3.6 Reproductive success

Reproductive success (RS) is one of the most important variables to measure as it lets you know how well your study species is faring. Changes in reproductive success can be used to assess how species are reacting to management actions or how environmental variables (such as climate change) are impacting them. Over the long term this calculation can be used to help understand overall population trends.
Put simply, RS is calculated as the number of chicks that fledged from the number of nests where breeding was confirmed (i.e., egg seen or bird sitting tight on nest incubating). Note nests used in these calculations should all be confirmed breeders, not prospectors, which would negatively skew the calculation.

RS = number fledged / total number of monitored nests with confirmed breeding & outcome known

While this calculation is relatively easy for surface nesting species where breeding can be directly observed, it can be much harder for burrow nesting species where it may not always be possible to access the burrow chamber itself. In these scenarios, sign around the burrow entrance can be used to help you sleuth out the outcome of a burrow, particularly if you have a good understanding of the breeding phenology of your seabird species. For example, guano, feathers and footprints outside the burrow throughout the season, followed by bits of down attached to the burrow entrance at the end of the season, could be used to infer a successful breeding attempt. Conversely, regular sign outside a burrow that ceases halfway through the season could be used to infer a failure (but beware prospecting birds and non-breeding pairs). Burrow cameras can also be used to gain a greater understanding of whether or not a burrow was successful.

In the case of burrow nesting seabirds, the above calculation of reproductive success is likely to be an overestimate of true reproductive success. This is because each year there will be a certain number of monitored burrows where breeding did in fact occur and the burrow failed, but these were not considered in the reproductive success calculations as it was not possible to actually confirm breeding (mainly because the burrow was too deep to assess whether an egg was ever laid). Therefore, another more conservative measure could also be considered as follows:

Number fledged / total number of monitored burrows that were confirmed active (not including prospectors) & outcome known 

In studies of burrow nesting species, both calculations could be considered to assess the true reproductive success, with the assumption being that the true figure lies somewhere between the two.

Another thing to consider is sample size. If you have calculated your RS with only three burrows then this clearly would be less indicative of the true RS of the colony than if you had sampled 60. As a general rule of thumb, the more burrows you monitor the more robust your estimate will be, within the confines of burrow availability, budget and logistics. A couple of other considerations should be taken into account. If your colony is scattered across a large area with multiple terrain features, make sure you are monitoring burrows across the colony in its entirety – predation can be very location specific, and birds doing well in one part of the colony may have fare differently from another area where the birds are being eaten by a localised predator.