Skip to main content

Supplemental Data on the Sex Ratio in Nestling Boat-Tailed Grackles

Robert K. Selander
6 (November-December)
From Field and Study
Online Text

Supplemental Data on the Sex Ratio in Nestling Boat-tailed Grackles.-In a recent paper (Condor, 62, 1960:34-44), I have shown that the nestling sex ratio in the Great-tailed Grackle (Cassidiz mexicanus flosopidicola) in Texas does not exhibit significant deviation from a normal Mendelian ratio of 5O:SO (table 1). This finding raised a serious doubt concerning the validity of an earlier report by McIlhenny (Auk, 54, 1937:274-295, and Auk, 57, 1940:85-93) of a nestling sex ratio strongly unbalanced in favor of females (2.51 females to 1 male) in the Boat-tailed Grackle (Cassidiz major major) in Louisiana. But, since it was recently discovered that the two grackles are separate genetic systems, being full species rather than merely races as previously supposed (Selander and Giller, Condor, 63, 1961:29-86), it seemed desirable to examine the nestling sex ratio in C. major.



      Form and locality                                                                    Number of nests                      Number of nestlings                              Males                     Females

Cassidix mexicanus prosopidicola                                                   

      Central and SE Texas                                                                        65                                             136                                             72                            64  

Cassidix major major

      Sabine Refuge, Louisiana                                                                   28                                              76                                              38                             38

Mixed sample’

       Near Vinton, Louisiana                                                                      14                                             32                                              14                              18

Totals                                                                                                    107                                           244                                           124                             120

 Nestling not identified to species; probably includes equal numbers of both species.



Between June 2 and 10, 1959, 21 nestlings of C. major major were collected at the Sabine National Wildlife Refuge, Cameron Parish, Louisiana. An additional sample of 55 nestlings was obtained in the same area on June 1 and 2, 1961. Dissection of these 76 nestlings showed a precisely equal sex ratio (table l), thereby confirming my previous supposition that McIlhenny’s figures were based on a faulty method of sex determination in which sex was judged by relative body size rather than by examination of gonads.

In both species of Carsidix it is now apparent that the existing imbalance in the tertiary (adult) sex ratio, in which females outnumber males, results not from a corresponding imbalance in the primary or secondary sex ratios but from differential mortality in the sexes beyond the nestling stage of development. The significance of this fact in relation to sexual dimorphism and the promiscuous, colonial breeding system of these grackles will be discussed in a forthcoming report on the biology of Cassidix.

I wish to thank Mr. Kent E. Myers, Director of the Sabine National Wildlife Refuge, for assistance in obtaining nestling grackles. This study was supported by the National Science Foundation (G15882) .-ROBERT K. SELANDER, Department of Zoology, The University of Texas, Austin, Texas, June 5,196l. 

Advanced Search