12. Threats on Breeding Grounds

ALAN impacting timing of daily activity at breeding ground

• if theres more light, would extend hours they sang at

• on average, birds sang an hour/day

• Larger eyes, migrants starting earlier, ending later with more ALAN (big eyes is adaptation for lower light levels, so ALAN would have stronger effects)

Possible effects of birds singing more due to ALAN

• if birds sing longer/earlier/later may affect reproductive output (spending more time on this as trade off to something else which could reduce output)

Why birds don’t have defences against cowbirds

• native birds have not evolved recognition or counter-strategies

Cowbird impact on forest birds

• Cowbirds find host nests in the forest, but feed in adjacent agricultural fields

• In small forest fragments it is common for > 50% of songbird nests to contain cowbird eggs

• Many nests contain 2-3 cowbirds eggs and only 1-2 host eggs

3 levels of evidence that cowbirds are linked to songbird population declines

1) Nest level

2) Population level (correlation)

3) Population level (experiment)

1) Nest level description

• parasitized nests have far lower reproductive success than non-parasitized nests

• cowbird parasitism causes a drop of ~ 50% in reproductive success

2) Population-level (correlation)

• when cowbird populations decline, reproductive success of host populations increases

• Cowbirds have declined in northern Missouri since the 1970s because forest cover has increased. Southern Missouri is control

• number of cowbird eggs per nest (intensity) also decreased sharply over time as cowbird pop decreased in north

• in south, as cowbirds decreased, fledglings increased

3) Population level (experiment)

• removed cowbirds and observed vireo recovery increased

Fragmentation and nest predators

• Agricultural matrix provides artificial source of food and increases population sizes of mice, chipmunks, raccoons, skunks, crows, blue jays, etc.

• In small forest fragments it is common for over 60% of nests to be eaten by predators

Edge effect and nest predation

• Small fragments have a higher proportion of edge habitat than large fragments.

• Overall predation rate decreases strongly as distance of the nest to the edge of the forest increases

• edge habitats had more predation by snakes and raptors

Source definition

• Average female produces more young than necessary to replace herself when she dies

Sink definition

• Average female produces fewer young than necessary to replace herself when she dies

survival bottleneck of fledglings

• if get to be about 10, survivorship is very high super vulnerable at young age (0-5)

Estimated of cats in U.S.A

• In the U.S. there are an estimated 84 million pet cats and an additional 30-80 million un-owned (ie stray/feral) cats

Name of cat monitoring program

The National Geographic & University of Georgia Kitty Cams (Crittercam) Project

Methodology and Results of Kitty cams to measure kill rate/cat

• Ask home owners to report dead animals that their cats bring home

• Put Kitty-cams on pet cats (n = 55) to monitor their behaviour; found that only 44% of these cats hunted wild prey

• Hunting cats brought back only 25% of their prey to owner’s house - so owners greatly underestimated kill rate

cat kill rate results and diversity in what they hunt

Kill Rate was 0.3 birds/hunting cat per week or about 12 birds/hunting cat/year

calculating US-wide bird mortality by cats

• 84 million pet cats in US, of which 50-80% hunt wild animals (according to owners)

• 30-80 million un-owned cats, of which 80-100% hunt wild animals

• Kill rate of birds varies greatly among studies due to different prey base & methods

• 1-34 birds killed/pet cat/year

• 24-51 birds killed/un-owned cat/year

Model number of birds killed by cats each year

• Ran the model thousands of times by randomly selecting different values for all the variables, within the observed range

• Most models predict 2-3 billion birds are killed by cats in the US alone

• Number of small mammals killed even higher

Total Bird mortalities by cats in Canada

200-300 million birds/year

Overall bird mortality stats

• Cats : 200-300 million birds/year

• Window Collisions: 25 million birds/year

• Wind Turbines: 24,000 birds/year