Seven Swans are Swimming

One of the local pairs of Mute Swans has successfully managed to rear 7 cygnets this year. Usually 2-3 pairs of Mute Swans breed on the Carrs (a lowland area of farmland with dykes and ponds) and surrounding land close to where I live. These usually successfully raise 2-5 cygnets on average each year. This pair seems to have done particularly well this season so I thought I would take a look in a bit more detail at it the ecology of Mute Swans in relation to clutch sizes.

Firstly Seven seems quite a large number but it isn't unusual for Mute Swans to lay this many eggs, in fact its around average for many populations. Typically Mute Swans lay between 3 and 9 eggs per clutch, with occasional clutches up to 13 occurring. These larger clutches usually involve two females laying in the same nest. Typically clutch sizes average at 5 or 6 eggs across most of the geographical range of this species. Mute Swans can be colonial in some parts of Europe (places like Denmark and Estonia), which is hard to believe after having seen a male see off intruders, at these colonies the average number of eggs decreases to around 4 to 5 egg per bird, with a high frequency of 1 to 3 egg clutches. 

In Mute Swan average clutch size is strongly linked to average winter temperature, warmer winters result in birds in better condition which breed earlier resulting in a larger clutch. Clutch size decreases with laying date, so birds which lay their first egg late in the spring tend to have a smaller brood than those which lay earlier in the year. Cold winters and springs can therefore impact the entire Mute Swan population resulting in a subsequent poor breeding season. In addition as one would expect larger females generally produce larger clutch sizes. Older females also tend to produce smaller clutches after the age of 14 years old, before this clutch size tends to slowly increase in the first 10 years of life.  

Clutch sizes in a study of 102 nest of non-colonial Mute Swans in Britain. 

So as can be seen above a clutch of 7 is around average for Mute Swans and not as unusual as it may perhaps first seem. Anecdotally the local swans here rarely seem to get to 7, this is undoubtedly influenced by the factors discussed above, but I also wonder how much habitat plays its part. In more productive habitats one would expect heavier better quality females, able to better withstand cold temperatures and produce lots of offspring.