Now here’s an amazing (coevolutionary) story…

Yellow-rumped Caciques (members of the blackbird family) enjoy a love-hate relationship with Giant Cowbirds in the Pantanal of southern Brazil.

The players in this story:

Yellow-rumped Cacique

A couple of Yellow-rumped Caciques near their woven nest, in a Cacique apartment house.  Caciques are highly social birds and nest together in high density, each individual female weaving multiple nests (to confuse the predators) in close proximity to her neighbors.  Males help defend the nests from intruders, but they are too busy attracting more females to their apartment house to help with the nest building.

The multiple, hanging, woven nests of the Yellow-rumped Cacique form a huge complex in tall trees in the Pantanal of Brazil.  There may be as many as 200 nest structures in one large “apartment house”.  Note the presence of a wasp nest in the middle of the image — looking like a gray loaf of bread.

Giant Cowbird

Giant Cowbirds, like other cowbirds, are obligate nest parasites, using many other (and smaller) bird species as hosts for their offspring. Female Cowbirds furtively explore potential nests when owners are absent, and then quickly lay an egg and leave. The chick is usually larger than the host chick and grows more quickly, outcompeting the host chick for food delivered by the female.

Well, it’s not exactly love between Caciques and Cowbirds, but the Caciques exhibit a great degree of tolerance for the presence of female Cowbirds who deposit an egg in Cacique nests, forcing the single mothers to raise their {adopted} large, greedy chicks along with their own.  BUT the benefit of this relationship is that the Cowbird chicks have a great fondness for botfly larvae that are ever-present in bird nests, and will pick them off themselves and the Cacique chicks as well, for a little extra nourishment.  In the end, Cacique nests with high degrees of botfly parasitism and Cowbird chicks in the nest actually fledged more offspring.

Yellow-rumped Caciques only lay two eggs in their long, hanging nest, but usually are only able to fledge one chick.  Between botfly parasitism and nest predation by larger birds or monkeys, there is low fledging success in this species in much of its range.

But that’s not the end of the story.  There is another player:  Polistine wasps, medium-sized wasps related to yellow jackets that build a large, papery nest, are aggressive hunters of flying insects, like botflies.

Caciques seek out trees with wasp nests, leaving a respectful distance between their nests and the wasp nest to avoid arousing the insects.  Wasp arousal, however, IS triggered by monkeys and larger-bodied chick predators like Toucans, Caracaras, or Great Black Hawks that move the branches and nests in their search, and most often, the predators are driven away by the wasps.

Yellow-rumped Caciques aren’t always so lax in allowing Giant Cowbird females access to their nest.  In fact, if the nest apartment house is built near a wasp nest, the wasps act like botfly exterminators, and with less risk of botfly larvae debilitating their chicks, the Caciques actively defend their nests against Giant Cowbirds.  The wasps have also been observed to drive away monkeys who try to raid the Cacique nests. So, there is a double benefit of nesting near wasps.

And there is still more to the story.

Yellow-rumped Caciques also seek out big trees on islands in their riparian habitat to build their apartment houses, because Caimans or Giant Otters in the rivers will attack and eat anything that tries to cross the river onto the island, such as snakes that are adept at climbing trees with an appetite for chicks.  In these situations, Caciques will also be intolerant of Giant Cowbirds presence.  Female Caciques stay close to their apartment house, foraging individually rather than in a group, so that there are always large numbers of them present to mob predators and Cowbirds and drive them off.

Adolescent Yellow-rumped Cacique females may not breed until they are three years of age, but stand guard at the nest apartment house ready to drive away predators and Cowbirds.

An adaptable breeding strategy, coupled with their high degree of sociality for nest defense, helps the Yellow-rumped Cacique survive in a world full of predators and parasites.

Be quiet, you big baby!

There is a lot of racket in the backyard these days — cowbird chicks have fledged and are once again driving their Chipping Sparrow parents crazy with their persistent chirping demands to be fed.  I have recently written about the Cowbird’s “Mafia strategy” for getting other species to raise its chicks (you can click here to (re)read that post).  The unfortunate Chipping Sparrows seem to be regular hosts for the Cowbirds in my backyard.  This is the third year I have seen the diminutive little sparrows foraging intensely to satisfy the appetite of chicks that are twice their size.

Cowbird chick waiting patiently (?) for its foster parent to return with something good to eat.

Cowbird chick waiting patiently (?) for its foster parent to return with something good to eat.

Doing a little preening while waiting.

Doing a little preening while waiting.

