Sex ratio variation among the Masked Lapwing

Balwyn meeting report
October 2015
Invited speaker, Daniel Lees reported on his research topic. The following is an abstract of his report.

In the absence of data on sex ratios, conservation managers assume a 1:1 ratio which may not be the case, as males and females may exhibit differential dispersal, mortality, size, feeding behaviour and habitat use. With the appropriate data, management authorities could focus on threats to the limiting sex allowing the implementation of more successful management strategies (predator control, sign posting, exclusion zones and education)

In this study we examine whether sex-ratio variation is occurring in the Masked Lapwing Vanellus miles at the primary, secondary or tertiary level.

We radio-tracked 50 masked lapwing Vanellus miles chicks (50 broods) and compared body condition (growth rates; grams per day) and mortality (whether the male chicks were more or less likely to perish than female chicks) We also compared the body condition of all chicks within broods and to establish whether the time of the breeding season affects the brood sex ratio.

Chicks were no more or less likely to be male (or female) as the breeding season progressed. Male radio-tagged chicks (n = 27) were no more or less likely to perish when compared to female radio-tagged chicks (n = 21). At hatching, male chicks were no lighter or heavier nor had shorter or longer tarsi than female chicks Male chicks also grew no faster or slower when compared to female chicks.

This study detected no sex ratio variation or sex-biased survival among broods in the sexually monomorphic Masked Lapwing. This result was as expected and is in line with the Trivers and Willard (1973) hypothesis and the literature which details such variation exclusively among sexually dimorphic species.

Sex ratio variation among the Masked Lapwing

Monitoring survival of free-living precocial avian young is difficult. Perhaps the most promising methods available to determine survival are: (a) a combination of radio-tracking and frequent investigator brood visits or (b) targeted visits timed to mark young after hatching and then again to confirm fledging.

Our aim is to understand if the process of radio-tracking and the associated frequent visits negatively impact chicks when compared to infrequent targeted monitoring visits.

We radio-tracked 50 masked lapwing Vanellus miles chicks and compared body condition (scaled mass index) and within-brood mortality to examine whether attached radio transmitters influenced chick body condition or survival. We also compared the body condition of all chicks from radio-tracked broods to chicks subjected to targeted monitoring to examine whether investigator visits influenced body condition.

Within broods, there was no difference in body condition or mortality between chicks with and without radio-transmitters. Similarly, there was no difference in body condition between broods subject to radio tracking or targeted monitoring.

In agreement with the literature on the ‘glue on’ method of backpack radio-transmitter attachment, the body condition of lapwing chicks was not affected by radio-tracking compared with the targeted monitoring technique. Smaller, less robust and possibly less habituated species may still be negatively affected by radio-tracking. Radio-tracking seems an ethical and practical method for attaining an improved understanding of cryptic life history stages such as chick-rearing in shorebirds.

Note

A Scaled Mass Index (SMI; a mass length relationship; Peig and Green 2009) was used to characterise body condition. All statistical tests were conducted in R (2015) with GLMMs of body condition conducted using the package ‘nlme’ (Pinheiro et al. 2014) and the Cox proportional hazard regression was conducted in the package ‘survival’ (Therneau 2015).

Peig, J. and Green, A. J. (2009). New perspectives for estimating body condition from mass/length data: the scaled mass index as an alternative method. Oikos 118, 1883-1891. doi: 10.1111/j.1600-0706.2009.17643.x.

Pinheiro, J., Bates, D., DebRoy, S. S., Sarkar, D., and Team, R. D. C. (2014). Nlme: Linear and nonlinear mixed effects models. (R Foundation for Statistical Computing.)

Team, R. D. C. (2015). R: A language and environment for statistical computing. (R Foundation for Statistical Computing: Vienna, Austria.)

Therneau, T. M. (2015). A package for survival analysis in R.

Trivers, R. L. and Willard, D. E. (1973). Natural Selection of Parental Ability to Vary the Sex Ratio of Offspring. Science 179, 90-92. doi: 10.1126/science.179.4068.90.

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