The Effects Of Cryptic Versus Non-Cryptic Eggs On Predation Rate Of Ground-Nesting Birds

The Effects of Cryptic versus Non-cryptic Eggs on Predation Rate of Ground-nesting Birds

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Determining the effect of cryptic, brown eggs versus non-cryptic, white eggs on the predation rate of ground-nesting bird species.

Abstract

Various interactions occur between organisms. Predation is a crucial interaction in which one species benefits by consuming another. I conducted this experiment to determine if crypsis affected the predation rate of eggs in ground-nesting birds. I hypothesized that cryptic eggs would reduce predation rate, therefore less cryptic eggs should be found. I created a simulation in which eight students randomly “laid” 96 eggs in a predetermined area, and seven predators hunted eight eggs each. The predators did not see the eggs being laid, and were given an egg quota so that all the eggs were not found. Also, they were directed to pick up the first eight eggs they noticed and not look for a particular kind. Half the eggs were white marshmallows representing the non-cryptic eggs, and the other 48 were the cryptic, cocoa covered marshmallows. The resulting values of eggs found were 37 non-cryptic and 18 non-cryptic. However, the expected amount for each type of egg was the total number of eggs to be found divided by two, or 27.5 eggs. I used a Chi-Square test to analyze my results further and found the observed chi-square value, which was 7.453. The p-value was between 0.01 and 0.005. The p-value supported my hypothesis, and with an alpha level of 0.05, I concluded that there was a significant difference between the number of cryptic and non-cryptic eggs found. Then I deduced that this difference was probably because the cryptic eggs were less noticeable to the predators, decreasing the predation rate of them. My results concurred with past experiments and promoted the notion that cryptic eggs may reduce predation rate among ground-nesting birds.

Introduction

Species of organisms can be associated due to biotic or abiotic factors, or a combination of the two. A species’ environmental niche is their habitat, or physical environment, and it also includes the interactions they participate in with other species (Vliet, 1993). In other words, all of the total biotic and abiotic resources that an organism uses in their environment represent their niche (Campbell and Reece, 2005). There are numerous types of interactions among species such as neutralism, amensalism, commensalisms, and mutualism (Vliet, 1993). However, three of the most prevalent interactions are competition, parasitism, and predation (Vliet, 1993). Predation occurs when one species benefits while another suffers because the first species kills and consumes the second species (Vliet, 1993). In this lab, I used mock predators and fake bird eggs to study the effect of crypsis on the interaction between birds and their predators. I replicated the predation of ground-nesting birds’ eggs (Vliet, 1993).

This topic is relevant because predation is often a leading cause of nest failure in birds; there is a strong force compelling species to adapt ways that reduce nest predation (Vliet, 1993). Predators profoundly impact the population of their prey, birds in this case (Vliet, 1993). The population growth rate of birds, or other species, can greatly decrease or the organism may not be able to persevere when the predator population is high (Vliet, 1993). For example, small rodents may significantly influence shorebirds’ reproductive success (Dobkin et al). Over an observed three year period, between six and 34 percent of Spotted Sandpiper eggs did not hatch because of damage cause by mice (Dobkin et al). Most commonly, damage to an egg occurred overnight in the way of two piercings the approximate width of mouse incisors, and subsequently, the parent disposed of the egg (Dobkin et al). The mice (predators) gravely affected the hatching rate of these shorebirds.

Prey must adapt or evolve in response to their predators to counter regulate their population (Vliet, 1993). Mimicry is very common among organisms. For example, the coloration of the Scarlet Kingsnake mimics the coloration of the coral snake (Gross et al). Both snakes have horizontal bands of red, yellow, and black. The difference is that the red and yellow bands are touching on the coral snake skin, and the red and black bands are adjacent on the Kingsnake. The relatively harmless Kingsnake intends to be recognized as an extremely poisonous coral snake by predators to avoid predation (Gross et al).

Similarly, nest predation rate can probably decrease by several means including inaccessible nests, nest defense and camouflage, or reducing nesting time. Predators use camouflage to blend in and attack their prey with the element of surprise, while prey use it to hide from their predators by blending in with their environment (Gross et al). This cryptic coloration makes it difficult for a predator to see their prey (Campbell and Reece, 2005). Different bird species utilize different camouflaging techniques to deter predators. The killdeer are ground-nesting shore birds known for their ability to disguise their eggs and nests, which are often constructed in relatively open areas (Corio, 2006). Their nests have minimal material because their eggs resemble pebbles because they are speckled and blotchy and need little to no further guise (Killdeer). They coax predators away from their nests by faking an injury, a broken-wing, which successfully draws predators away from their eggs (Corio, 2006). Another type of ground-nesting bird, the ovenbird, creates an oven-like nest covered with organic materials to make the nest blend into its surroundings (Corio, 2006). The eggs are successfully hidden from the predators view, even though they are not cryptically colored.

Based on past evidence, I hypothesized that cryptic bird eggs would reduce nest predation by making their detection by predators more difficult (The Wilson Bulletin). I predicted the number of cryptic eggs found or eaten by predators would be less then the number of non-cryptic eggs found by predators. According to the Wilson Bulletin, with continual incubation of fourteen days, the estimated success of a bird’s nest with cryptic coloration of eggs was 75% versus the 55% for nests with white eggs (The Wilson Bulletin). For the experiment, plain white Ringed Turtle-Dove eggs were used, and the cryptic eggs were the same white eggs spattered with brown tempera paint (The Wilson Bulletin). Also, on separate occasions Tinbergen and Montevecchi placed cryptic and white eggs in meadows to test the effectiveness of cryptic eggs in lowering predation rates of the avian predators (The Wilson