please help me out with these 4 questions.

AND COWBIRD HOST: THE YELLOW WARBLER

Study organism and methods

As we learned in our monarch butterfly lab, the costs of sexual reproduction can be

extreme – and may even result in death. Organisms that reproduce over a span of many years

may adopt different strategies of investment in current versus future reproducti ve effort. That is,

an organism may benefit by reducing their investment in current reproductive activities if that

reduction increases their lifetime fitness – for example, by allowing them to survive to breed

another year. This type of tradeoff is well i llustrated in migratory songbirds. Many species of

songbirds travel great distances between their wintering grounds and breeding grounds. These

birds are under intense time and energetic constraints during the breeding season because they

must produce and rear their young (an energetically demanding task) and regain energetic stores

before migrating to their wintering areas.

If we imagine two populations of the same species breeding in two distinct habitats (one

with abundant food, and the other with sev ere food limitation) we can imagine that where food is

abundant most birds will be able to rear a full brood of healthy offspring (e.g., 4 young) in time

to migrate successfully. However, parents facing food limitation may adopt a strategy of brood

reducti on , in which they reduce the number of offspring they raise to maximize the condition of

the offspring they do fledge, which should lead to a higher probability of survival. Brood

reduction may also maximize a parent’s chances of surviving to breed again b y reducing the total

amount of effort invested in a single brood. If parents are forced to limit their investment to only

some of the offspring in a brood, which offspring will provide the highest benefit from their

investment? Many studies have shown that larger offspring tend to receive more food from

parents than smaller offspring do. The proximate cause of this pattern may be that larger

nestlings are simply more competitive than smaller nestlings at receiving food from parents.

Parents may benefit fro m this scenario because it ensures survival of the fittest offspring (the

most competitive) when food is limiting, but allows for survival of lower -quality individuals

when food is abundant (as there are enough resources for all young).

Although larger nestlings often attain more food than their smaller siblings, male and

female parents may have some control over food allocation patterns, and each may benefit

differently from the ways in which they allocate food to offspring. For example, while a female

can be certain that all of the eggs in a nest are hers (because she laid them), males are left with

some uncertainty of their paternity. Birds that nest at high densities may share territory

boundaries with up to 4 other pairs of birds of the same species . In this case, mated males may be

able to increase their fitness by obtaining what are referred to as “extra pair copulations” with

already -mated females from neighboring territories. If they are successful in fertilizing that

female’s eggs, they have suc cessfully duped her mate into caring for offspring to which he is unrelated. Because of the large fitness cost associated with rearing unrelated offspring, males

frequently guard their females closely during the egg -laying period (females lay one egg per d ay

for a total of 4 -5 days typically). However, this mate -guarding tends to decrease as the egg laying

stage progresses, so that the late -laid eggs are more likely to be sired by an extra pair male than

are the early -laid eggs. How does this relate to food allocation patterns? Some species of birds

begin to incubate their eggs midway through the laying stage so that earlier laid eggs get a head

start on later laid eggs. The result is that later laid eggs, which are most likely to have been sired

by another male, are last to hatch and frequently are the smallest nestlings in a brood. Thus, if

male parents are forced to reduce their care to a brood they would benefit by focusing their effort

on larger nestlings rather than smaller ones, which are less likely t o be related. However, because

a female can be 100% sure of her maternity, she should provide care to all offspring equally, or

provide more to the smaller offspring to make up for the male’s skew toward larger nestlings.

Today we will observe the patter ns of food allocation by male and female Yellow

Warblers ( Dendroica petechia ) in broods containing offspring of different sizes. Yellow

Warblers breed throughout much of North America during the summer months, and are a

common host to the brood parasitic B rown -headed Cowbird ( Molothrus ater ), which lays its

eggs in the nests of other birds. Cowbird eggs hatch more quickly than host nestlings because

cowbird eggs are smaller than what is predicted for their adult size. This results in a large size

difference between cowbird nestlings and host nestlings. As a result, the presence of a cowbird

nestling frequently reduces the host parents’ ability to fledge their own young because its size

advantage allows it to monopolize the distribution of food. Even in unpar asitized nests, Yellow

Warblers often have size differences within a brood because females begin to incubate after

laying the 2 nd or 3 rd egg, so the last -laid egg may hatch 1 -2 days after the others. These birds

place their nest close to conspecifics (i.e. , others of their species) and extra pair copulations

occur regularly.