Then a foster parent arrives and the cowbird chick goes into a frenzy, fluttering its wings and tail, chirping loudly.  The Chipping Sparrow looks little intimidated, doesn't it?

Then a foster parent arrives and the cowbird chick goes into a frenzy, fluttering its wings and tail, chirping loudly. The Chipping Sparrow looks a little intimidated, doesn’t it?

The chick looks big enough to swallow its foster parent whole.

The cowbird chick looks big enough to swallow its foster parent whole.

And back the parent goes to find something else for this voracious eater it has mistakenly raised.  Chipping Sparrows feed their own young, as well as their foster kids, insects, even though they themselves eat a varied diet of seeds, fruit, and insects during the summer.  As I watched these host-brood parasite interactions, I saw the adults spend a lot of time hunting damselflies, flies, bees, etc. in the grass around the base of the buckeye tree in which this big baby was sitting.

On this trip, it looks like the sparrow nabbed a damselfly.

On this trip, it looks like the sparrow nabbed a damselfly, judging from the long, slender abdomen and wings sticking out of its beak.

No problem fitting this tiny little damselfly in that great big maw.

No problem fitting that tiny little damselfly in the chick’s great big maw.

Even though they get fed by both parents, they just keep screeching for more.

Even though the chick was getting fed by both parents, it just kept screeching for more.

The Chipping Sparrows didn’t spend all of their energy feeding the cowbird chick — I could hear them feeding a couple of their own babies that were hiding in the shrubs on the side of the yard.

Much smarter than a fifth grader

There is no doubt about it — Cowbirds exhibit ingenious methods of manipulating other bird species into raising their little brown-headed offspring.   Cowbird eggs have been found in the nests of 220 bird species, ranging in size from hummingbirds to raptors.

Brown-headed Cowbirds are n

By watching the nesting activities of particular target bird species, Brown-headed Cowbirds time their reproduction with that of their host, forcing the host to raise their chick for them.  A single female, laying just one egg in a host nest, may produce up to 36 eggs in a breeding season.

Cowbird chicks are usually larger than the host species chicks, hatch a little earlier, and grow a little faster, so they may consume the bulk of the food that parents bring to the nest, to the detriment of the host’s own chicks.

The end result is a single large Cowbird chick that follows its host parent (in this case, a Chipping Sparrow) around incessantly begging for food.

The end result is a single large Cowbird chick that follows its host parent (in this case, a Chipping Sparrow) around incessantly begging for food.

Some species, such as Robins, Gray Catbirds, and Brown Thrashers, physically eject cowbird eggs from their nest and seem less susceptible to nest parasitism. Blue-gray Gnatchatchers desert their nest if they find cowbird eggs in it and Yellow Warblers simply build another nest on top of the one that was parasitized, hoping the cowbirds don’t find them again.

However, Cowbirds have learned to retaliate against some egg rejectors with “mafia-like” behavior.

Ever watchful.  It's not enough to just find a nest in which to lay an egg

Ever watchful, Cowbirds not only monitor the nesting activity of the host, but watch what becomes of their eggs in the host nest.  If the host rejects the foreign egg, cowbirds return to the nest and destroy the host eggs!

And the “mafia” behavior works.  Prothonotary Warblers that rejected cowbird eggs managed to raise only one chick because cowbirds punctured the other eggs or threw the host’s offspring out of the nest,  Those that tolerated the presence of a cowbird chick in their nest raised three of their own chicks.  So, it pays to be tolerant of a nest parasite if you’re a Prothonotary Warbler.

Cowbirds have even learned to “farm” their host species by manipulating the hatch time of host eggs to match their own chick’s hatch time.  This is achieved simply by destroying those host eggs in the nest laid earlier than their own.

Who knew that becoming an accomplished nest parasite required such intelligence?

Let me introduce…

Venezuelan Troupials, Icterus icterus

The Venezuelan Troupial, which has been introduced to several Caribbean islands.

This striking member of the blackbird family is the national bird of Venezuela. It lives in dry scrubland, savannah, and residential backyards where it consumes, fruit, seeds, insects, and other birds eggs and chicks. In addition it is an obligate nest parasite, so it can only lay its eggs in other birds nests for them to raise. A real cheater, in other words.

Both sexes exhibit the distinctive orange and black markings, front and back, looking like an oriole.

Both sexes exhibit the distinctive orange and black markings, front and back, looking like an oriole.

Feed me!

