Gruiform Mating

To investigate the mating system of the Gruiformes, we will look at two species: The King Rail, in the family Rallidae, and the Sandhill Crane in the family Gruidae.

The Sandhill Crane

The King Rail

The Sandhill Crane, which is a much larger bird than the King Rail, has a slow life history.  They have been known to live over 20 years, and thus do not reach sexual maturity until between ages 3 and 5.  Some will find mates before they reach sexual maturity, but will not mate until maturity is reached. Sandhill cranes are monogamous and will generally only change partners in the case of the death of one partner.  The females select males based on plumage and call. During mating, they perform an elaborate dance ritual and engage in what is know as their unison call, where they call to one another during the dance routine.  Once copulation has occurred, both males and females will incubate the eggs for about a month.  The clutch size is, on average, 2 eggs/clutch for the Sandhill Crane.  The young are born precocial and leave the nest within 24 hours, usually one following one parent and one following the other.  Parents separate chicks because the more aggressive chick often will attack the more docile one.  The chicks fledge after 90 days and will remain with the parents until the following breeding season, which occurs one year later.

The mating dance of the Sandhill Crane

A typical clutch of 2 Sandhill Crane eggs, speckled for camouflage.

A precocial Sandhill Crane chick with an adult

The King Rail, in contrast, is a smaller bird with a shorter life history.  It only lives between 5 and 9 years, and thus much reach sexual maturity much earlier than its Gruidae cousin.  Mates are generally monogamous and philopatric from year to year.  The male attracts the female during mating by strutting and exposing white undertail coverts.  He also presents the female with gifts of food.  The male is also responsible for picking a next site and building the nest, in contrast with the more sharing nature of the Sandhill Crane in those duties.  After copulation, the female usually lays between 8 and 11 eggs, many more than the Sandhill Crane couple produced.  The eggsare incubated by both parents for between 21 and 23 days, and the young are born downy black and semi-precocial.  Despite this fact, they are also able to leave the nest after a day to follow their parents.  They fledge within about 2 months, which is a month sooner than the Sandhill Crane chicks, and by 7-9 weeks, the adults have stopped feeding and looking after the chicks, in contrast to the Sandhill Crane couple, which will provide some measure of care for about a year.

King Rails mating

A King Rail brooding a large clutch

A King Rail with three chicks in tow

Thus, with the King Rail and Sandhill Crane, we see two different life history types that lead to two different reproductive strategies.  The Sandhill Crane, with a slow life history, reaches sexual maturity at a late age and will raise only two chicks a year and care for them for the entire year.  In contrast, the King Rail, with a faster life history, reaches sexual maturity earlier, raises about ten chicks, and stops providing care after a couple of months.  The urgency displayed by the King Rail reflects the fewer opportunities that it has in its lifetime to breed as compared with the Sandhill Crane, which can have slow or no reproduction in some years but still have high lifetime fecundity.

Sources:

• Benedictine University Museum: http://www1.ben.edu/museum/kingrail.asp
• International Crane Foundation: http://www.savingcranes.org/sandhill-crane.html
• Michigan State University Extension: http://mnfi.anr.msu.edu/abstracts/zoology/Rallus_elegans.pdf
• National Wildlife Federation: http://www.nwf.org/Wildlife/Wildlife-Library/Birds/Sandhill-Crane.aspx
• Nebraska Game and Parks Division: http://outdoornebraska.ne.gov/wildlife/guides/migration/sandhill.asp
• Ohio Division of Wildlife: http://ohiodnr.com/Portals/9/pdf/pub382.pdf