Introduction

It’s a basic ecological question -- what factors influence wildlife abundance? -- but the answers are not always immediately obvious.  The number of birds at Klopp Lake can change dramatically.  Migration and breeding activity affect bird abundance between seasons and from day to day, but what causes the conspicuous changes in bird numbers from hour to hour?  Although there are many potential explanations, or hypotheses, for this phenomenon, we focused our study on how bird numbers respond to three factors: changes in tide height time of day  human activity at the lake. The major types of birds visiting Klopp Lake are shorebirds (e.g., Marbled Godwits), waterfowl (e.g., Bufflehead), and gulls (e.g., Western Gulls).  Klopp Lake is an important habitat for these birds, and their presence at the lake is a source of enjoyment for human visitors.  Therefore, understanding the factors influencing their abundance has recreational and management implications.

Methods

Data were collected 4 times per day for two weeks from February 25 to March 8, 2000.  Birds observed were recorded in one of three categories: shorebirds, waterfowl, and gulls.  We also recorded the number of humans and dogs at the lake and obtained tide height data from the National Oceanographic and Atmospheric Association.  Using tide predictions, we scheduled observations so that no single time period had more high or low tides than any other so that we could distinguish effects of time  and tide independently.

Results

Effects of tide

As tide height increased, so did the number of shorebirds.  This is probably because shorebirds rest on the shores and islands of Klopp Lake once their prime feeding areas have submerged under rising tides (Burger et al. 1977).  As tides fall, shorebirds leave Klopp Lake to feed on newly exposed mudflats in Humboldt Bay (Dodd and Colwell 1998).  There was relatively little effect of tide on shorebirds and gulls.

Effects of time of day

Shorebirds were more abundant during the morning than in the afternoon.  This result is difficult to interpret.  Although we made sure that the number of high tides was similar among all four of our time periods, morning high tides tended to be higher than afternoon high tides in our study, which could have contributed to especially large numbers of shorebirds on morning counts.  Alternatively, shorebirds may truly respond to time of day, perhaps by feeding elsewhere in Humboldt Bay during warmer afternoons.

 

Waterfowl and gull abundance was greatest during the late morning and early afternoon time periods.  In contrast to other ponds at the Arcata Marsh and Wildlife Sanctuary, Klopp Lake has relatively little aquatic vegetation, which is preferred food for “dabbling” ducks  (e.g., Mallards).  Therefore, the lake may have been used as a resting area by dabbling ducks at mid-day (Paulus 1988).  Gulls probably also used Klopp Lake for resting at mid-day; they fly to feeding areas in Humboldt Bay at others times of the day.  Diving ducks (e.g., Ring-necked Ducks) fed in Klopp Lake during the late morning and early afternoon and made regular flights at dawn and dusk, perhaps to and from nocturnal roosting areas (Higley 1989).

Effects of human disturbance

Human activity level had no effect on the number of birds at Klopp Lake.  This result was encouraging because it suggests that human enjoyment of the lake does not disrupt birds’ use of the habitat.  However, during the course of our study, visitors showed respect for wildlife by enjoying the lake in a non-disruptive manner.  It is possible that people acting disruptively could adversely affect birds at the lake.

Conclusions

In conclusion, fluctuating bird abundance at Klopp Lake seems to be a response to natural cycles such as tidal changes and daily movements of birds between habitats.  Human activity had little effect on the number of birds visiting the lake. These results may help visitors schedule trips to the marsh to view particular types of birds by understanding that: shorebirds are most abundant at high, morning tides waterfowl and gulls are most abundant during the middle of the day. Future students may wish to further examine effects of human activity as well as other factors such as weather and seasonal variations in bird numbers.

Literature Cited

Burger, J., M. A. Howe, D. C. Hahn, and J. Chase.  1977.  Effects of tide cycles on habitat selection and habitat partitioning by migrating shorebirds.  Auk 94:743-758. Dodd, S. L., and M. A. Colwell.  1998.  Environmental correlates of diurnal and nocturnal foraging patterns of non-breeding shorebirds.  Wilson Bulletin 11:182-189.  Higley, J. M.  1989.  Waterbird and raptor use of the Arcata Marsh and Wildlife Sanctuary, Humboldt County, California.  Thesis, Humboldt State University, Arcata, California, USA.  Paulus, S. L.  1988. Time-activity budgets of non-breeding Anatidae: a review.  Pages 135-152 in M. W. Weller, editor.  Waterfowl in winter.  University of Minnesota Press, Minneapolis, Minnesota, USA.

 

This study was conducted by the HSU Wildlife Techniques class, spring 2000.  Poster committee (l to r): Dr. Matt Johnson, Mitch Carlson, John Benson, Jeffrey Long, Rachel Smith, Matt Hamman, and Suzanne Schut.  Special thanks to Dr. Jeffrey Black, teaching assistant Sean Matthews, and the rest of the class.                            an HSU Wildlife publication