All over the backyard, a persistent squawking and chirping alerts us to the presence of newly fledged young birds.  In fact, the noise of these youngsters has entirely replaced the melodic singing of breeding adults.  Weaning is a difficult process as any parent knows, because the youngsters understand that the best way to get something is to annoy the parent until they finally give in (sound familiar?).  Some examples I have seen in the past couple of weeks:

Four fledgling House Finches bombarded their dad, urging him to find them some food.  At one point they all sat on the fence directly above him, noisily chirping away.  He looks unconcerned about it.

Four fledgling House Finches bombarded their dad, urging him to find them some food. At one point they all sat on the fence directly above him, noisily chirping away. He looks unconcerned about it.

Dad led them over to the bird feeder, showing them I suppose that this is where you eat.  But they wanted him to stick the seed in their mouths, by chirping continuously and fluttering their wings.

Dad led them over to the bird feeder, showing them, I suppose, that this is where you eat. But they wanted him to stick the seed in their mouths, by chirping continuously and fluttering their wings.  Two young sat next to him, two others perched right above him.

They haven't quite connected the appearance of seed with food -- still too fixed on being fed instead.  A cowbird looks on; she didn't have to worry about feeding her babies, because she left her eggs in someone else's nest!

They haven’t quite connected the appearance of seed with food — still too fixed on being fed instead. A female cowbird looks on; she didn’t have to worry about feeding her babies, because she left her eggs in someone else’s nest!

This is what happens to some of the local birds when cowbirds are in the area.

A Cowbird chick begs incessantly from its foster "parent", a Chipping Sparrow.

A Cowbird chick (left) begs incessantly as it follows its foster “parent”, a Chipping Sparrow around the yard.  Note that the Cowbird is is almost twice the size of the sparrow, and probably developed at the expense of some of the sparrow’s own chicks.  Chipping Sparrows apparently can’t tell the difference between their own eggs and one twice as large!

Brown-headed Cowbird egg in the nest of an Eastern Phoebe (By Frankie Rose (This is my own work.) [Public domain], via Wikimedia Commons

Brown-headed Cowbird egg in the nest of an Eastern Phoebe (By Frankie Rose [Public domain], via Wikimedia Commons).

Cowbirds are infamous as nest parasites, and lay their eggs in the nests of several different species of small birds.  This isn’t as easy as it sounds because they have to find the nests of suitable species, determine the incubation stage of that nest, and deposit one egg, at will, while parents are off the nest.  If they lay the egg too early, the parents might abandon the nest with a strange egg in it; if they lay the egg too late, their own egg might not get sufficient incubation time for full development.  Having found an appropriate nest, then they have to quickly lay their egg in it.  For comparison, it takes a hen more than 1/2 hour to get her egg out.

Why do they do it?  Brown-headed Cowbirds once followed bison herds across the prairie, moving nomadically with the herds and feeding on the insects flushed by many hooves and the seed heads of prairie grasses.  Nesting in one spot was thus impossible with such a nomadic lifestyle, and “egg-dumping” in other species’ nests became advantageous.

My question is...how does this little guy know who he is?  They obviously avoid the problem the ugly duckling experienced when he fell out of his swan parents' nest.

My question is…how does this little guy know who he is? A cowbird chick apparently avoids the problem the ugly duckling experienced when he got displaced from his swan parents’ nest.

Color me beautiful!

Need I say more?  Scarlet Tanagers out-red even the cardinals, and the black wings and tail put the gorgeous meter over the top.

Need I say more? Scarlet Tanagers out-red even the cardinals, and the black wings and tail put the gorgeous meter over the top.

This bird was busy hunting some big bugs in the buckeye tree, but posed briefly in a slightly open area so I could take his picture.

See bug...

See bug…

Grab bug...

Grab bug…

Eat bug...

Eat bug…

Pose, while digesting.

Pose, while digesting.

I feel fortunate to have gotten these photos.  Scarlet Tanagers (not members of the tanager family as the name would imply, but instead belong in the cardinal family!) are notorious for “skulking” among the large leaves at the top of tall deciduous trees.   They prefer to breed in large tracts of undisturbed forest, where they can escape the nest parasitism of cowbirds.  Cowbirds wait until the tanagers have left the nest unoccupied, then pitch out one of the tanager eggs and deposit one of their own.  The tanagers can’t tell the difference, and end up raising the interloper at the expense of one of their own young.

Like many of the spring migrants this year, the tanagers are late arriving in Minnesota to set up breeding territories.  By mid-summer, they will start moving south again, eventually making their way through Central America to western South America, where they merge with flocks of tropical (real) tanagers.