Quantifying brood parasitism with crow data

Overview

Species interactions play a key role in determining local species diversity.  Given interactions such as  parasitism have negative consequences for one species, their should be evolutinary pressure to minimized these impacts.  Here students will use previously collected data to consider how scientists use a combination of observation, field-based experiments, and lab trials to determine why some relationships persist.  

Objectives

Students should be able to

• Define parasitismand explain its impacts on hosts

• Discuss why host species may evolve to evade parasites (or not!)

• Use pivot tables and graphs to explore data

• Explain error bars

• Consider connectiosn between observation, field, and lab studies

Background

The Ecology of Avian Brood Parasitism 

Introduction

Parasitism is similar to predation and herbivory in that one species benefits from the interaction while the other does not. However, while predators attempt to kill and eat another animal and the ingestion of plant tissue by herbivores may lead to death, parasites do not typically kill their hosts, though hosts species may be weakened.  Parasitism is a type of symbiotic relationship, meaning organisms live in close contact with each other.  Parasites may live in (endoparasites) or on (ectoparasites) their host.  

While many parasitic relationships are directly trophic-based, meaning the parasite (think of a tick or tapeworm) recieves nutrients, other forms exist as well.  For example, some parasites required differnet hosts at differnet life stages, so in addition to receiving resources, some parasites change the behavior of their host to help them get to their next stop. For example, parasitized killifish are more likely to stay near the surface and thrash than non-parasitized individuals, and this eventually leads to a higher predation rate by birds on parasitized fish. It turns out birds are the next host of the parasite!  

Other examples include ants that take larvae from other nests. These larvae eventually grow and help their new nestmates, even though they are not related to them. Other ant species actually have queens that actively look for other nests to infiltrate and take over.  Another unusual example of parasitism is brood parasites.  Some bird species lay their eggs in the nest of other birds. These  chicks are then raised by the nest owners, which means less food for their nestmates. In some cases, the parasitic species will even evict their nestmates from the next.

Given these impacts, there should be strong pressure via natural selection to stop brood parasites.  Interactions among the host and parasite thus lead to an evolutionary arms race, with each species constantly evolving to prevent or promote brood parasites.  For example, brood parasites may produce eggs that look  like their hosts, spread eggs among  multiple host species, and make thicker eggs that hatch quickly to help get their offspring in the nest. Host species may abandon parasitized nests or mob adults they encounter.  However, another option  may be for nest parasites to actually offer tradeoffs to hosts in specific situations.

The group first analyzed 16 years of data from nests observations. This data (annual nest data tab in the above worksheet) notes 

• the year of collection

• nest success (failed/succeeded)

• nest success -numeric

  • recodes failed as 0 and success as 1

• # crow fledglings 

  • how many crows fledged (successfully left the  nest)

• Parasitized? 

  • was the nest parasitized by cuckoos or not

This data is in long format, with each row being one unit (nest) and related data. Notice the size of the dataset (550 observations), though relatively small, is hard to interpret immediately. Given that, we will summarize the data numerically and graphically to answer some questions. We will do this using pivot tables and connected graphs. To get started, review informaiton on charts and pivot tables at Data Summaries in Google Sheets .

The annual nest data is copied into several tabs.  While we could put all our graphs and pivot tables in 1 (and this may be a better idea in the long run, in case we need to update the data!), for now we will use each tab to answer specific questions.

Use the first tab (annual nest data) to get familiar with pivot tables. One has already been added for you. Note how you can manipulate rows, columns, and cell values. 

1. Update the provided pivot table. Take a screen shot and explain what is being summarized and what it means.

You can now duplicate that spreadsheet (to keep the pivot table) or use the annual nest data - impact on number tab.  It's the same dataset.  In this sheet, let's explore how the presence of parasites impacts the average number of crows that fledge from a nest.  

2. What is your hypothesis regarding the impact of parasites on the number of crows that fledge from a nest?

Make a pivot table that answers this question. Note you can filter the table to only include nests that were successful (since nests may fail for a number of reasons!) .  Make  a chart that displays your new data, and include error bars on it (for help, see Data Summaries in Google Sheets and/or Population Statistics (simulation) 

3. Take a screen shot(s) of  your pivot table and chart. Explain what is being summarized and what it means. 

4. Does this support your hypothesis?

You can now duplicate that spreadsheet again (to keep the pivot table) or use the annual nest data - impact on success tab.  It's the same dataset.  In this sheet, let's explore how the presence of parasites impacts the probability of success for a nest (likelihood of it producing at least one chick).  In this case we'll use all the data. You can also use the nest success - numeric column as a value in your pivot table; since its coded as 0 and 1, the average will give you the proportion that succeeded.  Make  a chart that displays your new data. It won't/can't/shouldn't have error bars on it this time (why?)

5. What is your hypothesis regarding the impact of parasites on the overall success of a nest?

6. Take a screen shot(s) of  your pivot table and chart. Explain what is being summarized and what it means. 

7. Does this support your hypothesis?

In addition to the observed data, the researchers manipulated nests in field experiment. They added parasites to some nests (Cuckoo added), removed them from others (Cuckoo removed), and left others (both parasitized and not parasitized), alone.  This data is in the experimental data tab.  Based on what you've already done, analyze this data.

8. For each treatment, determine if it is a control or manipulation. How do they relate to each other, and why were they needed?

9. Take a screen shot(s) of  your pivot table and chart. Explain what is being summarized and what it means. 

10. Does this support your hypothesis?

11. Compare and contrast the outcomes of your three analyses.  Do the results of the different datasets seem contradictory? Explain your reasoning. 

12. Do you want to update your hypothesis or have any new questions?

While  working with the cuckoos, the researchers noted disturbed chicks gave off a nasty secretion.  They decided to see how this secretion impacted feeding by common crow predators.  They carried out an experiment where  three predators (other crows, cats, raptors (predatory birds)) were offered meat. Some of the meat had been sprayed with the natural cuckoo secretion, while other samples had not.  The predator avoidance data tab contains data from these experiments.  Analzye this new data.

13. Take a screen shot(s) of  your pivot table and chart. Explain what is being summarized and what it means. 

14. Looking at all the experiments, can you think of how/why this parasitic relationship may be tolerated by crows?

15. In a follow-up experiment, Soler at al. (2017) could not replicate these results. They carried out their experiment in a different region. Can  you make any guesses about what might be different in this area?