AND COWBIRD HOST: THE YELLOW WARBLER
PATTERNS OF PARENTAL CARE BY A COMMON SONGBIRD
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?
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