and larger 2-week chicks (these older chicks need up to twice as much food). Conversely, the original parents of the older (2-week) chicks got a break and had a ‘new’ brood, which was younger, smaller, and less demanding.
As a measure of the costs of caring for their chicks – the researchers analysed blood samples to quantify several compounds. Several blood plasma metabolites were used to assess changes in levels of body reserves and physical exertion, associated with muscle activity and foraging. An additional measure (the ratio between two types of blood cells, heterophils and lymphocytes) was used as an index of stressors that might adversely affect immune function (e.g. inflammation, infectious disease, parasite infestation, food or water deprivation as well as temperature extremes). Blood was collected from the birds before and after the experimental manipulation (chick-swapping) so that any changes in these parameters could be related to the change in provisioning demands the parents experienced. The researchers also collected data on body weight for a more general assessment of their condition.
When parents experienced an increase in provisioning demand (i.e. thy were given bigger chicks) – both of them increased the number of trips to sea they made looking for food and they spent less time at their nest. When provisioning demand decreased (i.e. when parents were given younger, less needy chicks), surprisingly, the parents did not decrease their foraging effort. Although overall body condition was not affected by changes in provisioning demand, some of the blood plasma markers were: greater demand resulted in higher levels of biological markers indicative of physical exertion and potential stress on the immune system. Parents that experienced a decrease in provisioning demand had lower levels of these biomarkers, relative to control subjects suggesting that physiological condition tracked adjustments in foraging effort.
The costs associated with raising a brood can thus affect the physiological condition and possibly the future reproductive effort of the parent boobies. Yet, adults increased their effort to make sure that their chicks were well fed and had a better chance of reaching maturity. The flexibility in provisioning behaviour shown by the parent birds may be important given that the seas they inhabit can be very unpredictable in terms of food availability.
González-Medina, E., Castillo-Guerrero, J. A., Santiago-Quesada, F., Villegas, A., Masero, J. A., Sánchez-Guzmán, J. M., & Fernández, G. (2015). Regulation of breeding expenditure in the blue-footed booby, Sula nebouxii: an experimental approach. Animal Behaviour, 108(C), 9–16. http://doi.org/10.1016/j.anbehav.2015.06.025
You can find out more about this and related research on Erick Gonzalez-Medina’s Research Gate page.
Boobies in peril