Nest predation

History of deer colonization - Response of vegetation to deer - Response of animals to deer and squirrel - Deer biology

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Invertebrates - Songbirds - Nest predation and reproductive success

Nest predation and reproductive success

Aim & scope - Where & when - How - Main results - Conclusion - Scientific Publications

Aim and scope

We hypothesized that the presence of red-squirrel combined to the changes in habitat quality caused by the effects of deer browsing on the vegetation might have negative effects on nesting success for the songbirds that do breed on islands with deer.

To test this hypothesis we investigated 2 questions:

How is nest success affected by the presence of squirrel ?
How is songbird reproductive success affected by browsing history ?

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Where and when

The predation of artificial nests was studied in 1993, 1995 and 1996 on:

Lost, Low, South Low, (no deer/no squirrel)
Kunga, Ramsay, Reef (deer/no squirrel)
Limestone, Louise, Lyell (deer/squirrel).

Squirrel abundance was monitored between 1994 and 2000 on East Limestone (deer/squirrel).

Natural nests were monitored between 1995 and 2000 on:

Low (no deer/no squirrel)
Reef, West Skedans (deer/no squirrel)
Vertical Point (Louise Island), East limestone (deer/squirrel).

Mist nets were set up at fixed stations and monitored in 1998 and 2003 on:

Low (no deer/no squirrel)
West Skedans (deer present for less than 20 years/no squirrel)
Reef (deer present for more than 50 years/no squirrel)
Vertical Point, East Limestone (deer present for more than 50 years/squirrel).

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How

Nests monitoring
We monitored natural nests to assess the influence of predator assemblages on reproductive success. Losses of natural nests to predators were compared between islands with and without squirrel.
We baited 506 artificial nests with quail eggs on 9 different islands with different predator assemblages to assess the influences of predator assemblage as well as of habitat structure and quality on reproductive success.

Squirrel census
Squirrel abundance was monitored by means of censuses along transects.

Mistnet captures
We captured songbirds with japanese mistnets just after songbird reproduction and calculated the ratio of juveniles to adult among the birds that were captured. Sex age and body dimensions were recorded for each bird captured

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Main Results

Nest predation by squirrel

Evidence from natural nests	Evidence from artificial nests

Predation rate was higher in presence of squirrel.

Squirrel density

	Squirrel abundance varied with habitat type (Spruce/Hemlock forest or Alder/Spruce forest) and year.
Survival of artificial nests was lower in habitats dominated by Spruce and fluctuated in a cyclic fashion presumably in response to cycles in cone production, the main food source of squirrels.

Habitat quality, nest concealment and nest predation

In presence of squirrel, vegetation cover around the nest (and thus nest concealment) had little effect on nest predation by squirrels. Squirrels have a "smell and search" predation strategy which is independant from vegetation cover around the nest.

In the absence of squirrel, corvids were the main predators. They rely on visual cues. Predation risk was low and increased with decreasing vegetation cover.

Reproductive success

Preliminary results suggest that in the absence of deer and squirrel reproductive success is higher during warm years than during cold years. In presence of deer, reproductive success is low in both cold and warm years. The additional effect of squirrel on reproductive success is inconclusive.

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Conclusion

Temporal and spatial variation in risk of nest predation may prevent any tight adjustment of breeding strategies to predation risk.

Inconclusive results suggest that deer may prevent songbirds to take advantage of years with good weather to improve their reproductive success

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Scientific Publications

Jean-Louis Martin, Mathieu Joron, 2003, Nest predation in forest birds: influence of predator type and predator’s habitat quality, OIKOS 102: 641–653, 2003 – PDF

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