Avian Predation during Migration and in Mexico

[Mona Miller]

http://www.monarchwatch.org/update/2006/0430.html#8
April 30, 2006 Monarch Watch Update
"I love to get to get together with monarch biologists to share stories
about our favorite organism. The stories go better with beer of course and
it was while enjoying a local brew in the company of several monarch
biologists at the Entomological Society of America conference at Alisomar in
2004 that Bob Pyle related his observation of seeing a red tailed hawk chase
down a monarch. I'm sure I nodded in understanding, hopefully concealing my
disbelief - after all Bob has a reputation for veracity. A hawk chasing a
monarch! Would you believe it? The red-tails I watch catch rodents. The
young red tail pictured below is holding a wood rat it caught just outside
my window at Monarch Watch. Anyway, I was willing to cut Bob some slack but
now must apologize to Bob for ever doubting him - even silently. Recently, I
received a note from Doug Green, a hawk watcher from Cleveland, Tennessee,
who observed broad-wing hawks feeding on monarchs while in flight during
their migration in September. To quote Doug: "perhaps the broad-wing hawks,
flying in 'kettles' of hundreds or thousands, have chosen to migrate with
the monarchs, for an 'in-air' energy source". This is certainly an
interesting observation and it is plausible that both hawks and monarchs
would be using the same thermals in which to soar and glide as they migrate.
If you know of any hawk watchers, you might get the word to them that any
interactions they observe among hawks and monarchs should be noted and
reported."

In the roost along the way and in Mexico, Monarchs are preyed upon by birds:
http://www.monarchwatch.org/biology/pred2.htm

[dannus plex]

I thought Monarch adults remained somewhat noxious tasting and cardiotoxic due to the effects of the milkweed plants they consume in the larval stage. The quote states the hawk 'chasing' but not specifically consuming the Monarch. Are Monarchs 'unpalatable' to birds? In Dr. Urquhart's book, he questions Batesian mimicry. Could someone clarify all this for me??

[Mona Miller]

The hawks do eat them and so do many other birds, mice, other insects. My naturalist friend Denise Gibbs, told me that she has witnessed the hawks eating Monarchs. The toxins are concentrated in the wings. All they have to do is avoid the wings.

[Jim]

The toxins are concentrated in the wings. All they have to do is avoid the wings.

Actually, toxins are stored in the exoskeleton as well as the wings, so feeding on the body of the monarch is problematic for many would-be predators. Some predators seem to have some tolerance to the toxins and others have devised ways of "unzipping" the monarchs to feed on soft tissue while avoiding the toxic cuticle. Pretty amazing

[Mona Miller]

Doesn't the amount of toxicity in the monarch have to do with the milkweed it consumes. I read somewhere that the spring monarchs use the southern milkweeds which are more toxic and the fall use the northern milkweed which are not as toxic. But, then I also read that the sequester toxins from asters and goldenrods. I've seen preying mantis eat them without any problem. Birds eating or not is a learned behavior so at least one must be eaten before they decide not to eat anymore.

[dannus plex]

Thanks Jim,
Also in regard to predation in the species, is there a Batesian minimcry effect with Monarchs and Viceroys? I thought there was, but after reading Urquhart's book on Monarchs, it sounds like he wasn't sure. What is the thinking now?

[Mona Miller]

http://ufbir.ifas.ufl.edu/chap28.htm
"Many relationships involving what were once thought to be Batesian mimicry are being reevaluated. The most common example, the Viceroy butterfly (Limenitis archippus), once thought to mimic the Monarch (Danaus plexippus), has through further investigation proven to be as distasteful to birds as the Monarch (Ritland and Brower 1991). Thus, the Monarch and Viceroy are Mullerian mimics. That such a disparity could go unnoticed for so long shows to what extent mimicry needs to be studied. However, after sidestepping this non-Batesian example in every text and article reviewed, several pure Batesian examples stand out...."

[Mona Miller]

http://www.colostate.edu/Depts/Entomolo ... 9/pike.htm
"The high predation rates of monarch butterflies in overwintering sites in Mexico may also be due to the monarchs having low levels of these chemical toxins. Malcolm and Brower (1989) did research on monarch sequestration of cardenolides. They found that the most toxic milkweed plants (A. asperula, A. viridis, and A. humistrata) that the butterfly larvae fed on are found in the southern part of the USA, which is where the overwintering generation first migrates to and lays their eggs. This first generation of spring monarchs carry the most toxins. As these offspring migrate north they lay eggs on less toxic milkweed (A. syriaca) but these larvae can sequester cardenolides from this species of milkweed more effectively than from any other species of milkweed so their toxic levels are intermediately high. When the autumn generations start their migration to Mexico, they have fairly high levels of cardenolides, but after they have flown a considerable distance the levels drop dramatically. So when they arrive in Mexico, they have low levels of cardiac glycosides making predation more successful.

Alonso-Mejia and Brower (1994) found that there may be another reason cardiac glycosides are low in overwintering monarch butterflies. They showed that the concentrations of these toxins decrease as the butterflies age, regardless either of the initial amounts of cardiac glycosides or their chemical structures. Alonso-Mejia and Brower’s (1994) research shows that the butterflies that emerge in late May will be emetic, but their cardiac glycosides will dwindle below the emetic dosages. Nonetheless, since considerable overlapping of generations occurs over the summer, automimicry is possible: the freshly emerged individual monarchs can serve as noxious models for the older individuals that have lost most of their toxins. Therefore, as monarchs age during their individual lifetimes, they can progress from unpalatable models to palatable automimics."