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Hatching Patterns in Birds: Testing a Critical Assumption

Anne Ford, Mark Stanback, and Katie Lannon

Department of Biology, Davidson College

Abstract

Researchers have traditionally defined hatching asynchrony with respect to the difference in size between the largest and smallest chicks in the brood. However, this definition ignores the potential influence of the other chicks in the brood. To test the validity of this definition, and thus the validity of experiments in which asynchrony is artificially exaggerated, we manipulated Eastern Bluebird (Sialia sialis) broods into two distinct patterns (multiple runt and single runt) with identical total asynchrony. Treatment type did not affect the growth of either "large" or "small" chicks. Although our results tentatively validate assumptions and techniques of previous experiments, larger sample sizes are necessary to provide a critical test.

Introduction

Depending on when they initiate incubation during egg laying, birds can produce hatching patterns that vary from complete synchrony to extreme asynchrony. Because hatching asynchrony often results in brood reduction (stravation of the later-hatched chicks), various hypotheses have been proposed to explain the evolution and maintenance of different hatching patterns. To compare the adaptiveness of these alternative strategies, workers often examine chick growth in broods manipulated to be either synchronous or asynchronous. In such experiments, "asynchrony" is usually defined with respect to the size of the largest and smallest chick, without regard for the relative sizes of the other chicks in the brood. To test the validity of this practice, we compared the condition of large and small chicks, respectively, in broods containing single vs. multiple runts.

Methods

Manipulated Eastern Bluebirds broods into two patterns: Multiple Runt (MR) and Single Runt (SR)

Measured chicks every other day for nine days

Paired each Multiple Runt brood with a Single Runt brood based on date of manipulation

Compared chick size in the paired broods using a paired t-test

Artificially Created Hatching Patterns. A newly hatched chick was switched with a 2 day-old chick, resulting in a total age difference of approximately 48 hours in both broods.

Runts in paired SR and MR brood did not differ significantly in size. We compared single runt size to the mean size of the multiple runts. Error bars represent 95% confidence intervals, n=6 paired broods, paired t-test.

Large chicks in paired SR and MR brood did not differ significantly in size, despite a trend for average large chick body size in SR broods to be greater than that of single large chicks in MR broods. Error bars represent 95% confidence intervals, n=6 paired broods, paired t-test.

Conclusions

Chicks of equivalent size raised in broods with different size hierarchies did not differ significantly in their growth

Findings tentatively validate the use of relative size differences between the largest and smallest chicks as a measure of hatching asynchrony

An increase in sample size planned for 1998 should clarify these results

Acknowledgements: Thanks to Dawn Houseton and Janet Beamer for assistance and to the Howard Hughes Medical Institute for financial support.

This experiment was conducted as a summer undergraduate research project.