Abstract:
The Common Tern (Sterna hirundo) is a ground nesting colonial seabird. Terns
rely primarily on small prey fishes which they obtain through plunge diving for their
survival as well as the survival of their offspring during the breeding season. The zebra
mussel (Dreissena polymorpha) is a small bivalve mollusk that invaded North American
waters in the late 1980's. Through its suspension feeding, the zebra mussel has the ability
to alter the entire aquatic ecosystem, ultimately leading to a reduction in pelagic organisms
including small prey fish. The objective of the study was to determine what (if any)
indirect effects the invasion of the zebra mussel has had on fish prey captured by terns.
The study took place in two separate two-year periods, 1990-91 and 1995-96 on a
concrete breakwall off the north shore of Lake Erie near Port Colborne, Ontario. Daily nest
checks revealed clutch initiation dates, egg-laying chronology, hatching success and
morphological egg characteristics (length and breadth). Behavioural observations included
time each sex spent in attendance with its brood, the frequency of feeding chicks and the
prey species composition and size fed to chicks as well as to females (courtship feeding).
Egg sizes did not differ between study periods, nor did feeding rates to chicks,
suggesting that food was not a limiting resource. Terns spent less time with their broods
(more time foraging) in the 1995-96 period. However, they also had significantly larger
broods and fledged more offspring. The time of each individual foraging trip decreased,
suggesting that fish were easier to obtain in 1995 and 1996. Lastly, kleptoparasitism rates
decreased, suggesting that the costs of foraging (time, energy) actually decreased as fewer
birds adopted this strategy to compensate for what I assumed to be a lack of available food
(fish). The only significant difference between the periods of 1990, 1991 and 1995, 1996
was a change in diet. Terns delivered significantly fewer rainbow smelt and more emerald
shiner in 1995 and 1996. However, the average size of fish delivered did not change.
Thus, there was little impact on prey captured by Common Terns in Lake Erie since the
invasion of the zebra mussel.