We will observe parental food distribution patterns in nests that contain only Yellow

Warbler nestlings and in nests that contain Yellow Warbler nestlings with a much larger cowbird

nestling. Our objectives today will be to determine: (1) whether Yellow Warbler parents

allocate food disproportionately in favor of larger nestlings, (2) how the presence of a much

larger cowbird nestling affects these patterns, and (3) whether males diff er from females in their

allocation of food to the largest nestling. Our observations will be drawn from videotapes

recorded at nests in Montana. Each nestling was weighed before filming so they could be ranked

in terms of size and their bills were marked with black permanent marker so each nestling can be

identified on the video -tape. Worksheet pg 3 & 4 Protocol: Data Analysis

The data you collected from the videos represent one -hour samples of parental feeding

patterns among offspring in a parasitized and an unparasitized nest. Do the individuals in these

nests represent the population as a whole? To gain an accurate idea of how parental care

allocation varies across a population of Yellow Warblers you would need to sample a reasonably

large number (e.g ., 20) of parasitized and unparasitized nests. Also, by increasing the amount of

time each nest was observed you would increase your ability to accurately determine each

parents’ allocation patterns to each nestling.

We have provided you with data (found in the Warbler Worksheet pg 3&4) that were

collected from seven unparasitized and eight parasitized Yellow Warbler nests (some with three

nestlings and some with four). Each of these nests was filmed for three or more hours and each

feeding event was tran scribed in the manner we transcribed data on our videos in lab. For this

analysis we will focus on each parent’s allocation of food to the largest nestling – as measured

in the volume of food they each delivered to that nestling relative to others. (Note: this is NOT

the data you collected from the videos!)

For each nest the total volume of food that each parent delivered to the nest and the total

amount of food that each delivered to the largest nestling has been summed. These values can be

used to calcul ate the proportion contributed by each parent to the largest nestling. We can

compare these observed proportions to predicted values and determine whether parents are

allocating food evenly among nestlings. What value might we use as an expected proportion if

distribution were even? Remember, some of these nests have three nestlings and some have four.

When the expected proportion of food allocated to the largest nestling is subtracted from

the observed proportion the result is either a positive or a negat ive number. This number

represents the departure from the expected proportion (hereafter “departure”) of food that the

largest nestling received. A positive number indicates that the largest nestling received more

food than predicted if distribution were e ven, and a negative number indicates that the largest

nestling received less food than predicted if distribution were even. A zero value indicates that a

parent gave the largest nestling exactly what you would expect if food were distributed evenly

among n estlings.

Once the departure value has been calculated for each nest (and for each sex) in a

category (e.g. parasitized/unparasitized nests) we can calculate the mean across all nests to give

us an indication of the population average. Do female Yellow W arblers allocate food to cowbird

nestlings evenly, as predicted? Though proportions may not be appropriate to use in t -tests

(because they often do not meet a basic assumption – that data are distributed normally) we can

get a rough estimate of whether fem ale allocation patterns are significantly different from an even distribution by calculating the standard error (abbreviated as SE) of our mean. The

equation for calculating the standard error is:

- s/(sqrt(n))

where s is the sample standard deviation and n is the number of observations.

The standard error is a measure of dispersion around a mean – it is related to the variance (s2)

and sample standard deviation (s) and gives an idea of how the variance and mean are i nfluenced

by the number of observations (or the sample size, n). It can be used to compare a mean value to

a hypothesized value, such as our expected proportion of food attained. If the mean value is two

or more standard errors away from the hypothesized v alue (zero if distribution were even) then

the observed and hypothesized values are likely to be statistically different. Although this is not a

statistical test, this technique can allow one to describe how different two sample means are

which is appropri ate for our purposes. When comparing means, you should include error bars

that are equal to one standard error in either direction of the mean (+/ - SE). These means (+/ - SE)

will be roughly different if the difference between the means is greater than the sum of their

respective standard errors (i.e. their error bars do not overlap).

You can compare any two calculated mean values using the standard error as well. To help you

visualize, try calculating and plotting the mean departure in allocation between f emales of

parasitized and unparasitized nests. Although we may expect females to allocate food evenly in

both nests, is this prediction supported?

Now you can calculate the mean departure from expected values of food allocation to the

largest nestling for each of the other three categories (parasitized males, unparasitized females,

unparasitized males). Looking at the summarized data you might ask yo urself the following

questions:

- Does the cowbird have a different effect on allocation patterns than a large Yellow

Warbler nestling?

- Do males and females differ in their respective allocation patterns? Why might this

be?