Comparative Biology and Behavior of Monogamous Pairs and One Male-Two Female Trios of Herring Gulls

In northern Great Lakes Herring Gulls one male-one female pairs are the predominate breeding association. We found that 0.6% to 1.0% of the nests from 1978 to 1981 were attended by one male-two female trios in which both females laid eggs. Individually marked birds in trios remated in successive years in 77% of the instances which was not significantly different from pair fidelity. Trios were comprised of a male and a primary female, which may have been a pair in previous years, and a secondary female bonded to the male. We found that although a trio male might potentially benefit from breeding with 2 females because of a larger clutch-size, this does not increase his annual reproductive success. Reproductive success of secondary females was much lower than primary and pair females' success. Reasons for the poor reproductive output of secondary females relative to pair and primary females were significantly lower egg fertility and lower hatching success of fertile eggs. Lower hatching success of fertile eggs in trios' nests largely results from termination of incubation of secondary females' eggs when the earlier laid eggs of the primary females' hatched. In addition, eggs may be shifted between cups of a double-nest. Fertile eggs of trio females may not hatch because both cups are not incubated continuously. The hypothesis that a secondary female joined a mated male because the increase in annual reproductive success offsets the costs was rejected. It is also doubtful whether a female lives long enough to produce sufficient young to make secondary status adaptive. Secondary status may familiarize the secondary female with the male when there are too few males available.