Dr. Charles van Riper III

Professor, School of Natural Resources and the Environment
USGS Research Scientist Emeritus  

Sonoran Desert Research Station
125 Biological Sciences East ~ University of Arizona ~Tucson, AZ 85721-125
(520) 626-7027 ~ (520) 670-5100 fax ~ (520) 491-0721 call

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Breeding Information

PAIR FORMATION: Birds migrate north, arriving at breeding sites and forming pairs from June to mid-July.

NESTS:   In 1965, three nests in Arizona were described in detail (Hamilton and Hamilton 1965).  In the nests that they examined, there was practically no intertwining of components.  The anchoring branchlets (51 willow twiglets in one case) were laid across one another in a random pattern. The nest was then framed with additional twigs, again with little or no weaving. A lining was composed of strips of bark, leaves and more small twigs.  The overall dimensions were somewhat oblong and asymmetrical.  Frame size was 275 X 360 mm and 290 X 365 mm at two nests; Cup size was 102 by  275mm, 84 X 112mm and 115 X 140mm at three nests. Rosenberg et al. (1991) described the nests from the Lower Colorado River Valley as platforms of sticks placed on horizontal branches in dense foliage from 4.5 to 14 m above ground.  Willows were the preferred substrate, but nests were also found in screwbean mesquite and salt cedar. 

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EGGS: Eggs are ellliptical-oval to oval in shape and about equally rounded at both ends.  The smooth shell is without gloss and ranges from 'pale glaucous green' to 'pale florite green'. Measurements of 53 eggs averaged 30.4 by 23 mm, ranging from 34.64 by 23.11 and 33.53 by 25.40mm (Bent 1940)

CLUTCH: The clutch size is 3 or 4 eggs, sometimes one and rarely 5 (Bent 1940).

INCUBATION PERIOD:  Incubation period is 10 or 11 days (Hamilton and Hamilton 1965). Incubation is primarily by the female.

YOUNG: Young are altricial. Both adults care for young, feeding primarily insects.  Young leave nest when 6 to 8 days of age, crawling from branch to branch.  Parents continue care for an additional 3-4 weeks (Halterman 1991).


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