Sexual Selection in Barn Swallows:
Tail Feather Spots as an Indicator of Male Health
Abstract
The sexual selectivity of the monogamous barn swallow (Hirundo rustica), through manipulation of tail feather spots on males was investigated in the field in Hammonton, NJ. It has been well documented in past work that the avian world is very selective of their reproductive mates. In the barn swallow population tail spots are positively correlated with feather length and are sexually dimorphic. Male barn swallows were subjected to (1) full reduction of spot size, (2) partial reduction of spot size, or (3) no reduction of spot size. This study showed that tail spot size is positively correlated with the number of offspring produced. Thus, it appears that tail spots in the barn swallow can indicate the viability of males.

Introduction
Sexual Selection
Ø    Sexual selection is the process by which individuals of a given sex compete for access to individuals of the opposite sex. In most instances the female is the choosing sex (Moller & Szep, 2002).

Ø    It is theorized that the selection process is implemented in order to sire more viable offspring (Moller, 1991).

Ø    Certain male secondary sexual traits (i.e. tail length & horn size) are considered to be an expression of health and viability. Studies show that females prefer elaborate and exaggerated displays of these sexual traits (Badyaev & Qvanstrom, 2002).

Ø    In many bird species experiments have shown that females prefer males with larger tails (Fitzpatrick, 1997).

Test Subject

Ø    The barn swallow (Hirundo rustica) is a socially monogamous bird found worldwide, minus Australia and Antarctica, which colonizes in groups of up to 50 pairs (Moller, 1994).

Ø    The male swallow will arrive in the breeding ground prior to the female and establish a territory. This territory is usually several square meters and serves to attract females.

Ø    An average adult male barn swallow measures approximately 17 cm and weighs approximately 20 g.

Ø    Female barn swallows have been shown to prefer mates with longer and more symmetric tails (Moller, 1994).

Ø    During breeding season, males attempt to impress females with elaborate displays of their tales while in flight or while grounded.

Ø    Both males and females perform nest making together. However, males there is a known negative correlation between male parental investment and tail length

Method
Subjects


Ø    The study will be performed on the monogamous barn swallow (Hirundo rustica)

Ø    The study will be performed on three separate blueberry farms in Hammonton, NJ.

Ø    The barn swallows will be grouped into three separate colonies, one located on each blueberry farm. The colonies will colonies will consist of 10, 13, and 17 pairs of male and female swallows.


Procedures

Ø    Capture of the male swallows will be performed in early April as the birds begin to inhabit the breeding grounds through the use of mist nets.

Ø    Upon capture each male will be weighed through the use of a spring scale and measured in length via a standard ruler. A measurement will also be taken of the three longest tail feathers, and these measures will be averaged to provide a mean tail length.

Ø    For identification purposes, each male will be tagged with three separate metal bands. These bands will be of four possible colors: red, blue, silver & gold.  

Ø    Following the identification process each male swallow will be assigned to one of the following three treatments:

(1)    A full removal of tail spots through the use of non-toxic paint.
(2)    A partial removal of tail spots
(3)    No manipulation of tail spots

Ø    The males will then be set free and allowed to proceed with the breeding process.

Ø    Nests will be visited weekly over the course of two brood seasons, one year, to record start of laying, clutch size, hatch date and brood size.

Results
After gathering the data a repeated measures analysis of variance (ANOVA) will be ran to determine the correlation, if any, exist between spot size and reproductive success.  It is expected that there will be a significant different between treatment groups. While it is believed that brood size will not vary significantly, there is an expected difference between laying date and pairs having a second brood. It is believed that the control group will have an earlier date then either of the two treatments and will also have a higher percentage of having a second brood. These results will support the hypothesis that spot size is positively correlated to reproductive success.

Discussion
It has been established that in barn swallows tail length is a positive correlation of reproductive success (Moller, 1991). Females tend to mate with males who posses a larger tail then their small tail counter parts. It is believed the reason for this is that a large tail represents a male with more viable genes (Soler, Cuervo, Moller, & De Lopes, 1998). This study will show that this sexual selection process will hold true for tail spots as well. Males with larger, and more prominent tail spots will gain access to females more rapidly and more often. For this reason, females who mate with large spot males will mate sooner and thus sire a brood sooner then those females who mate with small spot males.

While this study will demonstrate the effects of spot size manipulation, further studies may analyze spot color. The normal spots on the barn swallow are white; a possible consideration would be if the spots were manipulated into different colors. That is, are females only concerned with the size of the spot or could there be a more underlying attraction to white only.

References
Badyaev, A. V. & Qvarnstrom, A. (2002). Putting sexual traits into the context of an organism: A life-history perspective in     studies of sexual selection. The Auk,     119(2), 301-310.

Dunn, P. O., Whittingham, L. A., & Pitcher, T. E. (2001). Mating systems, sperm competition, and the evolution of sexual         dimorphism in birds. Evolution, 55(1),         161-175.

Fitzpatrick, S. (1997). Patterns of morphometric variation in birds’ tails: length, shape, and variability. Biological Journal of         the Linnean Society, 62, 145-162

Moller, A. P. (1991). Sexual selection in the monogamous barn swallow (Hirundo rustica). I. Determinants of Tail         Ornament Size. Evolution, 45(8), 1823-1836.

Moller, A. P. (1994). Sexual selection in the barn swallow (Hirundo rustica). IV. patterns of fluctuating asymmetry and selection against asymmetry. Evolution, 48(3), 658-    670.

Moller, A. P. & Szep, T. (2002). Survival rate of adult barn swallows Hirundo rustica in relation to sexual selection and reproduction. Ecology, 83(8), 2220-2228.

Soler, J. J., Cuervo, J. J., Moller, A.P., De Lope, F. (1998). Nest building is a sexually selected behaviour in the barn swallow. Ecology, 56, 1435-1442.

Saino, N. & Moller, A. P. (1995). Testosterone correlates of mate guarding, Singing and aggressive behaviour in male barn     swallows, Hirundo rustica. Animal Behavior,     49, 465-472.