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Using Candlers to Determine the Incubation
Stage of Passerine Eggs


Table 1. Descriptions of the daily incubation stages of American Robin, Lark Bunting, and Red-winged Blackbird eggs with comparative information for Mourning Dove eggs.a

Day Yolk or embryo Air cell Blood vessels
0 Interior is uniformly light and clear; yolk is indistinct located equidistant from sides of egg. Small or possibly not visible if egg is still warm. None
1 Yolk becomes more clearly defined and more orange in color. Identifiable, but remains small. None
2 Small embryo visible, perhaps late in day. Embryo appears as small dark spot (red) in center of egg. Yolk rotates freely and floats toward upper side with embryo on top. Remains small. A few narrow vessels attached to embryo.
3 Embryo is now discernible as a curved dark expanse in the center of the egg. With good conditions the beating heart can be seen, but embryo moves little. Quite apparent in all species. Extend from embryo to form a network across one side of the egg below air cell.
4 Embryo expands in size; in good light embryo can be seen moving quite rapidly. Similar to day 3. Dark vesseled areas separated from clear areas by line which usually extends vertically mid-egg.
5 Egg interior is largely obscured by vessels, but embryo can be seen and it moves regularly. Increase in size for all species. Vesseled areas near air cell expand and clear areas decrease.
6 Embryo indistinct often appearing as two dark lobes. Larger and easily seen. Completely encircle the egg interior.
7 Embryo seen as a dark shape below air cell. Embryo still shows some movement. May tip slightly to side in passerines. Continue to expand within the shell.
8 Development similar to day 7; embryos of passerines show little movement. Similar to day 7. Similar to day 7.
9 Due to embryo growth, details become difficult to observe. Prominent for all species. Vessels can be seen at each end of egg.
10 Egg is almost totally opaque; only & light at air cell. Embryo does not At greatest extent. Mainly obscured.
11 move until just before hatching, when bill breaks inner shell membranes.    
12 Passerine embryos chip through egg shell and hatch.    

aEgg development for all three passerine species is similar throughout incubation period and for Mourning Doves through day 7. However, air cells of Mourning Doves tilt down the side of shell, whereas passerine air cells extend across the top of shell. After about 7 days, embryo development in Mourning Doves lags behind embryo development in passerines because Mourning Doves require two additional days to hatch. As a result, passerine embryos stop movement on day 9, whereas Mourning Dove embryos stop movement on day 11. Also, it is possible to view details in passerine eggs through day 9, but eggs become mostly opaque on days 10 and 11, whereas it is possible to view details in Mourning Dove eggs through day 11, but eggs become mostly opaque on days 12 and 13.

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