A Ph.D. research project by Christine Stracey
Many other animals eat the nests of birds. Common predators at our study sites include crows, blue jays, grackles, snakes, raccoons, and rats. Populations of all of these predators also vary between town and countryside.
Data from 2005 - 2006 are consistent with
our hypothesis that urban areas have lower nest predation rates than
non-urban habitats. However, in 2007 and 2008 there were no difference in
predation rates between urban and non-urban areas.
In order to resolve the nest predator paradox we have to figure out who's actually eating these nests! Check out the Mockingbird Video Clips for these results.
The best way to test our urban food enhancement hypothesis would be to experimentally manipulate food. In 2008 we tried a food supplementation experiment. We placed suet feeders in the territories of half the urban pairs and half the non-urban pairs. We wanted to compare how the urban versus non-urban birds responded to additional food in urban versus non-urban locations. We were planning on measuring how many eggs they laid, how much the eggs and nestlings weighed, etc. Unfortunately, none of the non-urban birds would eat from the feeders, so the experiment did not work! Why do you think only the non-urban birds would not eat from our feeders?
Because we were unable to experimentally manipulate food, we have to rely on correlations between habitat and food. Insects are the primary food resource of mockingbirds during the breeding season. Unfortunately, insect availability for mockingbirds is diffucult to quantify and so instead we measure a suite of reproductive parameters that reflect food availability including: territory density, clutch size, hatching failure, nestling mass, feeding rates, nestling survival (from causes other than predation), and fledging success.
When we look at each study site as a population we find that significantly more young are produced per hectare in town. This is because mockingbirds have smaller territories in town and thus there are more nesting pairs per hectare.
If we look at differences between urban and non-urban individual pairs of mockingbirds we find very few differences. We found no significant differences between urban and non-urban habitats in hatching success, nestling mass, nestling survival and feeding rates, or number of fledglings produced per successful nesting attempt. We found that in one year pastures had larger clutch sizes and non-urban nestlings were fed larger prey items. These differences, however, had no significant effects on nestling mass or on fledging success. Perhaps mockingbirds are able to feed their nestlings later into the night due to light pollution, enabling them to compensate for smaller prey?
What does all this mean?
The higher abundances of mockingbirds and higher production of fledglings per unit area likely reflect greater food abundance in urban areas. Mockingbird pairs, however, were distributed among habitats such that food was roughly equally available in all territories. These results are different from most previous studies, which have found lower food availability in urban areas as measured by these variables. Most previous studies have investigated species that are granivorous (seed-eating) and that are not territorial. We hypothesize that urban food mismatches may be rare in species that establish breeding territories each spring, but may be more likely in non-territorial species.
We have been color marking the adult mockingbirds at our study sites so we can recognize individuals. We place four colored leg bands on each bird we catch - three plastic bands and one aluminum USFWS band. Each bird receives a unique color combination, which allows us to track individuals from year to year. We can use return rates to calculate annual survival rates of mockingbirds, however our estimate of survival is a minimum because if a bird disappears from our study sites it didn't necessarily die - it might just have flown away (this is called dispersal).
In 2005 we were still working out the best way to catch the mockingbirds and so our sample sizes were low. It appears that in 2006 birds returned to the urban sites at a higher rate than to Ordway (the only non-urban site we banded at in 2005). However, we must very cautious saying this because our sample sizes were so low. The pattern for returns in 2007 was roughly similar to 2006. For the most part urban birds came back at a higher rate. However, there are a few notable exceptions: Very few birds returned to Butler Plaza and all four birds at the Beef Research Unit returned.
Mockingbird nests are sometimes parasitized by the botfly Philornis porteri. The adults look similar to house flies, but they lay their eggs in birds' nests. When the botfly eggs hatch the larvae burrow under the skin of the nestlings and feed on the nestlings. When the larvae are ready to metamorphose into adult flies they build a pupal case (like a coccoon) in the lining of the nest (see photo below).
We typically think of food and predators as the main factors that regulate population sizes of animals. Parasites, however, can also be very important in regulating population size. In addition, parasites are frequently the first species to be negatively affected by urbanization because of their complex life cycles and thus could potentially be used as bio-indicators to evaluate the health of an ecosystem. So, do botflies decrease with urbanization and could this be one of the reasons for the mockingbirds success in urban habitats? With that in mind we were interested in 1) the fitness consequences of botfly parasitism on nestlings and adults and 2) the distribution of botflies relative to urbanization. We collected mockingbird nests after the nestlings left the nest and dissected the nests for pupal cases of the botflies to get a measure of how many nests were parasitized as well as the parasitic intensity.
1) Fitness consequences of parasitism
We measured nestling body condition (mass regressed against tarsus), and nestling survival (as part of our food study) and related that to whether or not the nests were parasitized and the number of parasites per nestling. To investigate if there were any changes in parental behavioral as a result of parasitism we watched our nest videos and quantified feeding rates and adult pecks at the nest or nestling. We found no significant effects of parasitism on nestling body condition, survival, or feeding rates. We did, however, find that adults pecked at their nests more frequently as the number of parasites in the nest increased.
2) Distribution of parasites relative to urbanization
We found strong habitat effects on the intensity of botfly parasitism, but this parasitism was not related to urbanization in a direct manner. Parking lots and wildlife preserves had very low levels of parasitism, while residential neighborhoods and pastures had significantly higher parasitism prevalence. Unfortunately, we know very little about this species of botfly (we don't even know what they eat!) and can only speculate as to why we saw this pattern.
Urban environments are characterized by increased anthropogenic (human-produced) noise. This increased low-frequency urban noise may interfere with the song transmission of birds. Birds use songs for communication, territory defense, mate attraction, and predator avoidance. Thus, birds must be able to adapt their songs to be heard in noisy areas to keep their territories and retain their mates. Previous studies have deminstrated that some species of birds are able adjust their song according to their habitat. First, we are testing for differences in the pitch at which urban versus non-urban mockingbirds sing.
Now that we have established that urban mockingbirds do indeed sing at a higher pitch than non-urban mockingbirds, we are continuing to record mockingbirds, as well as the numerous species they mimic, in order to gain a greater understanding of the mockingbird s capacity to adjust its song in response to noise pollution.
The effects of light pollution on wildlife has received relatively little attention. Researchers have demonstrated that light pollution can increase mortality of nestling sea turtles and migrating birds, but the extent to which other species are affected in unknown. Preliminary data indicate that mockingbirds in parking lots feed their nestlings later into the night than mockingbirds in residential areas and non-urban habitats. Stay tuned...
We have been working on the
development of a series of lesson plans that incorporate active scientific
research into the middle and high school classroom. Our goal is to help
convey the process of scientific inquiry to students by exposing them to
the type of work that scientists conduct. Science is not simply about
learning all the information in a textbook, but rather about discovering
things that no one yet knows.