please help me out with these 4 questions.

please help me out with these 4 questions.
WEEK 10: 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?