Interview with Mike Bradbury answering questions about Central Valley Swainson's Hawk
Mike Bradbury is a wildlife biologist and member of the Swainson's Hawk Technical Advisory Committee. He has been doing research on Swainson's Hawk since 1992.
What is the current population? Where is most of the population located? The world-wide nesting population occurs from southern Canada to northern Mexico, from the Mississippi River to California’s Central Valley. The population has been estimated via migration counts in Panama and Vera Cruz, and is about 500,000 (average per year). The California population was determined through a State-wide nest survey to be 1800 to 2000 nesting pairs, down from the estimated pre-Europeans size of 20,000 nesting pairs.
When do they start to migrate? As per the Central Valley birds, they move throughout the Central Valley in August and September taking advantage of all the agricultural harvesting, which provides a lot of opportunities for finding and capturing prey. The species is highly adapted to agricultural practices and has abandoned most of its historic grassland habitat. In the beginning of October they begin to mass in large groves of eucalyptus trees in the south Valley, and from there move to their wintering grounds. The Central Valley birds primarily winter in western Mexico and Central America, although some go as far as Columbia and northern Argentina. By contrast, every Swainson’s hawk that nests outside the Valley (that has been tracked) migrated to Argentina. Because of the differences in wintering grounds, we implemented a genetics study to determine if our birds are an isolated population of Swainson’s hawk.
The 'book' said they will migrate to South America. Can you track their migration? As above, yes we can and have tracked our birds, and Swainson’s hawks outside the Valley have been tracked by other researchers. We use satellite tracked transmitters that fit on the bird like a backpack.
When they come back do they come back to the same area to start their own family? Or is Bakersfield close enough? Statistically speaking, adults that have nested already will return to a location as close to their previous nest as possible. Sometimes competitors get there first, and sometimes their nest site becomes a shopping center parking lot (for instance), so they have to use an alternative nest site if they can find one. Great-horned owls and red-tailed hawks usually only force the Swainson’s hawks to move a short distance, a few hundred yards, to another available tree, but urban development usually results in the loss of the nest site. Your birds are a rare exception to that rule, but over time, statistically speaking, that nest site next to your office building will be abandoned as well. The genetics study confirmed that most birds hatched in one area will return to that same area to nest. Most wildlife has that instinct. But in all populations, a small fraction has the instinct to move somewhere else, sometimes referred to as exploratory dispersal. That’s how species expand and find new habitat to occupy. But it is very dangerous to leave an area where you know where to find food and understand your competitors and predators. The vast, vast majority of exploratory dispersal leads to death and no new occupied range.
What is a typical number of eggs laid? At least 1 and up to 4. It varies from individual to individual, and from year to year, depending on the availability of food (prey). The same goes for the number of chicks that hatch and fledge (fly for the first time). In bad nesting years, 50% or more of nests might fail, and the average number of chicks per successful nest might be 1. In great nesting years, nest success might climb to 90% and almost every nest seemingly has 3 chicks; in those years we have found rare nests of 4 young. In an average year, most nests have 2 chicks, fewer have 1 or 3, and some nests fail.
Do both male and female take care of the babies? At what point do the parents leave them? Or do they ever fully leave them? Both parents incubate the eggs, although the female does the vast majority of that work. The male does the vast majority of hunting to feed the chicks and the female. The female takes the prey from the male and feeds the chicks (and herself). Both defend the territory, the nest, and the young equally (on average). The female also has the important role of shading the chicks. Even when the chicks are quite old and almost ready to fly, the female on hot days may still stand over them, wings spread somewhat, protecting them from the heat of the sun, especially if the nest is exposed to direct sunlight. Between 2 and 4 weeks after the chicks fledge, the adults and chicks go their separate ways. The chicks will find other first year Swainson’s hawks to congregate with, and as a group they will roost and forage together.
How long do the siblings stay together? That isn’t known, or at least I have never heard or seen that in a study. There is some speculation that adult males and females, as well as first and second year birds, may winter in slightly separate areas, but if they do, there is obvious overlap between the three groups.
How can you tell male from female? The females are almost always larger than the males within a given population. Individuals of both sexes in the north (or colder climates) are bigger on average than those in the south (or hot climates). Weighing them is the easiest method if you know the average weights for the area. If you see them side-by-side, you’ll usually notice the larger size of the female. In the Central Valley, the darker of the pair is usually the female (looking at the belly, chin and wing lining in combination). That rule breaks down, though, when both are very light or very dark. You can also use the cues for duties listed above to determine their sex, and of course, when they copulate, the male is above.
How does their prey die? Is it the impact of the talons? Or a killing bite like a cat? It just seems that they don't have enough mass or strength to make a very clean kill. If you’ve had a Swainson’s hawk grab your arm or leg with their talons as I’ve had, you’d be quite impressed with their strength. The kill is primarily from puncture during the grab, followed by biting/tearing vital organs and blood vessels. Mice are fairly fragile animals in contrast to larger mammals. If it is any consolation, mice, voles and gophers often get a bite in before they are dispatched, which is a good reason to do it quickly.
Do they eat any vegetation? Or are they strictly carnivores? Although Swainson’s hawk pellets (regurgitated indigestible matter compressed into a pellet) have been found with grass in them, I have never seen a study that indicated they purposely eat vegetation that wasn’t inside or stuck to a prey item, or picked up incidentally while eating their prey. Although they primarily eat small mammals, they also eat snakes, lizards, and small birds during the nesting season, and on their wintering grounds they eat just about anything they can find and kill, including lots and lots of insects. In Argentina they are known as the grasshopper eagle.
How long is the lifespan of a Swainson Hawk and do they mate for life? How many more years might we hope to watch this amazing pair of Swainson Hawks return to this site to raise their babies? As for the lifespan of a Swainson’s hawk, it varies greatly. Mortality can be quite high for pre-breeding birds, those less than 2 years old. I don’t know the exact numbers, but population biology theory would suggest the number of survivors that reach breeding age are a small fraction of those born. That’s nature. If they reach breeding age, they probably live an average of 10 years in the wild. We’ve re-trapped birds that were banded as youngsters, and in one instance, a male Swainson’s hawk was more than 20 years old. Yes, the species appears to mate for life, that is, the life of either partner. Once one of the pair dies, the second is likely to pick up a new mate. It is sometimes difficult to determine that unless the birds are banded, or the new mate is very different in color than the old one. It’s difficult to know how long this, or a subsequent pair combination, will return to this site. In rural nest sites, a site may be adopted immediately if abandoned by another pair (usually due to death). Urban nests are rare because the vast majority of Swainson’s hawks are unwilling to nest near that much human disturbance, and have to fly so far to find prey. So when this pair dies or otherwise abandons this site, it is unlikely another pair will adopt it amidst the human threat (in their mind), and as development continues and good hunting fields relatively near are lost.