Table 4.
Fitted parameters of a three-level (pair–nest–egg) model predicting Rock pigeon embryonic sex ratio (logit link function) for both eggs of the clutch as laid (egg number 1 or 2), assuming circannual cyclic variation of sex ratio, i.e., with a period of 12 months; additionally, the results are presented separately for egg 1 (showing significant circannual sex ratio variation), respectively, egg 2 (no significant circannual variation), based on two-level models (pair–nest)
| Predictors | B (coeff.) | Wald χ 21 | p |
|---|---|---|---|
| Including egg 1 and 2 (n = 478) | |||
| Constant | −0.877 | ||
| A = sine (2 × pi × month of laying/12) | 1.834 | 15.189 | 0.0001 |
| B = cosine (2 × pi × month of laying/12) | −0.874 | 2.878 | n.s., 0.0898 |
| C = egg number (1 or 2) | 0.613 | 6.428 | 0.0112 |
| A × C | −0.942 | 10.75 | 0.0010 |
| B × C | 0.285 | 0.815 | n.s., 0.3681 |
| Only egg 1 included (n = 239) | |||
| Constant | −0.427 | ||
| A = sine (2 × pi × month of laying/12) | 0.815 | 14.311 | 0.0002 |
| B = cosine (2 × pi × month of laying/12) | −0.540 | 5.252 | 0.0219 |
| Only egg 2 included (n = 239) | |||
| Constant | 0.133 | ||
| A = sine (2 × pi × month of laying/12) | −0.045 | 0.051 | n.s., 0.8213 |
| B = cosine (2 × pi × month of laying/12) | −0.277 | 1.579 | n.s., 0.2089 |
Cyclicity of the data was tested by harmonic modeling using a linear version of a sine wave (see “Materials and methods”). For fitted curves based on this model, see Fig. 2. The significant interaction between egg number and (sine-transformed) laying month shows that circannual variation in sex ratio varies significantly between egg 1 and 2