Larger groups provide more protection against egg predators.
The ani family shows up in groups of 2 adults (1M1F), 4 adults (2M2F), 6 adults, and 8 adults.
The groove-billed ani (Crotophaga sulcirostris) is an insectivorous black bird with a large, deeply grooved bill. It lives in marshes and open pastures in Central America and is related to the cuckoo. Family organizations of the anis may consist of twosomes with one female and one male, foursomes with two females and two males, sixsomes, and even eightsomes.
An ani foursome is two couples cohabiting a one-bedroom flat with one crib. Nests are built in thorny trees or vines. Each male guards one of the females. A female lays an egg every one to two days. The time from egg-laying to fledgling is three weeks.
Females lay fewer eggs each in 4some than in 2some by laying at different times but stop at same time.
Longer ovulator gets 37% of eggs raised, shorter ovulator gets 63% of eggs raised. Longer ovulator lays larger eggs that hatch earlier w/ better chance of survival.
The two females in a foursome wind up with four eggs total. A female in a twosome can also produce four eggs by herself. Thus the number of eggs laid per female is lower in a foursome than in a twosome.
In a foursome, the females start laying eggs at different times. The starter lays bigger eggs and has a longer time between successive eggs than the follower. Both females stop laying eggs at about the same time. Each female “tosses” out of the nest some of the eggs already laid by the other, with the follower winding up mothering an average 63 percent of the eggs and the starter only 37 percent of the eggs. After the tossing is over, four eggs are left in the nest.
Even though the follower mothers more of the eggs, she doesn’t necessarily successfully raise the most offspring. The starter lays larger eggs, which hatch earlier, so these chicks have a better chance of surviving than chicks from the follower.
All four birds in a foursome work together to provide for and protect the eggs.
Males divide day into unequal shifts based on oldest getting riskiest shift but also fathers most young.
Photo credit (C) Dave McCauley.
The males divide the day into unequal shifts. The oldest male incubates at night and most of the daylight hours. As a result, he also incurs the most hazard and the highest death rate. Yet he also fathers the most young.
The benefit from predator protection in a larger nesting group outweighs the disadvantages of a crowded family life.
The starter in a foursome produces the most young, a female in a twosome produces an intermediate number of young, and the follower in a foursome the fewest.
Photo credit (C) Joseph Morlan
By why should a female, who can lay four eggs in a twosome without worrying about a nest mate tossing her eggs out, bother living in a foursome? The answer is that the larger group provides protection against egg predators.
Once loss of eggs to predators is taken into account, the starter in a foursome produces the most young, a female in a twosome produces an intermediate number of young, and the follower in a foursome the fewest. For the anis, the benefit from predator protection of living in extended families of two couples outweighs the disadvantages of a rancorous life at home.
“It’s a cooperative soft goth bird!” —RXS
Citations
S. Vehrencamp, 1978, The adaptive significance of communal nesting in groove-billed Anis, Behav. Ecol. Sociobiol. 4:1-33.
B.S. Bowen, R.R. Koford, and S.L. Vehrencamp, Breeding roles and pairing patterns within communal groups of groove-billed Anis, Anim. Behav. 34:347-66.
Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. p. 57-8.