João L. Guilherme
Linked paper: Consistent nest-site selection across habitats increases fitness in Asian Houbara by J.L. Guilherme, R.J. Burnside, N.J. Collar, and P.M. Dolman, The Auk: Ornithological Advances 135:2, April 2018.
For birds that nest on the ground, discretion is everything. As they are especially at risk from predators, choosing where to nest may carry life or death consequences for themselves, their eggs, and their progeny.
We study the ecology of the Asian Houbara (Chlamydotis macqueenii) in the semi-deserts of southern Uzbekistan, as part of a long-term effort to gain insight into the dynamics of this wild population. The landscape has extensive low-density shrub coverage and tends to all look the same on first glance, but a closer look reveals subtly distinct habitats with shrub communities that differ in not just species composition, but also in the size and number of shrubs. The Asian Houbara is a highly cryptic ground-nesting bird inextricably associated with these habitats, breeding throughout. For 23 long days, females have the sole responsibility of laying, incubating, and protecting the eggs, and themselves, from the freezing cold and the strong sun, and from the desert predators such as foxes and monitor lizards.
This behavior of nesting in structurally different habitats made us question if females were choosing similar vegetation structure for nest sites and whether these choices had an impact on their nest success.
By following houbara tracks, we succeeded at finding 210 nests. Then we took it upon ourselves to identify and measure the height of the shrubs around all nests and at 194 random locations. Obviously, this was done after the nest was finished and the female and chicks had left the area. In the end, we identified 30 species and measured a total 35,853 shrubs! After running some statistical analysis we found that females were indeed choosing the same nest site features consistently across three structurally different habitats. Their selection was so fine-tuned that the optimal shrub height of about 30 centimeters had the greatest probability of being selected in all habitats. Furthermore, the scrape was consistently in the middle of shrubs that offered some degree of concealment, but enough visibility for the female to anticipate approaching predators.
So, females were choosing similar nest site across habitats, but we wondered if these features were helping them avoid nest predation.
To investigate this, we monitored the nests, placing temperature loggers inside the nest scrape and setting video cameras to collect information through the entire incubation so that we could classify if a nest was successful or if it had failed and, in that case, why (see video here). We found that nests in higher vegetation had a lower probability of being predated, with the likelihood being that the higher vegetation offered more concealment from predators. However, females would not nest in even higher vegetation, as this would eliminate their ability to see around and anticipate approaching predators. In fact, from more than 200 monitored nests, there was not a single time when a female was predated, which normally occurs in other ground nesting birds—females seem to value the old adage “run and hide, live to fight another day.” Nest camera footage showed us that females were eternally vigilant, with their heads extended so they could see just above the vegetation and surreptitiously leave the nest before a predator arrived. In this way, we found the connection between the choice of nest site and the chance of losing the nest to predation.
In a landscape where everything looks the same, there were in reality different habitats where nesting Asian Houbara had to find the “perfect spot” that maximized the chances of hatching while reducing the danger of being depredated. For a species of conservation concern, it is very important to maintain good productivity and minimize changes in vegetation structure away from the optimal choices, as these may lead to abandonment of previously suitable nesting areas, lower nest survival, or increased predation risk for the incubating female.