What makes the Northern Mockingbird (Mimus Polyglottos) an urban "winner"?

A Ph.D. research project by Christine Stracey

News:

We recently published a study experimentally demonstrating that mockingbirds are able to tell apart individual humans and aggressively respond to the person that they perceive to be a threat to their nest.  The study received quite a bit of media attention:

NY Times article: http://www.nytimes.com/2009/05/19/science/19obbirds.html
ScienceNOW article: http://sciencenow.sciencemag.org.lp.hscl.ufl.edu/cgi/content/full/2009/518/3
YouTube video: http://www.youtube.com/watch?v=BtoM-jiXT-Y
 

hatching

Background

Urbanization is one of the leading causes of species endangerment and most urban ecology studies focus on the species that don't do well in cities and towns.  However, there are some species that do very well in urban environments - the urban 'winners'. I am interested in determining what factors in urban areas allow these species to become so abundant. To address this I am comparing urban and non-urban populations of the Northern Mockingbird (Mimus polyglottos). I am specifically testing the following two hypotheses:

1.  Mockingbirds are abundant in urban areas because there are more food resources
and

2.  Mockingbirds are abundant in urban areas because nest predation rates are lower.

 

Urban
Residential Areas 
Parking Lots

Non-Urban
Pastures
Natural Areas

Data from the 2005 and 2006 breeding seasons indicated that nest predation rates were lower in urban areas relative to non-urban areas. However, in 2007 and 2008 there were no differences in predation rates between urban and non-urban populations. We will be continuing to collect data on nest predation rates this year and we will be placing video cameras on nests to determine the identity of the predators.

What we do:

  • First, we look in bushes and trees for mockingbird nests.

nest

  • When we locate a nest we check what's in there every 3 days.  We write down how many eggs and/or nestlings there are, describe what the nestlings look like, and weigh the eggs/nestlings.  With this information we can determine clutch sizes, the percentage of eggs that hatch, the number of nestlings that survive to leave the nest (fledge), and predation rates on the nests.

nest weighing nestling

  • We also catch the adult birds, weigh them, put leg bands on them, and then release them.  This gives us information on survival and movement of individuals.  To catch the adults we use mist nets near their nests that they fly into and get trapped.  This does not harm the birds!

      adult with bands

  • A large part of the study this year will be to determine who the nest predators are.  We know that a whole list of different things will eat mockingbird eggs and nestlings (crows, blue jays, grackles, raccoons, snakes, cats, rats, hawks, etc), but we don't know how frequently any of these are actually depredating (eating) the nest contents.  To figure this out we are placing video cameras on the nests to catch them red-handed!  See the video clips page for some videos from last year.

camera setup     camera

  • Last year we tried a food supplmentation 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 plannning 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 urban birds would eat from our feeders?

 

  • Over the past two years we have noticed that in urban areas we get attacked more frequently by the adults while we're checking their nests.  To experimentally determine if our city-slicker birds are more aggressive than the non-urban birds we present mockingbirds in both habitats with a model crow (a nest predator).  We then score the mockingbirds reaction to the model.

attack

  • Last year we started a new project on mockingbird songs. We want to know if mockingbirds in the city sing differently than mockingbirds in the country.  Other researchers have found that city birds sing at a higher pitch to overcome their noisy environment. However, these studies have been done with species that don't mimic sounds in their environment like mockingbirds do.  To do this we'll be walking around with recording equipment (parabolic dish, microphone, and recorder) and recording males singing. 

song recording 

  • Once we have digital recordings of the songs we use a sound software program (Raven) to create spectrograms of the songs.  Here's a sample spectrogram (the bird's song appears as the discrete bright colors): 

spectrogram

  • We are starting another new project this year looking at the effects of light pollution on when the birds start nesting in the spring and whether they change any of their behaviors in the evening or mornings when it would normally still be dark.

Four mockingbird eggs

Three mockingbird nestlings that just hatched and one egg that hasn't hatched yet.

It's hard work keeping the cows away from our mist nets